Content and licensingview original scan buy a printed copy

Geology of the country around Aberdeen: Memoir for 1:50 000 sheet 77 (Scotland)

By M. Munro

Bibliographic reference: Munro, M. 1986. Geology of the country around Aberdeen. Memoir British Geological Survey, Sheet 77 (Scotland), 124 pp.

British Geological Survey Natural Environment Research Council

London: Her Majesty's Stationery Office 1986. © Crown copyright 1986 First published 1986. ISBN 0 11 884390 7

(Front cover)

(Rear cover)

Other publications of the Survey dealing with this and adjoining areas

Books

Geological and geophysical maps

Notes

Much of Sheet 77 lies within the area of 100 km National Grid square NJ and grid references for localities within this square are quoted without grid letters. Grid letters are given, however, in references to localities in the northeastern corner of the map lying in the 100 km square NK, or in the southern, part of the map lying in 100 km square metres), but eight-figure (10 metre) references are given for most drill holes.

Specimen numbers refer to a comprehensive collection which is stored in the Department of Geology, Aberdeen University. A specimen from a drill hole or temporary exposure is designated thus*.

Preface

The lack of an up to date and readily available geological map and descriptive memoir for the Aberdeen area are deficiencies which have become increasingly significant with the passage of time as the geological investigation of north-east Scotland has progressed. In 1969 the Institute of Geological Sciences felt that it would be unable to proceed with the resurvey of the Aberdeen area in the foreseeable future and it was agreed that this should be undertaken by a team from the geology department of Aberdeen University. Following informal discussions between Mr G. S. Johnstone, who at that time was District Geologist in charge of the Highlands and Islands Unit of the Geological Survey and Dr M. Munro of Aberdeen University, a contract was negotiated between the university and NERC. Under the terms of this contract the university was responsible for the production of new geological maps (Sheet 77 solid and drift) and a descriptive memoir for the Aberdeen area.

A team of seven post-graduate research assistants was recruited to work under the direction of Dr Munro on the project over a period of five years. The re-mapping of the Sheet 77 area was mainly carried out by five members of this team-I. G. Duncan, F. McLean, W. M. Murdoch, W. G. Porteous and E. B. Walsworth-Bell, while lesser, but significant, contributions were made by L. J. Edwards and M. H. Smith who were employed on the contract for only a limited period of time. A major contribution was also made by R. Boyd, who conducted a detailed investigation of the Belhelvie 'Younger Basic' mass which lies wholly within the Sheet 77 area, although he was financed by a NERC research studentship, and was not formally employed on this contract.

The project was carried out under the direction of Dr Munro, but could not have been completed without the assistance and expertise of many members of the academic and technical staff of the Aberdeen University geology department. A particularly important feature of the reinvestigation was the use of geophysical techniques in conjunction with shallow drilling as aids to field mapping in the areas of poor exposure which are typical of much of the region near Aberdeen. The scale on which these techniques were employed, and the resultant improvement in the mapping, are largely a reflection of the energy and geophysical expertise of Dr W. A. Ashcroft. Other members of the academic staff provided invaluable assistance in the field and the laboratory, with the contributions of Drs A. Crane and M. A. Lappin being particularly significant. The support given by the technical staff was extensive and owed much to the direction of Mr G. G. Brebner in the early stages and of Mr R. Maitland when the project was nearing completion. Special mention must be made of the assistance provided by Mr Maitland (XRF analyses) and by Messrs G. Taylor (microprobe), A. Hawkesworth (geophysics) and A. Mathieson and I. Morrison (drilling and trenching).

A close relationship was maintained throughout the project with the staff of the Highland Unit of the Geological Survey. The success of the project owes much to the co-operative attitude of Mr G. S. Johnstone and to Dr J. D. Peacock, who has not only provided essential expertise in supervising the drift mapping, but has also been largely responsible for overseeing the final production of the Sheet 77 maps and memoir.

The memoir has been compiled by Dr Munro, largely from the PhD theses submitted by five of the research assistants and by R. Boyd. Contributions from Dr Boyd are incorporated in Chapters 6, 7 and 8; from Dr I. G. Duncan in Chapters 2, 3, 4, 5, 7, 8 and 11; from Drs F. McLean and W. M. Murdoch in Chapter 13; from Dr W. G. Porteous in Chapters 2, 5 and 9; from Dr E. B. Walsworth-Bell in Chapters 2 and 9. Chapter 9 also incorporates unpublished work by M. H. Smith and the geophysical maps and techniques owe much to the efforts of L. J. Edwards. In as far as has been possible credit for the individual contributions has been acknowledged in the text.

Following the completion of the field mapping and the submission of the PhD theses the production of the final maps was delayed so that information from four trenches for oil and gas pipelines which traversed the map area could be incorporated. These trenches have provided extensive sections through the rocks and superficial deposits in the inland area of the map which are unlikely ever to be exposed again, and at many localities have greatly clarified the understanding of complex relationships and have made it possible to produce much more detailed solid and drift maps. These results have wholly justified a delay of over three years in completing the project.

The acquisition of new information from these trenches and other sources (e.g. additional drillholes) has meant that some of the views expressed by the individual research workers in their theses have had to be modified. Further interpretative amendments have also been made in the final stages of the project when the author had had an opportunity to assess a much more extensive body of data than was available to any of the research students.

Dr Munro was assisted in the final compilation of the memoir by colleagues who have provided constructive criticisms of original draft manuscripts. The comments made by Dr W. Welsh have been particularly helpful. Messrs J. B. Fulton and W. Ritchie drafted the original diagrams and due acknowledgement must be made to the patience of Mrs W. Hadden in typing several drafts of the memoir.

Sir Malcolm Brown, DSc, FRS Director, British Geological Survey, Keyworth, Nottinghamshire. NG12 5GG 4th October 1985

Geology of the country around Aberdeen—summary

The study of the poorly exposed rocks of the Aberdeen area has been greatly assisted by the widespread use of magnetic surveying and by the availability of extensive temporary exposures in trenches for oil and gas pipelines.

The high grade Dalradian rocks which occur widely in this area probably belong to the Argyll Group and have been subdivided into the Aberdeen Formation, characterised by pelitic assemblages containing garnet, and the more strongly deformed Ellon Formation, in which pelites contain cordierite. Lower grade rocks of the Collieston Formation which appear to be members of the Southern Highland Group occur in the north-cast. An exceptional feature of the Aberdeen Formation is the presence of 'regional' corundum in certain pelites.

'Younger Basic' intrusions which were emplaced in the Dalradian rocks after the climax of regional metamorphism with the widespread development of hornfelses, have been much modified and disrupted by an episode of shearing and mylonitisation that also produced an important transgressive zone of steep structures in the country rocks. Granitic bodies were intruded over a lengthy period extending from prior to this deformation episode to the Devonian.

Old Red Sandstone sediments in the coastal area near Aberdeen have been affected by important NE–SW faults, which in part have also controlled the emplacement of late-Carboniferous quartz dolerite dykes.

A great diversity of erosional and depositional features associated with the Pleistocene glaciation are visible near Aberdeen, but although two major till units can be identified, nearly all these features appear to be associated with the Devensian glaciation.

The city of Aberdeen is in the middle distance. The low-lying sandy coastline extending northwards to beyond the mouth of the Ythan can be discerned in the cast, while much of the area inland has subdued relief and lies at an elevation of approximately 130 m. (D3359)

Chapter 1 Introduction

Location and area

The geology of Sheet 77 of the 1:50 000 geological map of Scotland, representing an area of approximately 470 km2 adjoining the city of Aberdeen (Figure 1), is described in the following account. The eastern boundary of this area is defined by the coastline between North Broad Haven, 1 km south of Collieston, and Blowup Nose, 2 km south of Cove. The western boundary lies 15 to 20 km inland and extends from near Meiklepark, 1 km north-east of Oldmeldrum, to Bogton in the valley of the Dee, 3 km south-west of Peterculter.

Physical features

The wide valleys of the rivers Dee, Don and Ythan, which flow in a general easterly direction towards the coast, are major topographic features of the area. Much of the landscape between these valleys is gently undulating, but there is a general tendency for elevation to increase towards the west, where the summits of Tyrebagger Hill (250 m above OD) and Brimmond Hill (265 m above OD) are the highest points within the map area (Figure 1). However, there are considerable areas where much of the land surface shows little variation in altitude, and it has been proposed (e.g. Walton, 1963) that the remnants of former plateaux may be preserved at elevations of approximately 70 m and 130 m (Plate 1).

On a small scale, the landscape shows irregularities, which, in many instances, can be ascribed to erosional or depositional events during the waning phases of the Pleistocene glaciation. Thus, deeply incised meltwater channels, such as the valley of the Leuchar Burn [NJ 827 025] ((Figure 1), (Plate 27), occur throughout the map area, while at many localities near the coast the landscape is dominated by mounds of sand and gravel deposited by meltwater.

There are no obvious relationships between the topography and the underlying solid geology in much of the map area, although the ridge of high ground extending SSW–NNE through Tyrebagger Hill can be related to the presence of a resistant unit of psammitic metasediments (Figure 2), and the low-lying areas to the east near Dyce and to the west near Blackburn (Figure 1) are largely underlain by granite. The lower valley of the Dee coincides with the line of a major fault (Figure 28), while the readily eroded nature of the Old Red Sandstone sedimentary rocks that occur near the coast immediately to the north of this fault (Figure 27) is made evident in drill hole records which show that rockhead is well below sea level under much of the eastern part of the city of Aberdeen (Figure 35). The Old Red Sandstone rocks are known to extend for at least 10 km north of Aberdeen, and it is possible that the low-lying nature of the entire coastal strip between the mouths of the Dee and the Ythan reflects the presence of outcrops of these rocks in the vicinity of this part of the present-day coastline. Certainly, there is a marked change in topography in the coastal areas north of the Ythan and south of the Dee, where Dalradian metamorphic rocks extend to the sea, and the coastline has rugged cliffs up to 30 m high.

History of the research

The original version of Sheet 77 published in 1885 contains only limited information on the nature and extent of the superficial deposits and is essentially a map of the solid geology of the area. Prior to the publication of this map, the solid geology of the area had been described only in a very general fashion (e:g. Nicol, 1860), but as no descriptive memoir accompanied the map, and no systematic studies have been undertaken subsequently, the solid geology of the map area has remained largely undescribed. Brief descriptions of the widespread regionally metamorphosed rocks have been provided by investigators who have been mainly concerned with the geology of the areas lying outside the boundaries of Sheet 77 (e.g. Hinxman and Wilson, 1890; Barrow, 1912; Read, 1955; Read and Farquhar, 1956; Phemister and others, 1960) and have conveyed the impression that the Sheet 77 area is occupied by high-grade gneisses (e.g. Chinner, 1966) at-id is traversed by the axis of a major anticline (the 'Buchan Anticline', Read and Farquhar, 1956) which trends N–S (e.g. Stewart, 1970). The only detailed investigations in the area have been restricted in scope, and include the study of the Old Red Sandstone sedimentary rocks near Aberdeen by Milne (1902), of certain of the granitic rocks by Cameron (1945), and of the unmetamorphosed basic and ultrabasic igneous complex at Belhelvie by Stewart (1947).

Although the superficial deposits were largely ignored when the original map was drafted, the study of these deposits has received a great deal of attention both prior to and after the publication of this map, and there is now an extensive literature on various aspects of the Quaternary geology of the Aberdeen area. The systematic description of glacial phenomena was initiated by Jamieson in 1858, and continued in a series of later publications (e.g. Jamieson, 1860; 1874; 1906) in which he developed the view that the glacial record could be best interpreted by postulating that north-east Scotland had been subjected to several episodes of glaciation. This hypothesis, which had a considerable influence on the development of ideas on the sequence of events during the Pleistocene glaciation, not only in the Aberdeen area, but in Scotland as a whole, was later further developed and refined by other investigators, notably Bremner (e.g. 1915; 1928; 1938) and Synge (1956). However, in recent years Jamieson's hypothesis has been increasingly questioned, and there has been growing support for the view that virtually all the glacial phenomena can be ascribed to the last (Devensian) glacial episode which has affected the Aberdeen area (e.g. Simpson, 1948; 1955; Clapperton and Sugden, 1977).

The relative neglect of the solid geology in the Sheet 77 area can be largely ascribed to the factor which has promoted the glacial studies, namely the presence of a widespread cover of Pleistocene deposits. Much of the detailed information acquired during the remapping was obtained only because techniques for identifying the nature of the bedrock beneath the drift were available (magnetic surveying, shallow drilling, trenching) or because trenches for pipelines provided extensive temporary exposures in areas devoid of natural outcrops (Plate 2).

Summary of geology

The superficial deposits and lithological units that have been recognised within the Sheet 77 area, can be listed as follows:

Superficial deposits (Drift)

Solid formations

All the regionally metamorphosed rocks have been affected by the Caledonian orogeny (sense lato), and all the igneous rocks, apart from the quartz-dolerite dykes, were emplaced within 100 Ma of the orogenic climax.

The distribution of the Solid formations is shown in a 'Solid' edition of the 1:50 000 map.

Metamorphic rocks

Three main groups of regionally metamorphosed rocks were distinguished in the original version of Sheet 77. The most important unit was described as 'gneiss' in the explanation and was shown as occupying much of the map area. The other units were described as consisting of quartz schists and quartzites in one instance and as 'knotted' schists and quartzites in the other, and were depicted as occurring in a limited area ((Figure 2), inset) in the north-east of the map area.

All of these units had been recognised during the earlier mapping of the Fraserburgh and Peterhead areas to the north (Wilson, 1882; 1886), and, when central Aberdeenshire was mapped at a later date (Hinxman and Wilson, 1890, p. 8) the gneiss unit was widely identified to the west of the Sheet 77 area. Read later suggested that this gneiss unit should be called the Ellon Series (1923a, p. 451; 1952, p. 271), and also proposed the names Mormond Hill Quartzite (1952, p. 270) and Collieston Beds (Read and Farquhar, 1956, p. 134) for the quartz schist/quartzite and knotted schist/quartzite units respectively.

The remapping of the Sheet 77 area has shown that this subdivision of the regionally metamorphosed rocks is inadequate and the following revisions have been made:

1. The rocks grouped in the original 'gneiss' unit have been found to show considerable diversity and, can be regarded as belonging to two distinctive lithological units:

The Aberdeen Formation—This occupies a considerable part of the map area and consists largely of interlayered metasediments, which are generally mainly psammites and semi-pelites, with only subsidary occurrences of pelitic rocks, and are often highly migmatised. This formation can be further subdivided into Northern and Southern units. The Northern unit is characterised by the presence of major sub-units which consist predominantly of either psammitic or relatively pelitic (pelitic and semi-pelitic) rocks (Figure 2) and by the relatively frequent occurrence of amphibolite bodies and minor calc-silicate horizons. In contrast, the Southern unit contains no large-scale metasedimentary sub-units dominated by particular lithologies, few amphibolites and virtually no calcareous horizons. Pelites and semi-pelites throughout the Aberdeen Formation often contain sillimanite and garnet, and occasionally contain andalusite and staurolite in the Southern unit, and corundum in the Northern unit.

The Ellon Formation—The rocks of this formation occupy much of the north-eastern part of the map area, and are mainly semi-pelitic and psammitic metasediments, although amphibolite is sometimes the predominant lithology over considerable areas. Typically these rocks lack the regular lithological banding that can be observed in the Aberdeen Formation, are poorly fissile and have a distinctive 'streaky' appearance. Characteristic minerals in pelites and semipelites include sillimanite, andalusite and cordierite. Read (1923a; 1952) described similar rocks from the Ythan Valley as being typical of his 'Hon Series' and although the name 'Elton Gneiss' has subsequently been widely used for this unit (e.g. Harris and Pitcher, 1975; Sturt and others, 1977) the term Elton Formation is preferred, partly because some of the rocks are not marked gneissose (e.g. the amphibolites), and partly because the term 'ElIon Gneiss' has been used as a designation, not only for the distinctive rocks in the Ythan Valley, but also for the rocks in other parts of Aberdeenshire (e.g. Read and Farquhar, 1956, fig. 2), which are now known to display the typical characteristics of members of the Aberdeen Formation (Figure 2).

2 The name Collieston Formation is given to the relatively low-grade, metamorphic rocks that occur within the Sheet 77 area in the coastal zone north-east of the Ythan. Metagreywackes and 'knotted' pelites containing andalusite and cordierite are important constituents of this formation, which consists largely of metasediments, with only minor amphibolites. As there is no evidence that a distinctive psammitic unit occurs between these rocks and the rocks of the Ellon Formation to the west, no 'Mormond Hilt Quartzite' is shown on the new map. Structural, textural and mineralogical evidence shows that the rocks in all three formations have had a complex deformational and metamorphic history, with the main period of recrystallisation taking place after much of the deformation had occurred. No major structures have been recognised, apart from a zone of rocks with steeply-dipping foliation that extends northwards from the mouth of the Don. The presence of deformed 'Younger Basic' igneous rocks in this zone suggests that it is a relatively late structure. The boundary between the Ellon and Aberdeen formations is partly defined by this zone, but the boundary between the Ellon and Collieston formations is not exposed.

Igneous rocks

In 1919 Read suggested that the igneous rocks associated with the Caledonian orogeny in north-east Scotland could be subdivided into an Older (pre-metamorphic) Series and a Younger (post-metamorphic) Series, and although later work has indicated that there are complexities in the relations of some of the igneous masses, this proposal still provides the basis for a broad scheme of classification. The two series include rocks ranging from ultrabasic to acid in composition, but it has become customary to use the terms 'Older Basic' and 'Younger Basic' for associations of igneous rocks that are predominantly mafic and ultramafic, and 'Older Granite' and 'Younger Granite' for predominantly acid assemblages.

Within the Sheet 77 area, the 'Older Basic' rocks are found in all three metamorphic formations and include amphibolites which seem to be the metamorphosed derivatives of a single tholeiitic suite. Metamorphosed ultramafic rocks also occur in the Northern unit of the Aberdeen Formation (Figure 2). The 'Younger Basic' rocks are represented by intrusions consisting predominantly of mafic and ultramafic igneous rocks that have thermally metamorphosed the surrounding regionally metamorphosed rocks of the Aberdeen Formation. The large Belhelvie mass lies wholly within the map area, and a small portion of another large intrusion, the Insch mass, extends into the north-west corner of this area (Figure 2). Both of these intrusions are shown on the original map but small bodies of 'Younger Basic' rock found to the west of Udny and to the east of Pitmedden (Figure 21) during the remapping have not been described before.

Most of the rocks in the 'Younger Basic' masses in the map area display textural features that suggest an origin by crystal accumulation in magma reservoirs (Wager and others, 1960), and this interpretation is supported by the presence of primary igneous layering in the Belhelvie mass. Evidence of hornfelsing occurs sporadically near these intrusions, being absent at many localities in the vicinity of the Insch and Belhelvie masses, and surprisingly widespread near the small bodies of mafic igneous rock near Udny and Pitmedden. The hornfelses east of Pitmedden contain an area of puzzling 'xenolithic gneisses' which may also be the products of thermal metamorphism (Figure 21).

At some localities, particularly in the Belhelvie mass, (Figure 22), there is evidence that the rocks in the 'Younger Basic' masses have been subjected to shearing and mylonitisation following consolidation. The anomalous relationships between the size of the 'Younger Basic' masses and the extent of the associated hornfelses suggests that large-scale disruption of igneous rocks and thermal aureoles occurred during this deformation episode, which probably also produced the zone of steep structures in the country rocks that extends northwards from the Brig O' Balgownie (Figure 5), (Figure 22). Isotopic age determinations (van Breemen and Boyd, 1972; Pankhurst, 1982) show that much of this deformation took place in the interval between the consolidation of the 'Younger Basic' masses (489 ± 17 Ma) and the crystallisation of a pegmatitic granite in Balmedie Quarry (462 ± 5 Ma).

The distinction of 'Older' from 'Younger' granitic rocks in the map area is less straightforward. The remapping has established that the granitic rocks in the vicinity of Aberdeen do not occur as a single, large, coherent mass extending from the city to the western limit of the map and beyond, as shown in the original version of Sheet 77, but as three separate intrusions (Figure 23). The Clinterty and Crathes masses in the west (Figure 2) display 'Younger' characteristics, as they consist of bodies of essentially uniform granodiorite that cut across structures in the surrounding metamorphic rocks. The third intrusion, the body of muscovite-biotite granite underlying much of Aberdeen city (the Aberdeen mass), has external contacts and internal foliation that are oblique to the trend of country rock structures and thus displays some 'Younger' characteristics. However this mass also displays affinities with granitic members of the 'Older Series' in being petrographically variable and in frequently containing numerous, highly-modified relicts of the country rocks.

Similar muscovite-biotite granite occurs as veins and sheets that form a migmatitic complex within the Aberdeen Formation and possibly also occur on a more reduced scale in the Ellon and Collieston formations. These subsidiary bodies of granite often have ill-defined, gradational contacts with the surrounding metamorphic rocks and are normally less than 10 m thick, although masses of similar migmatitic granite at Cove and Hill of Crimond (Figure 23) have dimensions of the order of 1 km. Other small granitic instrusions depicted on the original version of Sheet 77 have been deleted from the new map as it has been found that they represent areas where the migmatitic veins and sheets are particularly widespread and abundant, rather than coherent masses of granite. Although these granitic bodies display migmatitic features that are normally regarded as being typical of the 'Older Series', they are virtually undeformed and were probably emplaced after the metamorphic climax in the surrounding rocks.

A body of garnetiferous leucogranite near Kincorth (Figure 23) is probably related to the migmatitic vein complex, but a mass of tonalitic rock on the coast at Altens Haven (Figure 23) is cut by the granite sheets. Dioritic rocks near the boundary of the Ellon and Aberdeen formations may also be associated with the vein complex, or may represent modified 'Younger Basic' rocks.

Isotopic studies (Halliday and others, 1979; Pankhurst, 1982) suggest that the age of the granitic rocks in the Aberdeen area is probably comparable to that of other granitic rocks in north-east Scotland (c. 400 to 460 Ma).

The final stages of the Caledonian igneous activity in the area are probably represented by occasional small lamprophyre instrusions, by the body of explosion breccia at Souter Head and by felsite sheets (Figure 23).

The last igneous event was the intrusion of Late Carboniferous (c. 295 Ma) quartz-dolerite dykes. These intrusions are up to 15 m wide, trend ENE–WSW, and were sometimes emplaced on pre-exisiting lines of faulting and shattering (Figure 28).

Sedimentary rocks

Old Red Sandstone sedimentary rocks consisting largely of conglomerates, but including subsidiary sandstones and shales, underlie the eastern part of the city of Aberdeen and also occur at or near the coast for at least 10 km to the north of the Don. These rocks rest on a highly irregular surface and contain clasts of the dominant igneous and metamorphic rocks of the Aberdeen area, including fragments of the Aberdeen granite and 'Younger Basic' rocks. NE–SW faults limit the occurrence of these sedimentary rocks in the north near Belhelvie and in the south in the Dee valley (Figure 28).

Quaternary

Evidence of the effects of the Pleistocene glaciation is widespread, and includes features such as ice-moulded topography, glacially striated and abraded rock surfaces, and the layer of till that overlies much of the bedrock (Figure 31). No evidence of geological events in the interval between the Devonian and the Pleistocene glaciation has been preserved, apart from localised areas of deeply weathered rock, which may have been produced in the late Tertiary, and the plateaux postulated by geomorphologists, which may represent erosional surfaces produced during Tertiary uplift.

The till layer is usually no more than 1–4 m thick, although thicknesses of 15–20 m have sometimes been recorded. Three types of till can be recognised; a relatively argillaceous till with a strong NW–SE fabric which occurs locally above depressions in the bedrock; b grey-brown, relatively arenaceous till which overlies till a or rests directly on bedrock; c red-brown, argillaceous till with a S–N fabric which is found in the coastal area. This till may grade laterally into till band contains clasts of Old Red Sandstone rocks.

Other features associated with the glaciation include meltwater channels, which may be incised into cols and have arched longitudinal profiles, and which tend to be orientated W–E in the inland areas and S–N in the coastal area (Figure 33). Deposits of sand and gravel laid down by meltwater include a high proportion of ice-contact deposits, such as the terraces in the Dee and Don valleys and the eskers in the coastal zone. Clay and silt deposits are confined to the coastal zone and generally appear to have formed in glacio-lacustrine, ice-contact environments, although some deposits which are virtually at present sea level may be glacio-marine in origin.

In the past, supporters of the multiglacial hypothesis, notably Jamieson and Bremner, have explained the diversity of the glacial and melt-water features in north-east Scotland as being due to several successive periods of ice advance and withdrawal. However, no exposures containing an unambiguous record of several glacial and interglacial events have ever been found, and it is possible to explain virtually all the glacial and melt-water features in the map area as being due to a single event (Devensian) involving the convergence of an ice sheet moving eastwards from the Highlands with an ice sheet moving northwards at the coast (Figure 38).

A minimum age for the withdrawal of the Devensian ice (c. 11 500 BP) has been obtained from a plant bed in a quarry near Dyce. Features such as gravel-filled, ice-wedge casts suggest that periglacial conditions may have occurred subsequently, but there is no evidence of a glacial event postdating the retreat from the Devensian maximum. The presence of deposits of wind-blown sand in coastal areas, of peat in inland basins, and alluvium in river and stream valleys can all be ascribed to the agencies of erosion and deposition in operation at the present day.

Chapter 2 Regionally metamorphosed rocks—1

Metasedimentary terminology

The terms pelitic, semi-pelitic and psammitic, used throughout this memoir in the description and classification of compositionally variable metasedimentary rocks, have no universally accepted definitions. During the remapping, 'working definitions' of these terms were adopted for application to hand specimens and to rocks in outcrop for which no detailed quantitative information was available, but no attempt was made to devise a precise and rigid system, such as has been suggested by other investigators (e.g. Wallis and others, 1968). However, the 'Sheet 77' usage is similar to that employed by Mather (1970, p. 261) in his description of Dalradian metasediments in the Aberfoyle area.

The usage of the terms was determined by the proportion of non-quartzo-feldspathic constituents in the rock (normally biotite and other ferromagnesian minerals but also including muscovite and the Al2SiO5 polymorphs). Rocks with more than a minor amount (a few per cent) of hornblende, epidote or other calcareous minerals were excluded. Pelites were defined as containing more than 30 per cent of constituents other than quartz or feldspar, semi-pelites as containing between 10 per cent and 30 per cent, and psammites as containing less than 10 per cent of these constituents.

Many of the psammites approach quartzite in composition (> 80–90 per cent quartz), but feldspathic varieties are also common and although some of these rocks may have been produced by feldspathisation during migmatisation, many are probably derived from arkosic parents. The overall chemical characteristics of the pelites and semi-pelites suggest that most of these rocks had argillaceous parents (Figure 3). However, the Al2O3 to SiO2 ratio in several of these rocks is much higher than in most clays and shales (Figure 3)a. This is a feature that has been noticed in high-grade Dalradian metasediments from elsewhere (e.g. Chinner, 1960; 1966), and may reflect the influence of metamorphic rather than sedimentary processes.

The analysed rocks show an overall similarity to analysed Dalradian pelites and semi-pelites from elsewhere (Atherton and Brotherton, 1972; 1982), generally having M/MF ratios (molecular MgO/MgO + FeO) in the range 0.54–0.36, and being less aluminous than the muscovite-chlorite join in AK (FM) projection (Figure 3)d.

The Aberdeen Formation

The rocks of the Aberdeen Formation are exposed in the south and west of the Sheet 77 area and clearly extend southwards and westwards into the adjoining map areas (Sheets 67, 66 and 76) and northwards into the southwestern corner of the Sheet 87 area. A boundary between this formation and the Ellon Formation must occur in the north-east but cannot be located accurately from outcrop data, partly because there are few exposures in the critical area, and partly because the presence of hornfelses and the effects of late deformation complicate the interpretation of the large-scale geological relations. However, outcrops of psammitic metasediments at localities ([NJ 954 213], [NJ 956 211]) near Hillhead of Ardo and at Dubbystyle [NJ 948 212] show that the Aberdeen Formation extends northwards beyond the position of the 21 Grid line near the coast (Figure 4c). Further inland the boundary appears to trend northwesterly, with the outcrops of varied metasediments at [NJ 919 235] near Runnygurnal and at [NJ 912 253] near Monkshill and of psammitic rocks at [NJ 925 227] near Bridgefoot and at [NJ 905 261] near Mosshead providing some indication of the extent of the unmodified (unhornfelsed) rocks of the Aberdeen Formation. A wide zone of hornfelses occurs north-east of these last four localities, and includes many rocks that appear to be derived from the Aberdeen Formation (e.g. at Tillyfour [NJ 929 232] and at [NJ 935 243] near Hill of Fiddes), but unhornfelsed rocks of the Ellon Formation occur further to the east (approximately from the position of the 94 Grid line).

A ground magnetic survey in this area reveals a remarkable linear feature trending NNW–SSE near Grid line 94 (Figure 4a) which is defined by a zone of steep gradient separating discontinuous highs ranging up to 300 nT in the west from a continuous narrow low in the east. The magnetic profiles of (Figure 4b) show that the magnetic field is less disturbed to the east of this line than to the west. The outcrop data suggest that this feature can be interpreted as marking the western limit of undisturbed rocks of the Ellon Formation and that the boundary between this formation and the Aberdeen Formation lies within the zone of disrupted, hornfelsic rocks to the west.

This distinctive magnetic feature cannot be followed reliably south of Fiddesbeg, near the southern limit of the hornfelses, but magnetic mapping near Hillhead of Ardo [NJ 955 213] shows that the trend of distinctive horizons of magnetic amphibolite within the Aberdeen Formation changes from NNW–SSE to W–E as they are followed to the east (Figure 2), (Figure 4a), (Figure 4b), (Figure 4c). The foliation in the regionally metamorphosed rocks in this area shows a similar change in orientation, and the interformational boundary probably also trends eastwards from near Hillhead of Ardo to the sea.

General characteristics

Metasediments

The Aberdeen Formation consists predominantly of interlayered psammitic, semi-pelitic and pelitic metasediments with occasional minor calcareous horizons. The thickness of the individual metasedimentary layers shows considerably variations, but rarely exceeds 2 m, and more commonly lies in the range 0.25 to 1 m (Plate 4), (Plate 13)) Looking west, The Priest, south of Cove Harbour [NJ 954 004] (D3135)" data-name="images/P999811.jpg">(Plate 6). Finely-banded sequences consisting of layers no more than 10 to 20 mm thick sometimes occur, particularly where the lithologies are predominantly pelitic and semi-pelitic. Some of these finely-banded rocks have a highly-fissile character where thin (1 to 2 mm), micaceous partings separate thicker semi-pelitic layers. Individual layers are sometimes of constant thickness over distances of several metres, even when they are only a few millimetres thick, but many layers are impersistent and wedge-like in cross-section.

The lithological banding is presumably derived from original bedding, but no other primary sedimentary structures have been recognised. The prominent fissility or foliation that characterises most of the rocks is generally parallel to the lithological banding, except near the apices of certain tightly appressed folds where new axial-planar fabrics are developed (Plate 3).

Pelitic layers are generally highly micaceous and are often relatively coarse-grained (2 to 3 mm) and of limited thickness (less than 0.25 m). Semi-pelitic horizons show considerable variation and range from massive, fine-grained (< 1 mm) to coarser (2 to 3 mm), more fissile rocks. Psammites may be massive and quartzitic, with only minor biotitic streaks and laminae, but feldspathic or micaceous varieties with a more pronounced fissility also occur. The thickness of psammitic layers is usually greater than that of the associated pelitic and semi-pelitic bands and often exceeds 1 m. Some of the psammitic rocks are clearly derived from coarse-grained sediments and contain clasts of quartz and feldspar up to 3 mm in diameter. Minor calc-silicate and calcareous layers tend to be fine-grained and pale green and occur as thin (0.25 to 0.3 m) bands that are generally separated by several metres of non-calcareous rocks, even at those localities where lime-rich horizons are relatively abundant. However, rather thicker sequences of calcareous rocks occur locally-notably at the roadside [NJ 828 188] near Denmill and near Disblair (Figure 2), where the calcareous rocks have a thickness of several metres and include a thin (c. 0.25 m) limestone horizon at two localities [NJ 864 198]; [NJ 871 198]. These intermittent exposures suggest that the calcareous rocks in the Disblair area extend along strike (E–W) for approximately 1 km.

The three main types of metasediment are interbedded throughout the map area, and although psammites and semi-pelites generally predominate over pelites, the proportions of the different lithologies vary greatly from outcrop to outcrop. In well-exposed areas (e.g. part of the foreshore near Cove Harbour [NJ 954 005]) it can be seen that psammitic, semi-pelitic, or more rarely pelitic, metasediments sometimes predominate within parts of the metasedimentary sequence that are up to 5 to 10 m thick. This tendency for the rocks to be predominantly psammitic or relatively pelitic is expressed on a larger scale in the more northerly exposures of the Aberdeen Formation, and in the northern-western part of the map area (Figure 2) two major lithological subunits which have thicknesses of the order of 1 km or more have been distinguished.

The more northerly of these sub-units contains a high proportion of migmatised, biotitic pelites, abundant semi-pelites but only minor psammites. Typical pelite/semi-pelite assemblages are visible on the high ground c. [NJ 850 204] a kilometre or so west of Disblair and in the ditch exposures at [NJ 853 229] near Brunthill and [NJ 848 248] near Hattoncrook, but exposures of the dominant pelitic and semi-pelitic rocks are not numerous, presumably because these rocks are highly fissile and weather easily. However, observations made in four pipeline trenches excavated in this area demonstrate clearly that pelitic and semi-pelitic lithologies predominate.

The boundaries of this sub-unit are not sharply defined, but have been drawn with due regard to the prevailing trend of the foliation (Figure 2) and (Figure 5), through localities where it can be established that pelites and semi-pelites predominate over psammites. The presence of abundant amphibolites in some areas (e.g. between Burreldale Moss [NJ 830 240] and Pitmedden [NJ 895 275]), and of numerous sheet-like bodies of granite at others (e.g. near Hill of Crimond [NJ 825 230], (Figure 23)), makes the location of the northern boundary of this sub-unit a matter of particular difficulty. However, even in these areas, the position of this boundary can be located to within 3 to 400 m, and elsewhere the accuracy is probably within 50 to 100 m.

This pelite/semi-pelite sub-unit can be traced into the Sheet 76 area to the west, but, to the east, cannot be followed reliably beyond the position of Grid line 91 into the zone of hornfelsing and structural complexity adjoining the boundary between the Ellon and Aberdeen formations.

A second major metasedimentary sub-unit, approximately 1 km thick and consisting predominantly of psammitic rocks, has been recognised in the Tyrebagger Hill area (Figure 2). Here lithology is clearly reflected in the topography as the ridge of high ground extending from the outcrops of psammites at [NJ 847 103] on Elrick Hill in the south, over Tyrebagger Hill [NJ 844 127] to the outcrops of similar rock in the grounds of Pitmedden House in the north [NJ 862 147] coincides with an area where psammitic rocks predominate. Massive quartzitic and more fissile, 'streaky' feldspathic psammites, such as are exposed on the south slopes of Tyrebagger Hill, are particularly common, and other rock types, such as migmatitic semi-pelites (e.g. at [NJ 852 116]), are subsidiary. The boundaries of this sub-unit are ill-defined but have been drawn with due regard to the presence of outcrops of migmatitic semi-pelites and pelites to the east (e.g. near Dyce Quarries [NJ 863 136] (Figure 23); on Brimmond Hill at [NJ 857 094]) and to the west (e.g. at [NJ 851 144] near Woodlands; on Little Hill at [NJ 832 130] (Figure 2)), and to the prevailing foliation trends (Figure 5) and the topography. This sub-unit is truncated in the south-west in an irregular fashion by the Clinterty granitic mass, and, although exposures and topographic expression die out near the Don, it is likely that the psammites are cut by the Aberdeen granitic mass near the Chapel of St. Fergus [NJ 875 154]. Psammitic metasediments occur further north (e.g. in the railway cutting at [NJ 894 196] near Lower Rannieshill) but no major psammitic body can be recognised in the area northeast of the Aberdeen mass. Although the apparent absence of the psammitic sub-unit in this area may be an indication that the emplacement of the granite has disturbed the country rock structures, this sub-unit appears to narrow northwards and it is possible that the granite has not modified the original relations and that only a tenuous extension of the psammites occurs north of the Don. Alternatively, it is possible that the original relations in the metasediments east of the granite may have been greatly disturbed in the zone of discordant structures recognised in that area ((Figure 22); Chapter 8).

There is also evidence that the Aberdeen Formation may contain other major metasedimentary sub-units. Thus, psammites predominate in the areas immediately north and south of the main pelite/semi pelite sub-unit in the northwestern part of the map area and small bodies of calcareous rocks are relatively abundant in the area between this subunit and the psammitic rocks on Tyrebagger Hill (Figure 2). Exposures in a trench [NJ 835 175] near Castlehungry also suggest that a predominantly pelitic sub-unit occurs approximately 1 km south of the main pelite/semi-pelite sub-unit. However, insufficient information is available for the accurate delineation of the boundaries of these sub-units on the map.

The assessment of original lithological characteristics in the southern part of the map is particularly handicapped by extensive migmatisation, but the rocks to the south of the psammite on Tyrebagger Hill generally do not display large scale lithological variations, although it is possible that a major psammite sub-unit is exposed in the Leuchar Burn section c. [NJ 825 030] (Figure 1). However, no lithological horizons with thicknesses of more than 5 to 10 m can be recognised in the virtually complete coastal section south of Aberdeen, and it is concluded that the lack of thick sub-units is a feature which distinguishes the southern portion of the Aberdeen Formation.

The Aberdeen Formation has therefore been subdivided into a Northern unit in which sub-units consisting predominantly of certain lithologies up to a kilometre or more in thickness can be recognised, and a Southern unit, in which the sequences of any particular lithology are generally no more than a few metres thick. The Northern unit is further characterised by the more widespread occurrence of amphibolites and calcareous rocks, while andalusite and staurolite and abundant prograde muscovite appear to be confined to pelites in the Southern unit.

The boundary between the two units of the Aberdeen Formation is drawn at the southern margin of the major psammitic sub-unit on Tyrebagger Hill, and can be traced as far east as the contact between this sub-unit and the western margin of the Aberdeen granitic mass c. [NJ 873 145] (Figure 2). Relations further to the north-east are obscure because the psammitic sub-unit cannot be followed further. The overall characters of the metasediments, particularly the occurrence of rocks with abundant, prograde muscovite (e.g. in Lochhills [NJ 914 147] and Shielhill sandpits [NJ 938 130]), suggest that rocks of the Southern unit probably extend as far north as the position of the 15 Gridline between the Aberdeen mass and the Belhelvie intrusion. If this inference is correct, then the occurrence of calcareous rocks in the Perwinnes Moss area (e.g. at [NJ 929 123]) must represent one of the rare occurrences of such rocks in the Southern unit. However, original metasedimentary relations probably become highly disturbed in the vicinity of the Belhelvie intrusion (Chapter 8), and there is evidence from within the Belhelvie aureole and from the zone of complication further to the north which suggests that rocks of both units of the Aberdeen Formation may occur in complex association over a wide area near the boundary with the Ellon Formation.

Amphibolites

Amphibolites occur throughout the Aberdeen Formation but are more abundant in the Northern unit, and are the dominant rock type over considerable areas in the north-west near the northern margin of the major pelite/semi-pelite sub-unit (Figure 2). The amphibolites generally occur as sheet-or layer-like bodies (Plate 3)c that sometimes range up to 10 m in thickness (e.g. Cove Harbour [NJ 955 006]) and are generally concordant with the lithological banding in the adjoining metasediments. Many are fine-grained (c. 0.5 to 2 mm) although coarser (2 to 3 mm) varieties are found. Coarser and finer rocks are sometimes interbanded (as at Bareside Point [NO 947 990]), but in other instances coarser rock is confined to the areas adjoining leucocratic, migmatitic veinlets. The fine-grained amphibolites are generally massive and devoid of fissility, except where thin (c. 10 mm), widely spaced (100 to 200 mm apart), micaceous partings cut through the rock. However, extensive zones of micaceous, schistose rock occur at the margins of some of the amphibolites (e.g. at Cove Harbour [NJ 955 006]. Certain amphibolites are relatively highly magnetic (producing positive anomalies of c. 500 nT) and as noted above, at some localities (e.g. near Straloch [NJ 861 215] and Hillhead of Ardo [NJ 955 214]) form persistent, mappable horizons (Figure 2) and (Figure 4).

Large (c. 5 m diameter), erratic blocks of a particularly massive amphibolite with distinctive pock-marked surfaces are conspicuous at [NJ 890 215] on the high ground south-east of Whitlam. Similar rock is interlayered with metasediments in this area at [NJ 891 221] and at [NJ 891 216], where trenching has confirmed that the amphibolite occurs as a steeply-dipping band, 8 m wide (Figure 2). This type of amphibolite was also found in a trench near Tillymaud, 3 km to the north [NJ 896 248] and outcrops at [NJ 877 250] near West Coullie, a further 2 km to the west (Figure 2). Thin sections (see below) show that specimens from all of these localities differ from the typical amphibolites in being derived from ultramafic, olivine-rich parent rocks. Similar metamorphosed ultramafic rocks are also exposed immediately west of the Sheet 77 area on the southern slopes of Lawel Hill at [NJ 810 239], approximately 7 km west of West Coullie (Figure 1), (Figure 22).

Structures

The study of Dalradian rocks elsewhere in the eastern Highlands (e.g. Johnson, 1962; Harte and Johnson, 1969) has established that these rocks generally contain a record of a complex sequence of metamorphic and structural events, and it seemed reasonable to anticipate that comparable complexities would be recognised in the rocks of the Aberdeen Formation. However although the rocks in this formation have obviously been subjected to more than one episode of deformation and recrvstallisation, factors such as the lack of exposures in much of the map area, and the manner in which structural relations are obscured by pervasive migmatisation, have seriously handicapped the elucidation of the complete metamorphic and structural history.

This somewhat incomplete record of metamorphic and structural events for the portion of the Aberdeen Formation within the boundaries of the map area can be summarised as follows:

1 Coastal localities

First folds (F1)

At a number of coastal localities, notably at Blowup Nose [NO 947 987], the headland at the southern margin of the map, and at Cove Harbour [NJ 955 006], there is clear evidence that the rocks have been subjected to at least two periods of folding. An early fold set comprises tight to isoclinal structures with a well-developed axial-planar foliation (Plate 3). Pronounced attenuation on the fold limbs has led to the lithological banding being transposed into the attitude of the fold axial planes and has also produced marked thinning and disruption of the individual layers of different lithology. As a result, many of the folds are now rootless interfolials. These early folds generally have wavelengths in the range 1 to 2 m and show a variation in style from structures with long, straight limbs and angular, almost chevron-like hinge areas, to complex disharmonic varieties.

The axial planes are flat-lying at Cove Harbour, but dip at moderate angles to the west at the Brig O' Balgownie. The fold axes show considerable variations in plunge, but generally trend approximately N–S (between 330°–150° and 030°–210°).

Second folds (F2)

Small scale (amplitudes of a few mm) microfolds and crenulations are also conspicuous at these coastal localities (Plate 5), and are defined, not only by flexures of the lithological banding, but also by folding of the axial planar foliation of the tightly appressed F1 folds. At Blowup Nose and at Cove Harbour folds with amplitudes of several metres or more are coaxial with the microfolds (Plate 4), and, in many other coastal exposures (Plate 13)) Looking west, The Priest, south of Cove Harbour [NJ 954 004] (D3135)" data-name="images/P999811.jpg">(Plate 6) such folds are the dominant structural elements. These larger folds are generally asymmetric with the shorter limbs being steeply inclined while the longer limbs are sub-horizontal. The sense of asymmetry implies overturning towards the south (Phemister and others, 1960). Both the small-scale and large-scale F2 folds generally have rounded profiles, but examples with angular profiles occur which sometimes display new (S2) axial planar foliation in the hinge area. F2 axes are generally sub-horizontal and show a tendency to follow an E–W (080°–260° to 115°–295°) trend. The F2 fold axial planes and associated S2 foliation normally dip at moderate angles (30° to 60°) to the north. Thin sections (e.g. 2417; 2418 [NJ 955 006]; (Plate 9)c) show that the F2 microfolds are overgrown by mica crystals in metasediments and by amphibole crystals in amphibolites. The main period of recrystallisation in these rocks thus postdated, not only the S1 foliation, but also the F2 microfolds that have affected the latter structure.

Third folds (F3)

At Cove Harbour the foliation shows considerable variations in trend that cannot be entirely explained as being due to the superposition of the two fold systems already recognised in this area. Both here and elsewhere (e.g. near Cave of Red Rocks [NJ 965 028]) open folding about N–S axes has produced structures with wavelengths of the order of 5 to 10 m and with nearly vertical axial planes (Plate 7). These folds appear to have been superimposed on all the other structures and must therefore have formed at a late stage in the deformational history of the rocks.

Inland localities

In the inland areas, folds of small amplitude (less than 1 to 2 m) can be recognised at many localities but do not display a readily recognisable pattern of orientation. Microfolds and crenulations can also be recognised throughout the inland areas and, particularly in the northern part of the map area, show a pronounced tendency to trend N–S and to plunge at varying angles to the north. These microfolds trend approximately at right angles to the F2 microfolds in the coastal area, but generally resemble the latter in being overgrown by undeformed crystals of mica.

Orientation of foliation

The outstanding structural feature in most of the outcrops of the Aberdeen Formation is the foliation defined by the preferred orientation of minerals such as the micas and amphiboles which is generally parallel to the lithological banding. Evidence from the coastal exposures suggests that this structure has a composite origin, having developed initially prior to the second fold episode (the microfolding etc.) and being reinforced during a recrystallisation episode postdating that folding; there is no evidence to suggest that the mode of formation of the foliation has varied elsewhere within the map area. Three regions in which the foliation displays different patterns of orientation (Figure 5) are recognisable in the Aberdeen Formation.

Most of the area south of the Dee is characterised by relatively flat-lying foliation that generally dips gently to the south or south-west (Figure 5)r-v but at some localities, (such as Cran Hill [NJ 910 005], (Figure 5)s), or in the extreme south-western corner of the map) the foliation locally strikes N–S or NE–SW and dips steeply.

In the western part of the area north of the Dee, the foliation generally strikes approximately E–W or NE–SW and dips at moderate angles to the north and north-west (Figure 5)a, b, h, j, l, m, n, q). This structural trend is clearly reflected in the trend of the major lithological units in the Northern unit of the formation (Figure 2).

In the more eastern exposures of the formation in the area to the north of the Dee, the foliation becomes steeply dipping and generally strikes N–S or NW–SE as the boundary with the Eliot) Formation is approached (Figure 5)d, k. This change in trend can be recognised near the 91 Grid line in the northern part of' the map (Figure 5)c, but, in the area west of the Belhelvie igneous intrusion, occurs rather further to the west (near Grid line 89). Although exposures are few, it seems probable that much of the area between this intrusion and the Aberdeen granitic mass (Figure 2) is characterised by steeply dipping foliation with a N–S strike and that this structural trend prevails southwards to the Brig O' Balgownie [NJ 942 097] near the mouth of the Don ((Figure 5)p).

The assessment of the significance of these relationships, and particularly the recognition of large-scale structures, is handicapped by the absence of persistent and distinctive marker horizons in the Aberdeen Formation, but it is possible that major folding about NE–SW axes has occurred and has produced the prevailing trend of the foliation and lithological banding in the north-western part of the map area. The marked flexure in the eastern boundary of the major psammite sub-unit on Tyrebaggcr Hill (Figure 2) could also be ascribed to folding on a north-easterly plunging axis, with a synformal structure in the north (near Dyce quarries), being complemented by an antiformal structure in the south (near Elrick Hill). Support for this interpretation is provided by the unusual south-easterly dip of the foliation on Brimmond Hill [NJ 855 090]. The difference in the prevailing orientation of the foliation in the areas north and south of the Dee may also reflect the presence of an open antiformal structure with a westerly or south-westerly plunging axis in this area. However, as the valley of the Dee appears to have developed along a major fault (Chapter 12) it is possible that the difference in attitude of the foliation on opposite sides of the river valley is due to faulting and not to folding.

The areas of steeply dipping foliation with a N–S trend south of the Dee may show that the late, relatively small-scale folds with N–S axes identified in the coastal exposures (the F3 folds) have a larger scale expression here, and the curvature of the southern boundary of the major pelite/semipelite sub-unit in the Northern unit (Figure 2) could be explained by postulating that a major synformal structure with a north-plunging axis occurs in this area c. [NJ 850 200].

It has not proved possible to identify large-scale structures from an analysis of the geometry and orientation of small-scale structures. In particular microfolds and lineations with a NE–SW trend are virtually absent, and whilst the diagrams (e.g. (Figure 5)t) illustrating the orientation of the foliation in the coastal area suggest that the asymmetric folding (F2 folds) in that area has locally developed on a larger scale, there is no evidence that large-scale structures corresponding to the F1 folds of the coastal areas occur.

It might seem reasonable to assume that the steep dip and N–S strike of the foliation in the area of the Aberdeen Formation that lies to the north-north-west of the Brig O' Balgownie and adjoins the Elton Formation (Figure 5)d, k) is also due to folding about a N–S axis. However, there is abundant evidence that the relations of the rocks in this area have been greatly disturbed by shearing and mylonitisation after the Belhelvie intrusion was emplaced (Chapter 8), and it seems more likely that this structural trend is related to this late deformation episode rather than to earlier, large-scale folding.

Petrology and mineralogy

Metasediments

Petrography

The most widespread minerals in the psammites, semipelites and pelites that form the main rock types in the Aberdeen Formation are biotite, plagioclase and quartz. Potash feldspar and muscovite may also be important phases and many specimens also contain minor amounts of fibrolite. Garnet occurs on a more restricted scale, and andalusite, staurolite and corundum are found only in certain pelitic and semi-pelitic horizons.

The recognition of equilibrium assemblages is often handicapped by the finely-banded and highly migmatised nature of many of the rocks, but the following mineral assemblages occur widely:

Pelites

The amount of biotite in these rocks is generally 30 per cent or more, plagioclase is generally a major (30 to 60 per cent) constituent, and although the proportion of quartz is sometimes large (25 to 30 per cent), this mineral only occurs as a minor constituent (c. 5 per cent) in many pelites. Highly micaceous laminae occur locally, generally in migmatised rocks, and may be completely devoid of quartz or feldspar. Muscovite is normally subsidiary to biotite but is the most abundant mica (up to 60 to 70 per cent) in some pelites from the Southern unit.

Garnet occurs as a subsidiary phase (< 10 per cent) in a number of rocks, including the highly micaceous laminae, and garnet-bearing equivalents of the assemblages listed above can be recognised.

Parts of the Southern unit are characterised by the presence of andalusite and staurolite (Figure 2) and the assemblages include:

Muscovite-free rocks occur locally and consist of assemblages such as:

Muscovite is often present in only minor amounts in many other rocks and often seems to have crystallised after the other phases and to be replacive in origin. It is possible, therefore, that the assemblages produced in many pelites (and also in many semi-pelites and psarnmites) during the main episode of metamorphic recrystallisation were devoid of muscovite.

Corundum-bearing assemblages occur at a few localites (Figure 2) in the Northern unit. The most common assemblage is:

Other corundum-bearing assemblages contain only a single feldspar: either plagioclase or potash feldspar.

Semi-pelites

These rocks grade into the pelites and differ mainly in containing less biotite (generally 15 to 20 per cent) and more quartz (generally 30 to 40 per cent). The common assemblages are similar to the pelitic assemblages, and include corundum-bearing, garnetiferous and muscovite-poor rocks, but additional assemblages found in semi-pelites are:

Psammites

Many of the psammitic rocks seen in outcrops approach quartzites (>80 to 90 per cent quartz), and all of the sectioned specimens contain at least 60 to 70 per cent quartz. The proportion of feldspars often approaches 20 per cent, and may be as much as 30 to 40 per cent. Assemblages identified in these rocks include:

Calcareous rocks

These rocks are highly variable in character and there is a gradation from specimens consisting almostentirely of lime-bearing minerals into non-calcareous sediments or into amphibolites. Epidote occurs widely in the calcareous rocks, and is often the dominant constituent. It is normally colourless, but as it tends to occur in small, granular crystals which are often highly altered, cannot be identified with certainty as either zoisite or clinozoisite. Some rocks contain 30 to 40 per cent calcite, and, as noted previously, thin limestone horizons occur near Disblair [NJ 870 198].

Assemblages include:

Varieties transitional to pelites and semi-pelites include rocks that generally contain biotite, plagioclase and quartz, and sometimes contain potash feldspar and muscovite, in association with varying proportions of diopside, colourless epidote, amphibole and sphene. These transitional rocks include corundum-bearing varieties associated with the corundum-bearing pelites and semi-pelites. Transitions to psammites include rocks containing biotite, muscovite, colourless epidote, garnet and quartz while transitions to amphibolite are represented by rocks containing amphibole, clinopyroxene, colourless epidote and quartz.

Accessory minerals

Zircon, apatite and opaque constituents are the common accessories in the metasediments, but one garnetiferous pelite (2420 [NJ 956 006] (Plate 9)d contains a relatively high proportion of apatite (c. 5 per cent). Tourmaline also occurs frequently as an accessory mineral, particularly in the Brig O' Balgownie area, and, in general, appears to occur more widely in the rocks of the Southern unit. The tourmaline crystals often show zonation from blue-green cores to yellow-green margins but blue tourmaline is present in a talc-silicate rock (1379 [NJ 864 198]) from near Disblair. This rock, and another specimen from nearby (1382 [NJ 863 198]), also contain poikiloblastic crystals of highly birefringent scapolite.

Texture and mineralogical details
General pattern of textural relationships

Many of the metasediments of the Aberdeen Formation consist largely of an equigranular (c. 0.5 to 2 mm) intergrowth of biotite, plagioclase and quartz. The plagioclase crystals are sometimes lath-like and subidioblastic, particularly when the rock is coarser grained (2 to 3 mm) than normal, but generally occur in xenoblastic intergrowths with quartz. The biotite usually occurs in subidioblastic, platy crystals that are often concentrated in lensoid aggregates and laminae within the rock, and the foliation displayed by most metasedimentary specimens is normally largely due to the preferred orientation, not only of the individual biotite crystals, but also of these aggregates.

Subidioblastic crystals of muscovite that are of similar size to the crystals of the other main constituents, and display a similar pattern of preferred orientation to the biotite crystals, occur in some rocks, particularly in the Southern unit, where they occasionally form 60 to 70 per cent of micaceous laminae. A limited number of specimens (e.g. 34*[NJ 929 108]) contain crystals of potash feldspar that are intergrown with plagioclase, quartz and biotite and sometimes display microcline twinning. In a few specimens (e.g. 1383 [NJ 870 197]), leucocratic bands consist almost entirely of microperthitic potash feldspar.

Garnet generally occurs only as small (c. 0.5 to 1 mm) crystals, that are rarely idioblastic and generally form irregular intergrowths with quartz and plagioclase, being spongy and almost poikiloblastic in some psammites (e.g. 1083 [NJ 842 130]). Larger (up to 10 mm), better-formed garnet crystals occur in certain pelitic horizons (e.g. 2418; 2420 [NJ 956 060]; (Plate 9)d.

Andalusite is usually porphyroblastic, and often occurs as prismatic crystals ranging up to 20 mm in length that are concentrated in the melanocratic layers and locally form up to 30 per cent of the rock. These crystals generally show no preferred orientation, but the distinctive pink crystals of andalusite in the coastal outcrops extending 0.5 km north of Cove are apparently confined to the plane of the foliation, and sericite pseudomorphs of a prismatic mineral, possibly originally andalusite, that are widespread near the coast at the southern limit of the map area [NO 947 988], are sometimes orientated parallel to microfold (F2) axes. Although the andalusite crystals often appear idioblastic in hand specimen, thin sections disclose that most of the porphyroblasts have highly irregular margins and rarely display planar, idioblastic boundaries against biotite, quartz and plagioclase (Plate 9)f.

Within the map area staurolite occurs only on the north bank of the Don, near the Brig O' Balgownie [NJ 942 097] and at a locality [NO 835 992] near Peterculter, although this mineral is relatively abundant south of the Sheet 77 area near the Kincardine Coast (Porteous, 1973a). At the Brig O' Balgownie the staurolite crystals are highly sericitised, range up to 3 mm in length (Plate 9)e and occur in pelitic horizons (2412; 2413; 2414) while the Peterculter specimen (2446*) contains small (c. 1 mm) idioblastic staurolite laths.

Sillimanite (fibrolite) occurs widely, but in a markedly sporadic fashion. It is commonly very fine-grained (diameter < 0.01 mm) and fibrolitic, but coarser (0.1 to 0.5 mm) prismatic crystals occur in specimens from three localities (665, [NJ 856 074]; 2325* [NJ 880 214]; 2321* [NJ 856 202]). Typically, the fibrolite occurs as wispy aggregates of ill-defined crystals intimately associated with crystals of biotite and often orientated parallel to the cleavages of the latter (e.g. 12* [NJ 888 207]). This relationship is generally regarded as being due to the expitaxial growth of fibrolite on biotite (Chinner, 1961). However, in some rocks where fibrolite predominates over biotite, the mica appears to have been highly modified and occurs only as pale-coloured, relict grains. In many rocks idioblastic needles of fibrolitic sillimanite occur sparsely within crystals of quartz and plagioclase, as well as in association with biotite, and similar idioblastic crystals sometimes occur within platy crystals of muscovite. Fibrolitic aggregates are also found in association with andalusite crystals, either wholly enclosed within the latter (Plate 9)g, or as peripheral overgrowths (e.g. 609 [NJ 914 005]), but no obvious relationships between the orientation of the fibrolite needles and that of the associated andalusite crystals can be discerned.

The common association with biotite implies that fibrolitic sillimanite is most abundant in pelites and semi-pelites, but there are marked variations in the proportion of fibrolite from rock to rock, and even in different parts of a single thin section.

The relationships of the fibrolite suggest that it has crystallised after the other products of progressive metamorphism, apparently under higher-grade conditions (Figure 14), but these relationships appear to be exceptional, and the textural relations of the prograde phases in the metasediments generally are suggestive of simultaneous crystallisation. In many of these rocks it appears that a relatively simple texture was produced during a main episode of metamorphic crystallisation which largely obliterated pre-existing textures and which has subsequently survived with only minor modification. However, a considerable number of specimens display more complex relations, in sonic instances because structures and textural features from an early stage in the history of the rock have been preserved, in others because the texture produced during the main metamorphic episode has been modified during a later episode of retrogressive metamorphism.

Relict textural features          

Evidence suggesting that the foliation in the metasediments of the Aberdeen Formation often has a complex mode of origin, with recrystallisation emphasising a pre-existing structure has already been presented in the section devoted to megascopic structures (see above). Additional evidence of the overgrowth of earlier structures during the main metamorphism is provided by textural features in the thin sections of a number of other metasediments. In some rocks (c. g. 2306* [NJ 928 107]; (Plate 9)a the [001] cleavages in a considerable proportion of the biotite crystals are orientated at a large angle to the foliation (i.e. to the plane within which the cleavages of the majority of the biotite crystals lie), and in a number of specimens (e.g. 237 [NJ 948 213]) the 'oblique' biotites show preferred orientation and define a second foliation that is axial planar to microfolds affecting the first (main) foliation. Muscovite crystals sometimes show similar complexities (e.g. 2303 [NJ 914 147] (Plate 9)b and in some rocks (e.g. 2415 [NJ 942 097]) two types of muscovite crystals can be recognised; (a) small (0.5 to 1 mm) crystals elongated parallel to the [001] cleavage, which share a common pattern of preferred orientation with biotite and are one of the agencies defining the main foliation in the rock; (h) porphyroblastic (>1.5 mm), cross-cutting crystals, elongated at right angles to the [001] cleavage, which sometimes display preferred orientation with the cleavage of the crystals defining a foliation that is axial-planar to microfolds.

Porphyroblastic crystals often truncate the foliation and overgrow microfolds (e.g. garnet, 2418, 2420 [NJ 956 006]; andalusite. 633, [NO 856 990]; staurolite, 2412 [NJ 942 096]). Small (< 0.1 mm) crystals of phases such as biotite, quartz and opaque constituents included within porphyroblasts are sometimes aligned parallel to the foliation in the surrounding rock (e.g. in andalusite, 611 [NJ 908 002]), or are orientated at a considerable angle to the external structure (e.g. in garnet, 605 [NJ 911 007]; 763 [NJ 941 097]).

Similar, but unaligned, inclusions are visible in crystals of other minerals, notably plagioclase feldspar, and in some rocks (particularly in specimens from the Brig O' Balgownie area, e.g. 2402 [NJ 941 097]) quartz inclusions are so abundant that the plagioclase crystals have a spongy appearance. The widespread occurrence of the inclusion-filled crystals in these specimens suggests that original detrital grains have been preserved by being enveloped in growing feldspar crystals during prograde metamorphism. Elsewhere in certain psammites it is possible that the presence of large (c. 2 mm) crystals of quartz and potash feldspar within a finer matrix (e.g. 46 [NJ 858 263]; 1942 [NJ 840 203]) indicates that an original inequigranular sedimentary texture has also survived metamorphism.

Effects of retrogressive crystallization    

Many specimens of metasediment display features which suggest that, following the main metamorphic episode, these rocks have been partially recrystallised, not only under the high-grade conditions prevailing when fibrolite formed, but also under lower grade conditions favouring retrogressive alteration. The main phases involved are muscovite, quartz, potash feldspar and plagioclase.

Muscovite that has been identified as being a retrogressive product occurs in highly irregular, randomly orientated crystals, of markedly dissimilar appearance to the subidioblastic, prograde muscovite visible in specimens from the Brig O' Balgownie area and elsewhere. These irregular crystals, which are often present in very minor amounts, are sometimes small (< 0.5 m) and wholly enclosed within feldspar crystals, or are larger (1 to 2 mm) and display mantling, replacive relationships to phases such as biotite and andalusite. Crystals of fibrolite in some of the larger muscovite crystals often have round outlines (Plate 9)h which suggest that they have been partially replaced by the surrounding mica (e.g. 651 [NJ 965 025]).

Quartz also seems to have crystallised, or recrystallised, at a relatively late stage in many rocks. Thus, in some specimens (e.g. 1304 [NJ 836 212]) apparently separate quartz grains are in optical continuity, while the intricate, cuspate boundaries between the crystals of quartz and the other phases (e.g. 576 [NJ 854 053]) suggest replacive relationships. In other specimens (e.g. 650 [NJ 965 025]) the quartz occurs mainly in relatively large (> 4 mm) aggregates of strained, polygonised grains which have complex, sutured mutual boundaries. These aggregates are often lensoid and elongated in the plane of the foliation and sometimes coalesce to form vein-like bodies. When the rock is psammitic or semi-pelite, these relationships may merely indicate that there has been extensive recrystallisation of quartz crystals that were originally present, but, in other rocks which originally seem to have been quartz-poor, particularly calc-silicates (e.g. 99 [NJ 829 227]; 2318 [NJ 929 123]), it is possible that the formation of the aggregates required the extensive introduction of silica.

Potash feldspar is often abundant only in the vicinity of migmatitic leucosomes and may therefore be associated in origin with the latter. In some rocks highly irregular crystals of potash feldspar have interstitial, possibly replacive relationships to grains of plagioclase and quartz (e.g. 261 [NJ 948 187]), but in other specimens the potash feldspar has been largely replaced by symplectitic intergrowths of quartz and muscovite, or by myrmekitic quartz-plagioclase intergrowths. These relationships suggest that more than one episode of recrystallisation has occurred after the main metamorphism, with potash feldspar forming in association with the migmatic veins (see below), while the reactions involving its breakdown occurred at a later stage.

Detailed mineralogy

Many of the metasediments in the Aberdeen Formation are characterised by the presence of crystals of reddish-brown biotite, but in some specimens, particularly in the Southern unit, the crystals are very dark brown, or are olive green. These colour variations presumably reflect compositional differences, particularly TiO2 content and Fe2O3/FeO ratio (Chinner, 1960), but the only available microprobe analyses are all of reddish-brown crystals (Boyd, 1972; Porteous, 1973b). In five of the rocks in which the biotite has been analysed, the crystals show a general compositional similarity, despite marked differences in the mineral assemblages (Biotite-muscovite-garnet-plagioclase-andalusite-fibrolite-quartz; 604 [NJ 957 013]; 605 [NJ 911 007]. Biotite-garnet-plagioclase-potash feldspar-quartz; 260 [NJ 948 187]. Biotite-potash feldspar-corundum. 1383 [NJ 870 197], 2338 [NJ 852 116]), with the M/M + F ratios (100 Mg/Mg + Fetot as cations) ranging between 30.6 and 36.5. However, the biotite crystals in two specimens of biotite-plagioclase-potash feldspar-corundum pelite (1967* [NJ 9248 2406]; 2398 [NJ 929 232]) have appreciably higher M/M + F ratios, 46.1 and 47.6 respectively.

The composition of garnet crystals was also determined by microprobe analysis in three of these specimens (260, 604, 605) (Boyd, 1972; Porteous, 1973b). These crystals were all found to be iron rich, having almandine contents ranging between 68 per cent and 72.5 per cent. The other main divalent cations are Mn, Mg and Ca-generally in that order of decreasing abundance (Figure 6). The analysed garnets, not only from the Sheet 77 area, but from the Kincardineshire coast section to the south (Porteous, 1973b; Baltatzis, 1979) have appreciably higher Mn contents (Figure 6) than almost all the garnets from garnet to kyanite grade metasediments in Perthshire (Atherton, 1968) and from mainly sillimanite grade rocks in Glen Clova (Chinner, 1960; 1965). The MnO content of the Aberdeen/Kincardineshire coast rocks (0.05 to 0.25 wt. per cent) is no greater than that of the garnetiferous rocks elsewhere and, indeed, is less than that (0.27 to 0.38 wt. per cent) of rocks in Glen Clova containing relatively Mn-rich garnets ((Figure 6); Chinner, 1960; 1965). The garnetiferous rocks from the Aberdeen/Kincardineshire coast area also have relatively low oxidation ratios (mol. 2Fe2O3 x 100/2Fe2O3 + FeO = 10 to 20) and, unlike Glen Clova (Chinner, 1960), the Mn-rich nature of the garnets cannot be related to high values of this ratio in the host rock. It is probable, therefore, that the Mn-rich nature of the garnets within the high-grade rocks of the Sheet 77 area and the lower grade rocks in the coastal area to the south can be ascribed to the P, T conditions during metamorphism, rather than to compositional parameters.

The garnet crystals in individual specimens from the Aberdeen area generally show appreciable compositional variation (Figure 6) and microprobe traverses show that many of these crystals are zoned (Porteous, 1973b). In garnets from the coastal section to the south of Aberdeen, including specimens from outcrops south of the Sheet 77 boundary, Mg is usually enriched near the crystal margins, particularly in the lower staurolite-grade rocks, while Ca generally shows more marked zoning, with the content of Ca ions in the crystal cores being approximately double that of the margins. Mn often shows slight marginal enrichment in the higher sillimanite-staurolite- and sillimanite (andalusite)-grade rocks, but there is no tendency for Mn to be enriched in the cores of the crystals, as is often the case elsewhere in the Dalradian (e.g. Atherton, 1968; Wells and Richardson, 1979).

Optical and microprobe determinations show that the composition of the plagioclase crystals in the metasediments of the Aberdeen Formation generally falls in the range An20 to An28. Zoning is normally limited and rarely produces compositional variations of more than c. 2.5 per cent An. In general, there is little evidence of any relationship between the composition of the plagioclase crystals and the nature and proportion of the other phases, although the most albitic plagioclase identified (An14) occurs in a psammite (46 [NJ 858 263]) and relatively anorthitic feldspars (up to An36) occur in pelites associated with amphibolites (e.g. 1942* [NJ 840 203]) and in calc-silicate rocks (An40 to An70).

These rocks are considered in greater detail than any of the other metasediments in the Aberdeen Formation as corundum has not been found previously in regionally metamorphosed Dalradian rocks. Indeed, corundum has been recorded only rarely in regionally metamorphosed terrains elsewhere, and its occurrence in the Aberdeen Formation suggests, either that rocks of unusual chemistry have been metamorphosed, or that metamorphism has taken place under unusual conditions.

Corundum has previously been recorded within the map area from the hornfelses adjoining the Belhelvie 'Younger Basic' mass (Stewart, 1947; see (Figure 2)) and has also been identified in thermally metamorphosed rocks associated with other 'Younger Basic' masses in Aberdeenshire (e.g. Read, 1931). In these hornfelsed metasediments the corundum is associated with phases such as cordierite, spinel and sillimanite in rocks with a low SiO2 content (Table 1). The remapping has now shown (Figure 2) that corundum-bearing metasediments of entirely different character occur in the north-western part of Sheet 77, often in areas where there are no large igneous intrusions and where no evidence of thermal metamorphism can be detected (e.g. at [NJ 870 197] near Disblair and [NJ 829 227] near Nether Crimond). The corundum in these rocks is generally found as small relict grains, mantled by alteration products, in intimate association with biotite and feldspars, and although the rocks are comparatively aluminous (Table 1), the SiO2 content is relatively high (c. 50 per cent). At localities where the corundum-bearing rocks do occur in proximity to major intrusions, evidence of appreciable thermal metamorphism is either lacking (e.g. on Tyrebagger Hill at [NJ 852 116]), or any hornfelsed rocks that are present have an irregular distribution (e.g. in the area north of the Belhelvie mass near the boundary between the Aberdeen and Ellon formations). Thus there are good grounds for identifying these corundum-hearing rocks as being the products of regional metamorphism.

The outcrop distribution of corundum-bearing rocks (Figure 2) suggests that at least three different horizons of these rocks are present within the western part of the Northern unit of the Aberdeen Formation. Similar rocks occur further to the east in the zone of complexity between the Aberdeen and Ellon formations, and although the original field relations in this zone have probably been greatly disturbed (Chapter 8) it is likely that these represent the continuation of some, or all, of the corundum-bearing horizons in the west.

The corundum-bearing rocks occur in varied metasedimentary sequences which generally consist of pelitic, semi-pelitic, psammitic and calcareous horizons interbedded on a scale of c. 0.25 to 0.5 m. The interbanding of the corundum-hearing rocks and calcareous rocks is often particularly intimate e.g. at [NJ 829 227] near Nether Crimond), and calcareous rocks sometimes predominate in the nearby outcrops e.g. at the Disblair locality [NJ 870 197]). At other localities the associated metasediments are sometimes predominantly pelitic and semi-pelitic (at [NJ 915 265] near Cloisterseat) or psammitic (on Tyrebagger Hill at [NJ 852 116]). Amphibolites also occur in close association with the corundum-bearing rocks at several localities e.g. [NJ 829 227]; [NJ 891 240]; [NJ 930 232]), but, as described below, it has been deduced that these are metamorphosed igneous rocks unrelated in origin to the metasediments.

The crystals of corundum are often largely confined to pelitic horizons, but as this mineral is generally inconspicuous, even when it occurs as relatively large (1 to 3 mm) crystals or is made more obvious by alteration, the number and thickness of the corundum-bearing horizons at any particular locality cannot usually be determined by megascopic observation. However, examination of thin sections shows that corundum occurs in layers that are at least 50 to 100 mm thick at some localities e.g. in 2338 from Tyrebagger Hill at [NJ 852 116]), while a drill-core (1867*) from Cultercullen [NJ 9248 2406] contains several thin (4 to 5 mm) corundum-bearing laminae. None of the specimens of corundum-bearing rocks displays a well-defined fissility, and many which contain abundant, randomly-orientated crystals of muscovite e.g. 2338, [NJ 852 116]) are massive and virtually structureless.

Interpretation of relations in many thin-sections of the corundum-bearing rocks is handicapped by the widespread development of retrogressive alteration products, which has modified the original textures and largely obscured the nature of the original high-grade metamorphic assemblage. The corundum crystals, in particular, usually survive only as vestigial remnants enclosed within hydrous alteration products (Plate 10)a, c, and in most specimens it seems unlikely that this mineral could have been identified without the aid of a microprobe. In the majority of the rocks biotite and feldspar originally appear to have been the dominant constituents, with some rocks containing both plagioclase and potash feldspar e.g. 97 [NJ 829 227]), and others being virtually devoid of potash feldspar e.g. 2330* [NJ 914 269]) or of plagioclase e.g. 2338 [NJ 852 116]). Most of the corundum-bearing rocks show small-scale (1 to 3 mm) compositional banding that is made conspicuous by variations of biotite content (generally between 10 per cent and 50 per cent). The original content of corundum is difficult to assess because of alteration, but generally seems to have been less than 10 to 15 per cent, except in a specimen from Tyrebagger Hill (2338 [NJ 852 116]) where corundum may originally have formed 15 to 20 per cent of the rock.

The corundum crystals are frequently larger (1 to 3 mm) than the other constituents (0.5 to 1 mm) (Plate 10)b, are sometimes slightly elongated parallel to the foliation defined by the preferred orientation of biotite crystals, but show no tendency to be idioblastic. These porphyroblasts are concentrated within thin (1 to 2 mm) biotite-rich laminae in some rocks e.g. 2330* [NJ 914 269], but in other specimens are evenly distributed throughout relatively thick (50 to 100 mm) layers e.g. 1867* [NJ 9248 2406]). In specimens which contain interbanded corundum-bearing and calcareous layers e.g. 1867*), the corundum crystals in the pelitic layers become smaller and eventually disappear as the boundaries with the calc-silicate layers are approached.

Biotite in the corundum-bearing rocks is normally reddish-brown, but shows considerable variations in composition, with 100 Mg/Mg + Fetot (as cations) ranging from 30.6 to 47.6. The more ferriferous crystals occur in rocks in which potash feldspar predominates over plagioclase. Plagioclase in the corundum-bearing rocks also shows a considerable composition range, tending to be more albitic (c. An 15) in rocks in which potash feldspar predominates e.g. 1383 [NJ 870 1971], and to be more anorthitic (An35 to An50) in rocks in which plagioclase is the more abundant feldspar e.g. 2330*; 2398 [NJ 929 232]). The analyses of potash feldspars in three rocks (1383, 1867*, 2338) are all similar, being virtually free of lime and having a limited content of soda (Ab5 to Ab10).

Xenoblastic Fe-Ti oxide and subidioblastic tourmaline occur mainly in linear trails of small (0.1 to 0.2 mm) grains which are largely confined to the biotite-rich laminae in the rock, but are also intergrown with corundum, and thus appear to be members of the original high-grade metamorphic assemblage. The tourmaline crystals are generally yellow-green with pale margins and more intensely coloured cores, but larger (5 mm), spongy crystals of dark, olive-green tourmaline occur in the specimen from Tyrebagger Hill (2338). Zircon and apatite are common accessories in the corundum-bearing rocks, and also appear to be constituents of the original high grade assemblages.

Minerals produced by the alteration of the corundum crystals include margarite, colourless cpidote and muscovite, and in some specimens from the eastern localities e.g. 2330*, 2334* [NJ 914 269]) a possible sequence of retrogressive mineralogical transformations can be recognised. Thus, many of the corundum porphyroblasts are extensively replaced by sheaf-like clusters of margarite crystals which, in some instances, are wholly or partially surrounded by crystals of colourless epidote with a low birefringence. The epidote, in turn, may be partially mantled by muscovite, often in fine-grained, scricitic aggregates. Any of these zones may be lacking, so that simple margarite pseudomorphs occur, or the corundum porphyroblasts are replaced by epidote-muscovite intergrowths, or survive as small, rounded relicts in relatively large ragged (1 to 2 mm) muscovite crystals. Replacement of corundum by margarite and epidote is probably more widespread in the east and although such replacement can also be recognised near Nether Crimond (97 [NJ 829 227], (Plate 10)c, corundum relicts in specimens in the west (1383, near Disblair at [NJ 870 197]; 2338 on Tyrebagger Hill at [NJ 852 116]) generally occur only within randomly orientated muscovite crystals (Plate 10)a.

Apart from the alteration products of corundum, the rocks often contain irregular, fine-grained aggregates of pyrite and chalcopyritc which occur at the boundaries of biotite and feldspar crystals and probably formed after the main period of metamorphic recrystallisation. Quartz is confined to migmatitic leucosornes e.g. 2398, [NJ 929 232]) and is clearly not in equilibrium with the corundum-bearing host rock. Potash feldspar is often present within and around migmatitic veinlets, and in some rocks is probably largely a product of post-metamorphic crystallisation. However, in other specimens e.g. 2338) there can be little doubt that potash feldspar crystallised in association with biotite and corundum.

It is probable, therefore, that the dominant mineral assemblages in the corundum-bearing rocks originally ranged between biotite-plagioclase-corundum and biotite-potash feldspar-corundum. Two rocks approaching the limits of this range have been analysed. In one (2330*, near Cloisterseat at [NJ 914 269]), which is cut by thin migmatitic veinlets of quartz and potash feldspar (Plate 10)b but is otherwise virtually devoid of potash feldspar, the original corundum content was probably 5 to 10 per cent. In the other (2338, on Tyrebagger Hill at [NJ 852 116]), plagioclase is lacking, and the corundum, which now survives only as small relicts in muscovite (Plate 10)a, probably originally formed 15 to 20 per cent of the rock. These differences are reflected in the chemical analyses (Table 1) with 2330* being richer in silica and lime and poorer in alumina and potash. These rocks differ from the other analysed metasediments of the Aberdeen area (Figure 3), (Figure 7) mainly in being highly potassic, and it is clear that the corundum-bearing rocks have higher SiO2 contents and lower Al2O3 contents than some of the corundum-free pelites from the Kincardineshire coast. No ferric iron was detected in 2338, but this is not an exceptional feature as other pelites in the Aberdeen Formation which do not contain corundum are also highly reduced (e.g. 1816 [NJ 956 006]).

The corundum-bearing rocks in the Sheet 77 area thus do not display the highly aluminous, silica-deficient characteristics of the rocks that contain this mineral in many thermal aureoles (e.g. Smith, 1965) and in some regionally metamorphosed terrains (e.g. Jansen and Schuiling, 1976). It is possible that the original chemistry of the Sheet 77 specimens has been modified by metasomatism post-dating the main metamorphism, particularly as most of these specimens contain evidence of widespread, retrogressive alteration and in many instances are cut by veins of migmatitic granite. However, in specimen 2338 at least, it is clear that a highly potassic rock, which probably differed only slightly in bulk composition from the analysed specimen, recrystallised during the main metamorphism to give rise to the assemblage of biotite-potash feldspar-corundum. Furthermore, sedimentary rocks showing chemical similarities to the two analysed specimens of corundum-bearing rocks have been described (e.g. Bowie and others, 1966; see also (Table 1)), and it is possible that the corundum-bearing rocks have been derived from sediments rich in illite (Figure 3),(Figure 7)a.

It is probable, therefore, that as in some other regionally metamorphosed rocks (e.g. Serdyuchenko and Polunovskiy, 1971; Suzuki and Kojima, 1970), the presence of corundum in the Aberdeen Formation reflects unusual conditions of metamorphism rather than unusual host rock compositions. Experimental studies (e.g. Chatterjee and Johannes, 1974) have shown that muscovite breaks down at high temperatures in quartz-free rocks to yield corundum and potash feldspar. However, it has also been established (Figure 14) that, in more siliceous rocks, this reaction will be preceded at a lower temperature by a reaction between muscovite and quartz, yielded Al2SiO5 and potash feldspar. Although suitable silica-bearing rocks for this second reaction appear to be widespread in the vicinity of the corundum-bearing horizons, sillimanite is only a minor constituent in the Northern unit, and andalusite is found only in the Southern unit, which does not contain corundum-bearing rocks. This suggests that some factor other than a rise in temperature must have promoted the crystallisation of corundum, possibly a rise in the concentration of CO2 in the vapour phase in the vicinity of the layers of calcareous metasediment that are often (Figure 2) intimately associated with the corundum-bearing rocks (Kerrick, 1972)

Amphibolites

Most of the amphibolites consist almost entirely of amphibole and plagioclase, but biotite and quartz are also present in many specimens. The proportions of amphibole and plagioclase are often variable, particularly when the rock contains melanocratic and leucocratic bands and layers and has a laminated appearance, but amphibole is usually the dominant phase (typically 60 to 70 per cent) and almost monomineralic amphibole-rich bands of rocks are not uncommon. The amphibole is invariably monoclinic and usually displays a yellowish-green to dark green pleochroism but some crystals are pleochroic in shades of reddish-brown or brown. In some specimens the amphiboles have colourless cores and pale green margins, a few rocks contain pale brown and colourless amphiboles as separate phases, while only colourless amphibole occurs in other specimens.

Biotite is the dominant ferromagnesian mineral in some rocks, often being accompanied by quartz, while clinopyroxene occurs in a few rocks, commonly in crystals that are mantled by amphibole. Garnet has been found in two specimens in the Southern unit. Olivine occurs as relict grains in ultramafic rocks that now consist largely of colourless amphibole, but also contain orthopyroxene and spinet in some instances.

Plagioclase is sometimes more abundant (c. 50 to 60 per cent) than amphibole, and is only lacking in the ultramafic, olivine-bearing rocks. Quartz occurs most commonly in the feldspathic amphibolites, but is never a major constituent except in highly migmatised rocks where there is often textural evidence to suggest that the quartz (and the associated biotite) crystallised after the other minerals.

Opaque constituents and sphene are common as minor phases, and, together, sometimes form approximately 5 per cent of the rock. Zircon and apatite are common accessories, but although epidote occurs in many rocks, the textural relations invariably suggest that it is a product of retrogressive alteration.

Texture and mineralogical details
Textural relations

The texture of many amphibolites is dominated by intergrown crystals of amphibole and plagioclase which are of comparable size (generally between 0.5 and 2 mm) and tend to be subidioblastic and lath-like in the case of the amphibole and more nearly xenoblastic in the case of the plagioclase. Ill-defined handing due to variations in the proportions of the plagioclase and amphibole crystals can be recognised in many specimens and is generally parallel to the imperfect foliation defined by the preferred orientation of the elongated amphibole crystals. Porphyroblastic (c. 3 to 4 mm) crystals of amphibole occur in some specimens, and frequently have a core of clinopyroxene and are set in a relatively fine-grained (c. 0.1 to 0.2 mm) matrix of amphibole and plagioclase. Such specimens are generally more strongly deformed than most of the amphibolites (e.g. 2389 [NJ 916 2371].

Biotite is frequently present only as small grains that are intergrown with the amphibole crystals, but forms sub-idioblastic laths when present as a major (>10 to 15 per cent) phase (Plate 5)d. Many of these more biotitic rocks are schistose due to the strong preferred orientation of the {001} cleavage of the mica crystals. The crystals of biotite are often larger (c. 3 to 4 mm) and more abundant near migmatitic veins, and it is possible that this mineral is partly a product of metasomatism post-dating the main metamorphism in these specimens. These migmatised rocks often also contain quartz crystals that show replacive relations to the plagioclase, and thus also appear to be late-stage products. However, in other specimens quartz occurs throughout the rock in small grains (c. 0.1 to 0.2 mm) that are intergrown with the plagioclase and amphibole crystals and appear to be an integral component of the mineral assemblage produced during the main metamorphism (e.g. 509 [NJ 970 051]).

Clinopyroxene in the amphibolites generally occurs as turbid kernels to amphibole porphyroblasts in strongly deformed, inequigranular rocks (e.g. 2389) but colourless pyroxene grains are poikiloblastically and granoblastically intergrown with yellow-green amphibole and plagioclase in a specimen (27*) of unusual chemistry (see below) from Burreldale Moss [NJ 836 241]. Orthopyroxene occurs in the specimen (59) of ultramafic rocks from Lawel Hill immediately west of the map area [NJ 807 238] (Figure 1), (Figure 21) as poikiloblastic crystals which have been largely replaced by colourless amphibole and contain small (c. 0.05 m) irregular grains of green spinel. Inclusions of spine] also occur within the olivine crystals which originally formed the main constituent of this rock, but other metamorphosed ultramafic rocks (e.g. 2343 [NJ 877 2501] from within the Sheet 77 area to the east appear originally to have been dunitic and almost devoid of pyroxene. In all of these ultramafic specimens the original textural relations have been largely obscured by replacive, colourless amphibole, but large (up to 5 mm), relict olivine grains can still be recognised, which sometimes define a crude foliation when the long axes of elongated grains are sub-parallel.

Garnet is present in an amphibolite from near Maryculter (2433 [NO 853 990]) (Plate 10)e as large (7 to 10 mm), irregular porphyroblasts which are intergrown with quartz and form approximately 10 per cent of the rock. Another, highly migmatised amphibolite from south of the Dee (631 [NJ 921 0251] contains garnet as small (0.5 mm) irregular grains which generally occur in intimate association with biotite and hornblende and have a very uneven and sporadic distribution in the rock.

Many amphibolites contain linear 'trails' of small (0.1 to 0.2 mm) irregular grains of sphene which are aligned parallel to the foliation and banding and are often highly altered. Opaque grains, which are frequently associated with the sphene in these 'trails', also appear to be products of the main episode of metamorphic recrystallisation, but highly irregular opaque grains found at the margins of the main rock-forming minerals seem to have crystallised at a later stage in the history of the rock.

Relict igneous or sedimentary features cannot be reliably identified in any of the amphibolites, although it is possible that the mantled clinopyroxene crystals in some rocks (e.g. 2389) are relicts of an original igneous assemblage, and the olivine, orthopyroxene and spine] crystals in the ultramafic specimens appear to have been original constituents of dunitic and harzburgitic rocks of possible igneous origin. Some of the amphibolites contain rootless isoclinal folds and others display microfolds (e.g. F2 structures at Cove Harbour, 2417 [NJ 955 006]). These structures are generally overgrown by the crystals of the minerals formed during the main episode of metamorphism (particularly amphibole) as in the metasediments.

Detailed mineralogy

Green amphiboles in six rocks of the Aberdeen Formation, including the amphiboles in the two garnetiferous rocks (631, 2433), have been analysed by microprobe (Duncan, 1974). All of these amphiboles are calcic and hornblendic (Leake, 1978). Two (509 [NJ 964 044]; 2433 [NO 853 990]) are ferro-hornblendes with Mg/Mg + Fe2 + cation ratios of less than 0.5 (Table 2), the other four are magnesio-hornblendes with Mg/Mg + Fe2 ratios of between 0.53 and 0.84. All six minerals have relatively high contents of Aliv (Table 2), and thus show similarities to tschermakitic hornblendes (Leake, 1978, fig. 3a). Compositional differences between the six amphiboles are largely due to variations in Mg/Fe ratio, and can be correlated with variations in the Mg/Fe ratio of the host rocks (Figure 8)a. This compositional variation is also expressed in colour differences, with the Mg-rich minerals generally being relatively pale coloured.

The amphiboles in the garnetiferous rocks (631, 2433) have relatively high contents of Alvi (and of Al2O3) (Table 2), and this characteristic, together with the presence of garnet, could be taken as an indication of relatively high-pressure crystallisation (Leake, 1965; Wiseman, 1934). However, as both these rocks have relatively low Ca + Na + K/Al ratios (Figure 9)a and thus have a relatively large excess of Al2O3 over the requirements of feldspar, it is probable that the aluminous nature of the amphibole, like the occurrence of garnet (see below), can be related to the bulk chemistry of the host rock.

The garnet in specimen 2433 has been analysed and has been found to be Fe-rich and to contain a smaller proportion of Ca and relatively subsidiary amounts of Mg and Mn (Figure 8)c. The crystals are zoned and have margins enriched in Fe and Mg, and cores enriched in Mn and Ca. The overall chemistry of these crystals is similar (Figure 8)c to that of garnets from Dalradian amphibolites in the central Highlands (Pantie, 1956; Shido and Miyashiro, 1959).

Leake (1972) has shown that the crystallisation of garnet in amphibolites is promoted by certain chemical characteristics of the rock, notably high MnO, low Mg/Fe and Fe2O3/FeO, and that the appearance of garnet in amphibolites need not indicate that the grade of metamorphism has increased. The two garnetiferous amphibolites from the Aberdeen Formation are relatively Mn-rich (weight per cent MnO c. 0.30 as against 0.15 to 0.2 in the other amphibolies, (Table 3)), and Mg-poor and in a diagram which combines Leake's three chemical parameters (Figure 9)b these rocks plot in the compositional field defined by the garnetiferous amphibolites from Connemara, suggesting that the presence of garnet in these two specimens is due to compositional rather than physical control. The crystallisation of garnet may also have been facilitated by the low Ca + Na + K/Al ratio in these rocks, as this should promote the crystallisation of aluminous ferromagnesian minerals generally, garnet as well as aluminous amphibole (see above).

The three chemical variables plotted in (Figure 9)b have been combined by Leake into a single parameter g (Niggli mg x W/MnO) that decreases in magnitude as the rock composition becomes more suitable for the crystallisation of garnet. At any particular grade of metamorphism, theg values of garnetiferous amphibolites will be less than a maximum value but variations in grade in metamorphism, particularly rise in pressure, will lead to an increase in the maximum g value of these rocks. The g value can therefore be used as a general geobarometer.

The two garnetiferous amphibolites in the Aberdeen Formation have g values of 0.14 (631) and 0.17 (2433), while the associated garnet-free rocks have values in the range 0.15 to 0.95 (average 0.48). The g values of 0.23 to 3.07 (average 1.31) for garnetiferous amphibolites from the area south-west of Glen Clova quoted by Leake (1972) suggest that crystallisation took place at considerably higher pressure in that area than in the vicinity of Aberdeen. Support for this view also comes from the Alvi values of the amphiboles, (Figure 8)b which are consistently lower for the Aberdeen area over a considerable range of mineral and rock compositions (Figure 8)a.

Most of the amphibolites contain plagioclase lying in the range An30 to An58, with the crystals in any particular rock often showing appreciable normal zoning (c. 10 to 15 per cent An), but one of the garnetiferous specimens (631) contains a more calcic plagioclase (c. An85). The olivine in the harzburgitic rock (59) from Lawel Hill [NJ 807 238] is more fayalitic (Fo72) than the olivines in ultramafic bodies in most regionally metamorphosed terrains (e.g. Jackson and Thayer, 1972). The spinel in this rock is aluminous.

Geochemistry of the amphibolites

The study of field relations, textures and mineralogy provides little information on the possible nature of the parental rocks of the amphibolites in the Aberdeen Formation, although the presence of relict pyroxene crystals (e.g. in 2389 [NJ 916 237]) suggests an igneous parentage, and fine interbanding with calcareous metasediments (e.g. 98 [NJ 829 227]) may be indicative of sedimentary origin. Geochemical criteria have been identified (notably by Leake, 1964), however, that enable groups of related amphibolites to be categorised as being of igneous or sedimentary origin with a fair measure of certainty, although considerable problems remain when dealing with single, isolated specimens. When these criteria are applied to be amphibolites from the Aberdeen Formation (Duncan, 1974) (Figure 9), (Figure 10) the majority show chemical characteristics (see also (Figure 11)) which suggest that they are the metamorphosed derivatives of related basic igneous rocks. Three specimens are distinctive, however; the ultramafic specimen from Lawel Hill (59 [NJ 807 238]), and two specimens from Burreldale Moss (27*, 29* [NJ 836 241]). The exceptional chemical characteristics of specimen 59 in conjunction with the unusual mineralogical and textural features displayed by this rock and the other ultramafic rocks, suggest that they are derived from a group of igneous rocks with a different mode of origin and history from the other amphibolites (Figure 9), (Figure 10), (Figure 11). The Burreldale Moss specimens also display unusual mineralogical features, with 27* containing clinopyroxene, while 29* is biotitic and contains a colourless amphibole (Plate 10). Specimen 27* is chemically distinctive in being richer in CaO and poorer in MgO than most of the other amphibolites ((Figure 9)a; (Table 3), while 29* is relatively poor in CaO, but rich in alkalies and SiO2 (Figure 9)a, b; (Figure 11)a; (Table 3). Both of these rocks have relatively low Ni and Cr contents (Figure 10) and as both are intimately interbanded with metasediments it is possible that these unusual features are indicative of a sedimentary origin.

The remaining amphibolites form a reasonably coherent group with clearly defined tholeiitic characteristics (Figure 11), showing similarities to metamorphosed basic igneous intrusions from the S. W. Highlands (Graham, 1976). Although the original field relations of the Aberdeen Formation amphibolites cannot be deciphered, there is no evidence of discordant relations at the boundaries of these rocks and none of the features used by van de Kamp (1970) to identify Dalradian Green Beds as metamorphosed tuffs, such as gradation from amphibolites into pelitic metasediments, is readily recognisable. It is probable, therefore, that the amphibolites represent the metamorphosed derivatives of basic lavas or concordant intrusions.

Migmatitic granite

Field relations

Bodies of migmatitic granite are associated with the metasediments and amphibolites in the majority of the exposures of the Aberdeen Formation and sometimes are the dominant lithology within considerable areas (>10m2) of outcrop. Although these granitic masses are often highly irregular in form and frequently have ill-defined, gradational contacts against the associated rocks, they tend to occur as lensoid or sheet-like bodies ranging from a few tens of millimetres to ten metres or more in thickness (Plate 13)) Looking west, The Priest, south of Cove Harbour [NJ 954 004] (D3135)" data-name="images/P999811.jpg">(Plate 6). These bodies have a widespread, but variable, distribution within the Aberdeen Formation in the Sheet 77 area, being relatively abundant in much of the area to the south of the Dee and also near Udny and Pitmedden, and less common between Hatton of Fintray [NJ 840 165] and Straloch [NJ 860 210] (Figure 23).

Larger masses of granitic rock apparently identical to that forming the migmatitic sheets have been identified at two localities; south of Cove c. [NJ 950 000], where a body of granite at least 70 m thick is exposed for 2 km in the coastal cliffs, and near Hill of Crimond c. [NJ 820 230], where granite extends over a considerable area (c. 1 km2) of elevated ground apparently as a single mass at least 60–70 m thick. The body of granitic rock with dimensions of approximately 5 to 10 km underlying Aberdeen city (the 'Aberdeen Mass', see (Figure 23)) also shows marked similarities in field relations and in petrographical and geochemical characteristics to the rocks in the migmatitic sheets and veins. It may be related in origin to the smaller bodies, but, for convenience in description, fuller discussion of this mass is deferred to a later section (Chapter 9), and only the smaller bodies are considered here.

The Cove and Hill of Crimond masses are shown on the 1:50 000 map, but none of the other vein or sheet-like granitic masses is of sufficient size to be depicted at this scale, apart from an irregular body of pegmatitic granite near Kincorth (c. [NJ 935 020], (Figure 23)), which is leucocratic and garnetiferous, but is probably related in origin to the other migmatitic granites as it has similar field relations. Small granitic masses shown elsewhere in the original version of Sheet 77 (e.g. Craigingles Wood [NO 880 995]; Cults [NJ 885 035]; Tillygreig [NJ 885 235]) are now known to correspond with areas where small, sheet-like bodies of migmatitic granite are abundant and not with larger coherent bodies of granite. Temporary exposures in trenches have also shown that the outcrops in the Udny–Pitmedden area convey a misleading impression of the abundance of granite in this area (see original version of Sheet 77), partly because the granite appears to be more resistant to erosion than the associated amphibolites and metasediments, but mainly because a sub-horizontal sheet of granite (c. 10 to 15 m thick) has been widely exposed at the surface in an area of low relief.

In detail the field relations of individual bodies of migmatitic granite are very variable. Many are lensoid or sheet-like and are roughly concordant with the lithological banding in the surrounding rocks, particularly in areas where this structure is flat-lying (e.g. at Allathan Quarry [NJ 898 272]; Craigingles Wood [NO 880 995], and in much of the coastal area between Aberdeen and Cove). However the boundaries of individual granitic bodies may be highly irregular, cross-cutting the country-rock structures at one point, or extending as vein-like apophyses along the foliation at another. In addition, the boundaries of a granitic mass may be sharply defined in one part of an outcrop and highly irregular and gradational in another. These features are visible in bodies of all sizes, but transgressive relations and gradational contacts are more characteristic of the larger masses.

Gradational contacts are particularly common where the granite occurs in contact with pelitic or semi-pelitic metasediments and at many localities (e.g. virtually throughout the Aberdeen-Cove coastal section) granitic material appears to have preferentially exploited bands of these rocks, and extensive areas of metasediments have become gneissose with a permeated, feldspathised appearance, and contain small pods and lenses of granitic material and show only relict metasedimentary banding (Plate 11),(Plate 12). The larger and more coherent granitic bodies frequently contain highly permeated and modified metasedimentary xenoliths, with the transformation process being so complete in some instances that the only trace of the xenolith is the presence of ill-defined leucocratic and melanocratic areas corresponding with the original lithological banding. However, several features suggest that a straightforward transformation hypothesis is incapable of explaining all of the characteristics of the migmatitic rocks. Many of the contacts of these rocks are transgressive (Plate 13), and at the southern limit of the Cove mass (Bareside Point [NO 948 992]) the structure of the surrounding metasediments is locally disrupted where a series of sheet-like apophyses of granite extend into the country rocks. In addition, although the foliation in many of the granitic bodies appears to be partly a relict metasedimentary structure, at many localities (e.g. Crawpeel Shore [NJ 957 008]) the mica crystals defining the foliation are orientated parallel to the margins of the granitic body and are oblique to the structures in the surrounding rocks.

Sharply defined contacts are more abundant where the country rocks are psammitic metasediments or amphibolites (Plate 13)) Looking west, The Priest, south of Cove Harbour [NJ 954 004] (D3135)" data-name="images/P999811.jpg">(Plate 6), and although xenoliths of pelitic or semi-pelitic metasediments are often transformed into vague 'ghosts' or are reduced to biotitic relicts, xenoliths of psammite and amphibolite frequently have sharply defined, angular boundaries.

The overall impression from a study of the field relations of these rocks, therefore, is that while processes of transformation have played an important part in the formation of the migmatitic bodies, in many instances the granitic material was sufficiently mobile to intrude and disrupt the surrounding rocks.

The migmatitic rocks were emplaced into country rocks that were already extensively deformed and, indeed, the full range of structures in the latter is recognisable in disorientated xenoliths in some of the granitic bodies (Plate 13) At some localities (e.g. Blowup Nose [NO 947 987]; Robin Hood Yawns [NJ 965 027]) granite sheets display 'pinch and swell' structures. These may have been formed either when intrusion occurred under stress or during a post-consolidation deformation episode, although there is generally no evidence of intensification of the foliation in the granite in the 'necks' and the attenuated sections of the sheets sometimes transgress country rock structures (Plate 13) The 'necks' frequently trend N–S and in many instances are the site for the emplacement of vertical quartz veins, also with a N–S strike.

Many of the migmatitic bodies, particularly the larger masses, consist almost entirely of equigranular, medium-grained biotite-muscovite granite, and have a relatively homogeneous appearance. Others show greater petrological diversity and frequently contain potash feldspars ranging up to 40 mm in diameter, or show patchy variations in grain size and in the proportions of melanocratic and leucocratic constituents, generally where metasedimentary xenoliths are abundant. Coarse pegmatitic variants also occur within the granitic bodies, sometimes forming irregular masses with ill-defined, gradational boundaries, in other instances having sharp contacts and a vein-like form. The large sheet-like body near Kincorth ((Figure 23) [NJ 935 020]) consists almost entirely of pegmatitic granite with little or no associated medium-grained rock.

A more mafic and relatively fine-grained rock (549) of 'igneous' aspect occurs at Altens Haven [NJ 964 024] as a mass with dimensions of one hundred metres or more, and similar rock (692) occurs in the quarry in Craigingles Wood [NO 880 993] on a more limited scale. These rocks are extensively cut by irregular veins of typical migmatitic granite, many of the contacts being gradational or having biotitic selvedges.

Examination of thin sections shows that the granite veins consist almost entirely of potash feldspar, plagioclase, quartz, biotite and muscovite, with opaque constituents, apatite and zircon as common accessory minerals. The proportions of the main constituents are highly variable, particularly in granitic bodies displaying gradational contacts with the metasediments. Modal analyses of the more uniform specimens show that many rocks contain between 30 to 37 per cent quartz, 24 to 28 per cent plagioclase and 28 to 38 per cent potash feldspar with the total content of micas normally lying between 6 per cent and 7 per cent. These rocks can thus be classified as granites sense stricto (Streckeisen, 1967).

Under the microscope the rocks often have a granular appearance, with relatively few of the crystals showing an approach to idiomorphism. The texture is often dominated by the presence of quartz crystals that display intricate boundaries against the other constituents and occur in lensoid or irregular masses ranging up to 5 to 7 mm in dimensions. Crystals of plagioclase are sometimes subidiomorphic, range up to 5 mm in size, and generally display complex oscillatory zoning superimposed on normal core to margin zoning in the range An30 to An15 (e.g. 1015 [NJ 824 184]). Round inclusions of quartz occur in many of the plagioclase crystals and sometimes have a vermicular form, particularly in rocks in which potash feldspar has been replaced by myrmekitic quartz-plagioclase intergrowths (e.g. 648 [NJ 964 023]).

Potash feldspar is generally present as crystals with dimensions of less than 2 mm which are allotriomorphic (e.g. 527 [NJ 969 035]), but larger crystals ranging up to 40 mm in size (e.g. 529 [NJ 968 035]) also occur and are generally microperthitic and display microcline 'cross-hatch' twinning. X-ray diffraction studies (Walsworth-Bell, 1974) show that there is some variation in the nature of the potash feldspars in the bodies of migmatitic granite, with microcline invariably predominating in the veins in the north, while either microcline or orthoclase may be the dominant form of potash feldspar in individual veins on the Kincardine coast.

Biotite occurs as subidiomorphic or irregular plates which often form aggregates. These crystals generally show a certain measure of preferred orientation and the foliation visible in most of the bodies of migmatitic granite is largely defined by the planar orientation of biotite crystals. In most rocks the biotite is grey-brown but red-brown crystals occur occasionally (e.g. 596 [NJ 842 042]). Microprobe analysis (Porteous, 1973b) shows that the M/M F ratio (100 Mg/Mg + Fetot as cations) ranges from 24.35 to 26.89 in biotites from two specimens in the north (1007 [NJ 897 272]; 1029 [NJ 915 245]), but is appreciably higher (38.5 to 41.5) in a specimen (570 [NJ 967 031]) from the Kincardine coast.

In many of the bodies of migmatitic granite muscovite occurs as small (c. 0.5 mm) laths intergrown with opaque phases at the margins of biotite crystals, or as irregular flakes that are wholly enclosed within feldspar crystals and appears to have formed mainly as a replacement product. In some of these specimens (e.g. 647 [NJ 964 0231] potash feldspar has also been extensively replaced by symplectitic quartz-muscovite intergrowths. However, many rocks contain larger (1 to 2 mm), idiomorphic crystals of muscovite that cut across biotite laths and penetrate feldspar crystals, and seem likely to be the products of magmatic crystallisation (e.g. (656 [NJ 969 036]).

Quartz sometimes occurs in small (c. 1 mm) grains with relatively simple, straight or slightly curved boundaries (e.g. 434 [NJ 896 0391], but, as noted above, this mineral frequently occurs in larger masses which are composed of single crystals up to 5 to 7 mm in dimensions or of several intergrown crystals (e.g. 491 [NJ 969 055]). Irregularities at the boundaries of the quartz crystals often suggest that the formation of the quartz has involved replacement of the adjoining minerals (e.g. 1806 [NJ 964 023]). In the aggregates, the mutual boundaries of the quartz grains are generally highly complex and sutured, while the individual crystals are often strained and polygonised.

Variants of the normal migmatitic granite include leucogranites, which are generally not only deficient in ferromagnesian minerals but are also rich in potash feldspar. These rocks sometimes contain round or sub-idiomorphic garnets up to 4 to 5 mm in diameter and tend to be pegmatitic (e.g. 620 [NJ 934 0181], but otherwise show no exceptional mineralogical or textural features. Other pegmatitic areas in the granites are mainly characterised by the larger size of the crystals, but sometimes contain large (4 to 5 mm) crystals of black tourmaline. The dark, fine-grained rock from Altens Haven (549 [NJ 964 024]) which appears to predate the main development of migmatitic veins (see above), is relatively rich in biotite and poor in potash feldspar. This rock shows marked chemical differences from the veins (Figure 12), and approaches a diorite in composition.

Many of the migmatitic granites show considerable mineralogical and textural similarities to the adjoining metasediments, particularly the semipelites. As a result, the modified metasediments at gradational contacts generally show no distinctive mineralogical features, and the gradation is reflected more in changes of mineral proportions and in a general elimination of inhomogeneities in the metasediment, such as lithological banding, than in the development of new phases or of modified textural relations. In general, the transition from granitic rock to pelitic and semi-pelitic metasediment involves a reduction in quartz and potash feldspar and an increase in biotite and plagioclase.

These relationships are reflected in variations in the chemical characteristics of analysed specimens (Porteous, 1973b) of migmatitic granite and of migmatised and unmigmatised metasediments from the Kincardine coast (Figure 12). Thus, the migmatitic granites generally contain a higher proportion of alkalis ((Figure 12)a) and a lower proportion of ferromagnesian oxides (Figure 12)b than the pelitic and semi-pelitic metasediments. The conversion of such metasediments to a granitic end product (Figure 12)c would involve silicification and the introduction of alkalis, particularly potassium (Figure 12)a. However, some of the highly migmatised metasediments are more siliceous than the granites (Figure 12)b, c, suggesting that relatively psammitic parent rocks have also been modified by migmatisation.

The formation of the bodies of migmatitic granite appears to post-date all of the main structural and metamorphic events recognised in the surrounding metasediments, but it is possible that migmatisation occurred only shortly after the metamorphic climax when the country rocks were still at elevated temperatures. Indeed, it is normally implicit in the discussion of the metamorphism of the Dalradian (e.g. Chinner 1966; Atherton 1977; Harte and Hudson 1979) that metamorphism and migmatisation are events that occurred simultaneously or within a relatively short time interval. However, the field relations and petrological and geochemical characteristics of the bodies of migmatitic granite and of the major Aberdeen granitic mass are similar, (see below, Chapter 9). This suggests that the formation of these migmatitic rocks and the emplacement of major bodies of granite are related. If this is the case and the tentative isotopic age for the Aberdeen granite mass (Halliday and others, 1979) is confirmed, then the migmatitic rocks must have formed long after (c. 460 Ma) the metamorphic climax (c. 500 Ma, Pankhurst, 1982).

Some of the smaller migmatitic leucosomes are highly siliceous and contain oligoclase but no potash feldspar, although allowance has sometimes to be made for the possible replacement of potash feldspar by myrmekitic intergrowths. A separate migmatitic event involving the production of such trondhjemitic leucosomes may therefore have occurred, possibly during or shortly after the main metamorphic climax. If this is the case, then the later, potassic migmatitic event was so widespread and pervasive that the relations of the trondhjemitic leucosomes have been largely obliterated.

Chapter 3 Regionally metamorphosed rocks-2

The Ellon Formation

The Ellon Formation is virtually confined to the area east of the 94 Grid line and north of the 21 Grid line with the most westerly outcrops of this formation being at [NJ 947 246] near Fiddesbeg and at [NJ 943 258] near Mill of Rannieston, and the most southerly outcrops at [NJ 967 219] near Aikenshill and at [NJ 973 211] near Newtyle House (Figure 4). However, poor exposure prevents precise location of the boundary with the Aberdeen Formation. In the west, rocks showing similarities to typical specimens of the Ellon Formation occur in the zone of metamorphic and structural complexity adjoining the interformational boundary (e.g. at [NJ 934 256] near Auchindarg), but evidence of deformation and disruption is widespread throughout this zone, and there is a high probability that the original field relations of such rocks have been much disturbed. The western limit of the coherent area of rocks of the Ellon Formation is therefore taken as being defined by the linear magnetic anomaly near the 94 Grid line (Figure 4)a.

The position of the boundary between the Ellon and Collieston formations is also uncertain because of the complete lack of exposures in the area now occupied by the windblown deposits of the Forvie Sands (Figure 30). Undoubted gneissose metasediments and amphibolites of the Ellon Formation occur immediately north of the northern margin of the Sheet 77 area at Waterside [NK 002 280], while the most westerly occurrence of the Collieston Formation is an outcrop of psammitic metasediments between tidemarks to the south-west of Rockend at [NK 018 260] (Figure 13). The boundary between the Ellon and Collieston formations has therefore been arbitrarily drawn parallel to the prevailing strike direction (Figure 2), midway between the closest outcrops of the two formations.

Within the map area, rocks of the Ellon Formation are well exposed only near Waterside Bridge [NK 002 269], north of Newburgh, and at the northern margin of this area near Meikle Tarty [NJ 990 280]. Scattered exposures elsewhere include the outcrops near Foveran Church [NJ 984 242], at [NJ 999 238] and [NJ 997 233] near Mains of Foveran, at [NJ 963 251] near Kincraig and at [NJ 945 265] near Rannieston (Figure 2). However, much information on the characteristics of the rocks of the Ellon Formation was obtained from trenches and drill holes within the Sheet 77 area and from the extensive exposures in areas lying immediately to the north e.g. at Hill of Logic [NJ 977 296] and Waterton Gorge [NJ 985 300].

General characteristics

The Ellon Formation consists largely of psammitic, semipelitic and pelitic metasediments, with occasional calcareous horizons. Amphibolites also occur widely in this formation and locally (e.g. near Newburgh (Figure 2), andnear Cairn Hill [NJ 941 278] (Figure 4)c are the predominant rock type.

Unlike the Aberdeen Formation, in many of the exposures of the Ellon Formation regular, penetrative fissility or foliation is lacking and distinctive lithological horizons are infrequent or are absent. As a result, the rocks superficially have a rather massive, structureless appearance (Plate 15). However, close inspection usually reveals an imperfect fissility and a 'streaky', lensoid banding due to the presence of impersistent layers of differing lithology ranging up to approximately 0.5 m in thickness. Finer banding, defined by leucocratic or melanocratic laminae a few millimetres thick can be recognised in many of the metasedimentary rocks, and often serves to indicate the presence of structural complexities, such as tightly appressed folds (e.g. at [NJ 947 247] near Fiddesbeg (Figure 4)c. Pods, lenses and veins of quartz and bodies of migmatitic granite occur widely in both metasediments and amphibolites. The granitic bodies are sometimes sharply defined with lensoid or vein-like form, but are often highly irregular in outline with gradational contacts, particularly when they occur within the metasediments. In many exposures these ill-defined granitic bodies occur in such abundance that the entire outcrop has a permeated, gneissose appearance, and in some instances (e.g. north-west [NJ 995 268] and west [NJ 996 254] of Newburgh) the granitic rock predominates and forms a relatively homogeneous matrix to disorientated lenses and blocks of metasediment and amphibolite ranging up to. 0.3 m in size.

The small size and widely scattered distribution of most of the outcrops of the Ellon Formation handicaps the identification of major lithological sub-units, but no particular type of metasediment predominates in the outcrops within the Sheet 77 area, except near Meikle Tarty [NJ 991 280], where psammitic rocks are more abundant than elsewhere. The amphibolites are usually subordinate to the metasediments (e.g. at [NK 001 243]), and even where amphibolites predominate, as in the area to the west of Newburgh (Figure 2), outcrop distribution and magnetic surveys show that they are interstratified with subsidiary metasedimentary horizons.

Structures

Although there are difficulties in identifying and in interpreting the detaili of the structures in the rocks of the Ellon Formation, certain broad relationships can be recognised. Thus (Figure 5)f, the foliation defined by the planar orientation of minerals and the streaky banding in these rocks generally strikes approximately N–S and dips gently (usually towards the east). However, as the western limit of the Ellon Formation is approached, steeper westerly dips become more common (Figure 5)e. The tightly appressed folds visible in many outcrops generally have an associated axial planar foliation with the same trend as the regional foliation, and have subhorizontal axes trending N–S. A lineation defined by microfolds or by crystals displaying preferred orientation, such as the hornblende laths in the amphibolites, can sometimes be recognised trending parallel to these fold axes (e.g. at [NJ 998 268]).

The predominant foliation is affected by small-scale (a few mm) crenulations at several localities (e.g. [NJ 997 268]). These microfolds have subhorizontal axes trending approximately E–W or NE–SW and an axial-planar foliation dipping S or SE at moderate angles. Examination of thin sections discloses that undeformed crystals of biotite in the metasediments (e.g. 70 [NJ 965 259]) and of hornblende in the amphibolites (e.g. 2430 [NJ 995 266]) (Plate 10)f overgrow, not only these microfolds, but also the tight N–S folds, showing that the main episode of recrystallisation occurred after both episodes of folding.

Open folding about subhorizontal axes trending N–S has been recognised (e.g. Read and Farquhar, 1956) in the Waterton Gorge [NJ 985 300], immediately north of the map area, and the scatter of the readings plotted in (Figure 5)f suggests that such folds may affect much of the Ellon Formation within the Sheet 77 area. These folds seem to have developed late in the history of metamorphism and deformation as they affect all the other structures, and in the Waterton Gorge have wavelengths of a kilometre or more. However, there would seem to be no grounds within the Sheet 77 area for recognising a larger-scale, N–S-trending, anticlinal structure in the Ellon Formation (the Buchan Anticline) as has been suggested by Wilson (1886, p. 7) and by Read and Farquhar (1956, p. 135).

Petrology and mineralogy

Metasediments

Petrography

The recognitition of original metamorphic assemblages in the metasediments of the Ellon Formation is hampered, as in the Aberdeen Formation, by extensive migmatisation, fine-scale (1 to 2 mm) lithological banding, and retrogressive crystallisation post-dating the main metamorphism.

However, in most specimens the main features of the original metamorphic assemblages can be recognised with reasonable certainty.

Plagioclase and biotite are major constituents in the pelites and semi-pelites. Cordierite is abundant in many of these rocks, but garnet is virtually lacking in the metasediments of the Ellon Formation within the Sheet 77 area. Potash feldspar and andalusite often occur in appreciable amounts, and subsidiary fibrolite is frequently present. The metasediments are generally fine-grained (0.5 to 2 mm), but highly migmatised rocks are often coarser-grained.

Pelites

Common assemblages include:

The content of plagioclase generally approaches 50 per cent. Biotite is usually the next most abundant phase and, with cordierite, forms much of the remainder of many of the rocks.

Muscovite is only a subsidiary phase in many of these rocks, and muscovite-free specimens occur. Potash feldspar is sometimes present but often appears to have formed after the main episode of metamorphic recrystallisation. A number of the pelites also contain andalusite.

Semi-pelites

The assemblages include:

Biotite is usually the main ferromagnesian mineral but muscovite is generally only a minor constituent which often overgrows the other minerals. In some semi-pelites potash feldspar is a subsidiary phase, found mainly in the vicinity of migmatitic leucosomes, which overgrows and appears to replace plagioclase and biotite. In other specimens it is abundant (20 to 30 per cent), occurs throughout the rock, and appears to have crystallised in association with the adjoining minerals. Andalusite occurs locally, generally in rocks that also contain potash feldspar and cordierite.

Apart from the lower content of ferromagnesian minerals (10 to 25 per cent), the semi-pelites differ from the pelites mainly in being richer (25 to 50 per cent) in quartz.

Psammites

These include quartzitic rocks and rocks which have an appreciable content (15 to 20 per cent) of minerals other than quartz. Assemblages in the latter group include:

Calc-silicates

Calcareous rocks are uncommon in the Ellon Formation and no specimens containing carbonate phases were found within the Sheet 77 area. Calc-silicate assemblages often contain colourless epidote and garnet in association with biotite and plagioclase. These rocks generally contain only subsidiary quartz, and, in more quartz-rich specimens (e.g. 64 [NJ 944 273]) the textures often suggest that silicification has occurred.

Accessory minerals

Common accessory minerals include zircon, apatite and opaque constituents, but no tourmaline has been found in Ellon Formation metasediments within the map area. Garnet is present as a minor constituent (c. 1 to 2 per cent) in a psammite (30* [NJ 969 283]) but has not been found in any other metasedimentary specimen. A gneissose biotite-plagioclase-quartz rock associated with amphibolite near Newburgh (89 [NJ 999 268]) also contains garnet, but cannot be identified with certainty as being a metasediment, and probably represents a highly migmatised marginal facies of the amphibolite.

Textural and mineralogical details

The 'streaky' banding visible in outcrop is represented by alterations of biotitic melanosomes and plagioclase or plagioclase-quartz leucosomes in the thin sections. The foliation is generally parallel to this banding, and is mainly defined by the preferred orientation of the cleavages in the subidiomorphic crystals of biotite. However, many specimens are poorly foliated, partly because the biotite crystals are small (generally less than 0.5 mm) and frequently show a very imperfect preferred orientation, and partly because equant crystals of cordierite are commonly associated with biotite in the melanosomes (Plate 10)g. Microfolds affecting the foliation are often overgrown by undeformed crystals of mica (e.g. 70 [NJ 965 259]), and it appears that the foliation is a structure with a complex history, as in the Aberdeen Formation.

The biotite crystals in the metasediments are almost invariably red-brown, and are generally smaller than the plagioclase (An22 to An29) crystals which range up to 2 to 3 mm in dimensions. These feldspar crystals occasionally show a lath-like form, but are generally equant and xenoblastic. The textural relations of quartz and potash feldspar crystals frequently suggest that these two phases have crystallised (or recrystallised) after the main metamorphism, but in other specimens (e.g. 70 [NJ 965 259]) microperthitic feldspar and quartz are intergrown with the other metamorphic minerals and show no evidence of replacive relations. Much of the muscovite in the metasediments is probably also a product of relatively late-stage crystallisation, but subidioblastic laths of this mineral which are intergrown with biotite in micaceous laminae and which display the same preferred orientation as the biotite appear to have crystallised during the main episode of metamorphism.

The relatively large (1 to 2 mm) crystals of cordierite in the melanosomes are accompanied by smaller (< 1 mm) crystals in the leucosomes which are intergrown with quartz and feldspar (e.g. 1347 [NJ 947 247]). Fibrolite laths are abundant within cordierite crystals in some rocks (e.g. 62 [NJ 946 265]), but, more typically, occur in wispy aggregates of fine needles that are intimately associated with biotite, and are often aligned in the plane of the cleavage of the mica. As in the Aberdeen Formation, much of the biotite associated with the fibrolite appears to have been reduced to pale-coloured, vestigial crystals. Fibrolite also occurs within plagioclase and potash feldspar crystals, sometimes as relatively large (0.2 to 0.3 mm), idioblastic needles (e.g. 62 [NJ 946 265]), but there is no evidence that the association of fibrolite with potash feldspar has been produced by the breakdown of muscovite.

Small (c. 1 mm), irregular crystals of andalusite are found in a number of the pelites and semi-pelites (e.g. 2363 [NJ 943 258], (Plate 10)g, and are generally extensively overgrown by biotite and frequently appear to be the relicts of crystals that were originally much larger. These andalusite grains have intricate boundaries against quartz and feldspar and are sometimes penetrated by small aggregates of fibrolite needles.

The relationships of fibrolite suggest that an episode of high-grade recrystallisation occurred after the main metamorphism, and in many rocks textural relations established during metamorphism have also been extensively modified by relatively late-stage recrystallisation, principally involving quartz, potash feldspar, plagioclase and muscovite, as in the Aberdeen Formation. Thus, in many specimens the texture is dominated by the presence of lensoid aggregates of quartz with a strained, polygonised internal structure that often appear to have replaced the other minerals in the rock (e.g. 2363 [NJ 943 258]). In other specimens, quartz and potash feldspar are clearly more abundant in the vicinity of migmatitic leucosomes (e.g. 1353 [NJ 973 221]). Plagioclase crystals often contain vermicular quartz inclusions, particularly in rocks in which potash feldspar crystals have been replaced by myrmekite and by muscovite-quartz symplectites. Many of the muscovite crystals are highly irregular and appear to have formed by the replacement of biotite or feldspar, and laths of fibrolite with round terminations, which may be relict grains, occasionally occur in these irregular muscovite crystals (31* [NJ 970 278]).

The cordierite crystals commonly display evidence of extensive retrogressive alteration, either to a fine-grained, greenish product ('pinite'), or to red-brown, vitually isotropic material, that may also have developed when the reactions involving muscovite and the quartzo-feldspathic phases affected the rock.

Amphibolites

These rocks consist largely of amphibole (normally c. 50 to 60 per cent) and plagioclase, but biotite is also found in significant amounts (5 to 10 per cent) in a number of specimens. Quartz also occurs widely, but is generally a subsidiary constituent, and often has a replacive relationship to the other phases, and cannot always be identified with certainty as being a product of the main period of metamorphic recrystallisation. Opaque constituents, zircon and apatite are common accessories but sphene is much less abundant than in the amphibolites of the Aberdeen Formation.

Many amphibolites are fine grained (c. l mm), massive, and devoid of regular fissility. Other specimens display imperfect banding defined by amphibole-rich lenses and laminae. None of the rocks shows features indicative of a sedimentary origin and primary igneous characteristics cannot be identified with certainty, except in a specimen from the east bank of the Ythan, immediately north of the Sheet 77 area (1338 [NK 003 280]), in which the outline of former plagioclase phenocrysts can still be recognised (Plate 10)h.

Texture and mineralogical details
Textural relations

Amphibole generally occurs as small (c. 0.5 to 1 mm), irregular to subidioblastic laths which may either be evenly distributed throughout the rock or largely restricted to ill-defined planar or lensoid aggregates. The foliation displayed by many specimens is generally parallel to the banding defined by these aggregates and is due mainly to the preferred orientation of the {001} axes of the elongated amphibole crystals. The plagioclase usually occurs as xenoblastic crystals of comparable size to the crystals of amphibole with which they are intergrown. In general the rocks are equigranular, except in the vicinity of migmatitic leucosomes where larger (c. 1.5 to 2 mm) crystals of amphibole and plagioclase sometimes occur.

Red-brown biotite is usually found only as a subsidiary (c. 5 per cent) phase intergrown with the crystals of amphibole. Thin (c. l mm) horizons with a relatively high content of biotite crystals displaying a marked preferred orientation occur in some rocks, however, and the margins of many of the amphibolite bodies are biotitic and schistose. Quartz occasionally occurs in small (< 0.5 mm), evenly distributed crystals which are intergrown with plagioclase and amphibole (e.g. 90 [NJ 995 268]) and appear to be products of the main episode of metamorphic recrystallisation. Irregular grains of opaque constituents are often concentrated in bands or 'trails' parallel to the foliation, and the compositional banding defined by the amphibole-rich lenses.

The sites and form of original plagioclase phenocrysts are preserved in specimen 1338 [NK 003 280] (Plate 10)h, although the original crystals are now replaced by aggregates of small plagioclase grains. Relict igneous texture may also be preserved in other rocks which are equigranular (e.g. 66 [NJ 942 281]) and in which the plagioclase crystals are subidioblastic against xenoblastic amphibole crystals displaying little or no preferred orientation. In other specimens textural complexities which can be ascribed to retrogressive changes and recrystallisation following the main metamorphism include the coarsening in grain and evidence of replacement of other phases by quartz in the vicinity of migmatitic leucosomes (e.g. 90 [NJ 995 268], and the formation of epidote as a replacement product of plagioclase and amphibole (e.g. 91 [NJ 996 268]).

Mineralogical details

The amphibole in different specimens display marked differences in colour, being pleochroic in shades of brown in some rocks and showing variations from brown to green or from green to yellow in others. A brown amphibole (66 [NJ 942 281]) and a yellow-green amphibole (90 [NJ 995 268]) have been analysed (Table 2) (Duncan, 1974). Both minerals are calcareous and hornblendic (Leake, 1978), with the yellow-green mineral being a magnesio-hornblende and the brown mineral an edenite with a composition very close to that of a magnesio-hornblende (Na + K = 0.53 in this mineral; the edenite/magnesio-hornblende boundary is drawn at Na + K = 0.5 (Leake, 1978, fig. 3)). The brown amphibole is more titaniferous than the green, and its greater iron content reflects a variation in parent-rock chemistry (Figure 8)a. Both of these amphiboles are more siliceous and less calcareous than the amphiboles from the Aberdeen Formation (Table 2), and also have a somewhat lower Alvi content than the latter (Figure 8)c, and may thus have crystallised at rather lower pressures.

Plagioclase crystals in the amphibolites are essentially unzoned, but vary in composition from rock to rock within the range An30 to An45. There is no obvious relationship between these compositional variations and other mineralogical features of the rocks, such as the proportion of amphibole or the presence of quartz and biotite.

Geochemistry of the amphibolites

Ten amphibolites from the Ellon Formation have been analysed, four from localities lying within the Sheet 77 area, six from localities in the area immediately to the north (Duncan, 1974). These specimens show a marked similarity to the analysed amphibolites from the Aberdeen Formation (Figure 9), (Figure 10), (Figure 11), and although some of the Ellon Formation rocks display similarities to metasediments (Figure 10), the analyses of the amphibolites from both formations plot as a coherent group in all the diagrams, and follow trends that cut obliquely across the composition field of metasedimentary amphibolites (Figure 9), (Figure 10).

The Ellon Formation amphibolites thus resemble the comparable rocks in the Aberdeen Formation in appearing to be derived from tholeiitic igneous rocks (Figure 11). The analysed specimens from the two formations display some differences, e.g. the Ellon Formation rocks have higher SiO2 and total Fe-oxides, somewhat higher TiO2 content, lower total alkalies, MgO/MgO + FeO + Fe2O3 ratios, and lower Cr and Ni contents (Figure 9), (Figure 10), (Figure 11). These features suggest that the group of analysed rocks from the Ellon Formation contains a greater number of more differentiated specimens than the group from the Aberdeen Formation. However the specimen with a relict porphyritic texture (1338) from the Ellon Formation, is distinctive in being poor in SiO2 and rich in MgO, and is the only analysed amphibolite from this formation to contain normative olivine (Figure 11)b, (Table 3), and a much more extended programme of analyses would he required before it could be established that there are significant differences between the amphibolites in the two formations.

Migmatisation

The migmatitic leucosomes that occur widely in the Ellon Formation are generally small (e.g. thicknesses of c. 10 to 20 mm), with a lensoid or an irregular outline and frequently grade into the surrounding rock. These irregular bodies show considerable diversity in character, sometimes being very coarse-grained, with crystals up to c. 10 mm in diameter, sometimes containing abundant feldspar, while in other instances they consist almost entirely of quartz. Oligoclase is the dominant feldspar in some of the feldspathic examples, potash feldspar in others. Muscovite and biotite are also common constituents. Quartzose leucosomes generally have sharply defined margins, but many rocks with a highly permeated appearance contain ill-defined quartz-rich areas (e.g. near Fiddesbeg [NJ 947 247]).

Every gradation can be observed from rocks in which original lithological components predominate and migmatitic leucosomes are relatively minor constituents, to rocks which have the appearance of a xenolithic gneiss with disorientated, irregular and round fragments of metasediments and amphibolite set in a 'streaky' foliated, leucocratic matrix. Such variations can be seen in a single exposure (e.g. in the area to the west of Waterside Bridge c. [NJ 995 2681] and are more common in metasedimentary rocks than in amphibolites. At one locality [NJ 996 254] west of Newburgh, the leucocratic matrix to metasedimentary and amphibolite 'xenoliths' consists of a plagioclase-quartz-biotite rock (2348) containing an appreciable proportion (c. 5 to 10 per cent) of cordierite.

At a few localities (e.g. [NJ 996 254], [NJ 999 268]) leucocratic veins up to 0.5 m wide with sharply defined boundaries cross-cut the structures in the metasediments and amphibolites of the Ellon Formation. These veins generally contain abundant potash feldspar and have a composition approximating to that of a true granite.

Chapter 4 Regionally metamorphosed rocks—3

The Collieston Formation

Exposures of the rocks of this formation are separated from the Ellon Formation by an extensive area devoid of outcrops where there is a thick overburden of glacial deposits and wind-blown sand. The most south-westerly outcrop of the Collieston Formation lies on the foreshore, 400 m south of Rockend at [NK 0185 2600] (Figure 13), but is only occasionally exposed when wave and current action remove the uppermost layers of the present-day intertidal deposits. To the north of Rockend [NK 022 264] exposures at the coast extend in a virtually continuous strip to the northern margin of the map area and beyond.

General characteristics

Within the area of Sheet 77 the Collieston Formation consists predominantly of interbanded psammitic, semi-pelitic and pelitic metasediments, with subsidiary calcareous and calc-silicate units. Psammites are generally the most abundant rock type and probably form over 50 per cent of the succession.

These siliceous rocks commonly form persistent lithological layers up to 1 to 2 m in thickness and show considerable diversity, sometimes being massive and poorly fissile, in other instances having a flaggy character because of the presence of numerous, thin, micaceous laminae. Coarse psammitic horizons containing quartz and feldspar clasts up to 4 to 5 mm in diameter elongated in the plane of fissility also occur. A continuous progression can be recognised from such psammites into rocks containing Oasts enclosed in a relatively abundant dark coloured matrix, which is essentially semi-pelitic or even pelitic in character. Examination of thin sections shows that many of these rocks are the metamorphosed derivatives of greywackes. Many of these inequigranular rocks contain thin, impersistent micaceous laminae and display a complex, 'streaky' internal structure (Plate 18).

Pelitic and semi-pelitic horizons include, not only metagreywackes, but also rocks that are uniformly fine-grained (c. 1 mm grain size). However the most conspicuous, and often the most abundant, pelitic and semi-pelitic rocks contain dark, ellipsoidal or spherical porphyroblasts in a fine-grained (< 0.5 mm) matrix and have a distinctive 'knotted' appearance (Plate 17). Elongated, roughly lath-shaped andalusite crystals form the larger porphyroblasts (up to 50 to 60 mm in length). Smaller, more-nearly spherical crystals of andalusite also occur, but in many rocks the smaller porphyroblasts consist mainly of cordierite. The 'knotted' rocks display variations in colour, degree of fissility and proportion of the porphyroblasts that can be related to variations in bulk composition, with the more pelitic rocks being darker, more fissile and having a higher proportion of porphyroblasts. The porphyroblasts generally show no preferred orientation and, in most instances, the pelites and semi-pelites display only an imperfect fissility and are rarely slaty or schistose.

In most exposures the pelites and semi-pelites occur in relatively thin layers (0.15 m or less in thickness) inter-banded with the predominant psammites, but certain parts of the coastal section consist predominantly of pelitic and semi-pelitic rocks (e.g. at [NK 029 272] near Corbie Holes). At some localities (e.g. at [NK 028 267] near Black Hole) individual pelitic and semi-pelitic horizons show considerable variations in thickness across the outcrop.

Calcareous rocks generally occur as thin (< 0.25 m thick) layers that are often impersistent and lensoid. They are generally fine-grained (< 0.5 mm) and frequently show 'streaky' lithological banding on a scale of a few millimetres. Occasional bands of limestone occur, being particularly conspicuous at The Smithy [NK 026 265] and on the foreshore to the south-west (Sanyne), where a band of limestone and calc-silicate rocks up to 3 m in thickness can be traced along strike for approximately 40 m (Figure 13). Calcareous rocks are also conspicuous at the southern limit of the strip of continuous exposures near Rockend [NK 022 264], where much of the foreshore consists of dark-coloured, fine-grained metasediments containing numerous thin (< 50 mm thick), impersistent bands, lenses and pods of pale-coloured calcareous rock. A major psammitic unit is delineated in the earlier version of Sheet 77 (inset to (Figure 2)) in the area west of Collieston, while a composite knotted-schist/ quartzite unit is shown as occuring at the coast. This interpretation was refined by Read and Farquhar (1956, p. 140) who suggested that quartzites ('Mormond Hill Quartzite') predominated south-west of The Smithy, while more varied rocks ('Collieston Beds') were thought to occur to the north-east of this locality. However, the only distinctive feature of the Smithy-Rockend foreshore is the relative importance of calcareous rocks and there are no strong grounds for the identification of a major psammitic unit here. Psammitic rocks, particularly coarse grained varieties and metagreywackes, are particularly abundant on the western shores of Hackley Bay [NK 027 270] and it is possible that a lithological sub-unit a few tens of metres thick consisting predominantly of siliceous rocks occurs in this area. Similarly, the predominance of 'knotted' rocks at [NK 029 272] near Corbie Holes north-east of Hackley Bay, suggests that a major sub-unit of such rocks may be present in this area, but as metasediments of more varied lithology appear to the north-east within 200 m, and as the lithological banding is flat-lying (see below), any such sub-unit can be no more than a few tens of metres thick. Massive, fine-grained (c. 0.5 m) amphibolite occurs at the headland [NK 023 264] immediately north-east of Rockend as a sheet-like body approximately 1 m thick. A more extensive, thicker (2 to 3 mm), body of similar rock occurs a few hundred metres to the north-east and occupies much of the foreshore immediately south of The Smithy (Plate 16). These amphibolites are roughly concordant with the lithological banding in the surrounding metasediments, but exposures at the western contact of the body near Rockend suggest that the boundaries of the amphibolites may be characterised by complex, small-scale interdigitation with the metasediments. Another body of mafic rock occurs nearby on the Sanyne foreshore [NK 025 264], but differs in having a more 'igneous' aspect and in forming a north-trending dyke-like mass, 1.5 m thick. Similar rock, displaying slight foliation, occurs further to the north [NK 0305 2730], immediately south-west of South Broad Haven as a relatively thick mass (c. 5 to 7 m thick). The metasediments and amphibolites in this coastal section are cut by bodies of granite which sometimes have regular dyke-like form and contain numerous angular xenoliths of metasediment, (e.g. at [NK 022 263]) or have a highly irregular outline (e.g. at [NK 025 265]). These bodies are widely scattered and form only a minor proportion of the exposures. They consist largely of medium-grained biotite granite, but coarse-grained pegmatitic varieties occur locally. Quartz veins occur more frequently, and, indeed, at some localities (e.g.[NK 024 265]) are so abundant that the rock has a breccia-like appearance with wispy relicts and irregular, angular fragments of metasediment or amphibolite set in a quartz matrix. The selective weathering of the altered reddish or greenish material (carbonate in part?) associated with the quartz gives many of these veins a cavernous appearance.

Structures

The foliation in the Collieston Formation is normally parallel to the prevailing trend of the lithological banding, and generally strikes approximately N–S and dips at low angles (< 20°) to the east ((Figure 5)g). Locally, dips are to the west, as in South Broad Haven [NK 031 273], and the angle of dip is often greater in the more northerly exposures.

Folds with subhorizontal, N–S axes and flat-lying axial planes are conspicuous in many of the outcrops. These folds occasionally have an asymmetric profile (e.g. at [NK 026 266]), but in most instances are tightly appressed and virtually isoclinal. Their amplitude is sometimes as much as 1 to 2 m (e.g. at [NK 023 263]), but most examples are much smaller and have amplitudes of only a few tens of millimetres, and, indeed, at many localities this set of structures is represented merely by crenulations with dimensions of the order of a few millimetres. In general, these folds display a well-developed axial planar foliation parallel to the fissility in the surrounding rocks, and, in thin section (e.g. 2437 [NK 0275 2705]), are seen to be overgrown by the main metamorphic minerals, the micas in particular. Many examples of this set of folds are rootless and are confined within layers of a particular lithology that have subparallel, apparently unfolded, external boundaries. It is probable that much of the structural complexity recognisable within many of these layers (e.g. in the calc-silicate-pelite sequence at Rockend [NK 022 264]), such as the widespread occurrence of impersistent, lensoid pods and layers, can be ascribed to ductile extension on the limbs of isoclinal folds on planes parallel to the layer boundaries.

Many of the bands of massive, coarse-grained psammite contain thin (2 to 3 mm) fine-grained (< 0.5 mm), biotite-rich layers (e.g. at [NK 027 266]) which cannot be readily explained as being due to original sedimentary heterogeneities, and, indeed, may cross-cut original lithological banding (Plate 18). These biotite-rich layers resemble structures described from elsewhere in the Dalradian (Harris and others, 1976) which are thought to represent a spaced cleavage developed from fracture cleavage sets through pressure solution processes. The development of these biotite-rich layers may have been associated with the isoclinal folds discussed above, but at some localities (e.g. the foreshore south of The Smithy [NK 025 265]), biotite-rich laminae in coarse-grained psammites are affected by the isoclinal folds, suggesting that they are the products of a period of deformation that preceded the folding.

The axes of the folds generally trend N–S parallel to the strike of the lithological banding, and are effectively horizontal in most outcrops. However, sometimes (e.g. immediately north of Black Hole [NK 027 267]) the trend of the fold axes is slightly oblique to the strike of the banding and the folds plunge gently to the north or south. At a few localities where the lithological banding in the metasediments dips westwards, the axial planes of the folds show a corresponding change in inclination. These relationships may have been developed when the banding and the axial planes of the isoclinal folds were flexed about later folds with subhorizontal N–S axes trending at a small angle (5 to 10°) to the axes of the earlier folds. Examples of these later structures are probably present at Sanyne [NK 023 264] where gently undulations about N–S axes with wavelength of 5 to 10 m or more have developed in metasediments with subhorizontal lithological banding.

Evidence of other structural complexities can also be recognised at a few localities. Thus, a fold with an amplitude of 1 to 2 m, and a subhorizontal, NW–SE axis which occurs in a sea stack on the foreshore between Sanyne and The Smithy INK 025 265] affects, not only the lithological banding, but also the axial planes of tightly appressed interfolial folds. Folds with E–W axes can be recognised on the northern face of Hackley Head [NK 029 268] and at Corbie Holes [NK 030 272]. The foliation in the metasediments is also deflected until it strikes approximately E–W in the vicinity of the southern contact of the body of amphibolite at [NK 031 273] near South Broad Haven. However, as the metasedimentary banding at this contact is effectively parallel, not only to the foliation developed in the amphibolite, but also to the contact it is probable that this deflection merely reflects the localised variation in strain in the vicinity of the amphibolite when this relatively resistant body was subjected to post-consolidation deformation. Evidence of the varying response of different lithological units to stress is also visible at other localities where boudinage structure has developed, as in the calc-silicates near Black Hole [NK 027 267] (see also the description of areas to the north in Read and Farquhar, 1956, pp. 143–147). Boudinage structures may also be seen in quartz veins (e.g. at Marywalls [NK 032 274]).

Petrology and mineralogy

Metasediments

The semi-pelites generally contain quartz, reddish brown biotite and plagioclase. Cordierite and andalusite are additional phases in many of these rocks, generally as porphyroblasts. The pelites differ mainly in containing less quartz and in being richer in biotite, andalusite and cordierite, with the content of cordierite approaching 40 per cent in some specimens (e.g. 2428 [NK 031 274]). Potash feldspar is a constituent of a number of pelites and semipelites, several of which also contain idioblastic crystals of muscovite which appear to have crystallised in association with the other metamorphic minerals. Garnet occurs in small, round grains in the groundmass of a semi-pelitic metagreywacke from Hackley Bay (2437 [NK 0275 2705]).

The andalusite porphyroblasts have highly irregular boundaries against the other phases (Plate 19)a, even when the crystals appear to be relatively well-formed in hand specimen, and are extensively replaced by fine-grained white mica in many rocks. The cordierite crystals are also markedly xenoblastic, although the original textural relations are often largely obscured by the development of fine-grained (possibly micaceous) alteration products. A specimen from near Hackley Bay (2427 [NK 028 271]) is notable because it contains abundant unaltered porphyroblasts of cordierite with sector twinning (Plate 19)b.

In many pelites and semi-pelites the andalusite and cordierite porphyroblasts are set in a fine-grained (about 0.1 to 0.2 mm) groundmass consisting largely of intergrown crystals of biotite, feldspars and quartz. The preferred orientation of these groundmass biotite crystals is generally largely responsible for any fissility displayed by these rocks. Very fine-grained (< 0.01 mm) inclusions of biotite, plagioclase and quartz are often visible within the porphyroblasts and in some specimens (e.g. 2427) these inclusions are not always aligned parallel to, and continuous with, the external foliation, and thus provide a record of complexities in the structural history of the rock.

The psammites include even-grained, quartz-rich rocks with only a minor content of biotite and feldspar. Many of the inequigranular, coarse-grained rocks are also highly siliceous, but there appears to be a continuous gradation from such speciments into rocks with a ferromagnesian-rich matrix which are probably metagreywackes. The clasts in all of these rocks are predominantly of quartz, though feldspar, mainly plagioclase, but occasionally potash feldspar, is sometimes abundant. The quartz clasts are often aggregates of grains with sutured mutual boundaries and both quartz and feldspar clasts are often elongated within the plane of the foliation. The matrix sometimes forms more than half of the rock (e.g. 2424 [NK 025 265], Plate 19)c, and may contain cordierite and andalusite as well as biotite, quartz and feldspars. In some rocks (e.g. 2437 [NK 0275 2705]) complex folded structures are overgrown by matrix biotites, while in others (e.g. 2436 [NK 027 267]), fine-grained biotitic zones which form a spaced cleavage, contain small (c. 0.5 mm) crystals of andalusite intergrown with the biotite foliae. The calc-silicates are generally fine-grained with typical assemblages including colourless epidote, biotite, plagioclase and amphibole. Garnet and diopside are occasionally present (e.g. 2421 [NK 022 263]), while a calc-silicate from The Smithy (2442 [NK 026 265]) contains scapolite.

Widespread retrogressive alteration has affected most of the important rock-forming constituents and greatly impedes the identification of the metamorphic minerals in many of the metasedimentary specimens of the Collieston Formation. The feldspars, in particular, are often largely replaced by sericite or by very fine-grained, brown alteration products, and although it has been established that oligoclase (c. An20 to 25) occurs in a number of rocks, in many specimens the composition of the plagioclase cannot be determined reliably, and potash feldspar cannot be recognised with certainty.

Amphibolites

The metamorphosed basic rocks are generally fine-grained (c. 0.5 to 1 mm) and essentially massive. In most instances recrystallisation to an assemblage of plagioclase and amphibole has been accompanied by the obliteration of the original texture and the development of an imperfect foliation, mainly defined by the preferred orientation of the amphibole crystals. However, in a specimen from near The Smithy (1324 [NK 026 265], (Plate 19)d, an original ophitic texture is still recognisable even although the original large (3 mm) laths of plagioclase have been replaced by aggregates of fine-grained (c. 0.1 mm) plagioclase and large (3 mm) amphibole crystals have pseudomorphed the original ferromagnesian minerals. Relict igneous features are also recognisable in the mafic rock from South Broad Haven (1330 [NK 031 273]), in which plagioclase (An45) phenocrysts, up to 1.5 mm in length, occur in a slightly foliated groundmass consisting mainly of fine-grained (c. 0.1 mm) plagioclase, amphibole and opaque constituents.

Many of the amphibolites are extensively altered, with the feldspars being replaced by turbid alteration products and the amphiboles by chlorite. Several specimens (e.g. 2422, Rockend [NK 022 264]) also contain epidote but this mineral invariably appears to be the product of post-metamorphic alteration, and never displays features suggesting that it has been formed during the regional metamorphism.

The yellow-green amphibole in the specimen (1324) from the foreshore south of The Smithy has been analysed (Table 2) and shows a general similarity to the amphiboles from the Aberdeen and Ellon formations (Figure 8), being calcareous, and, although identified as an edenite, being compositionally similar to magnesio-hornblendes as Na + K = 0.52 (Leake, 1978). The Alvi content of this mineral is relatively low (Figure 8)b, and is similar to that of the Ellon Formation amphiboles, but it is appreciably richer in Mg and poorer in TiO2 than the latter, presumably reflecting the composition of the host rock (Figure 8)a.

The above rock (1324) and the amphibolite from South Broad Haven (1330) have been analysed (Table 3). Both specimens show a general similarity to the analysed amphibolites from the Ellon and Aberdeen formations, being tholeiitic (Figure 11) and plotting within the range of composition of the other amphibolites in the variation diagrams (Figure 9), (Figure 10). Specimen 1324 has a relatively high content of normative diopside and a relatively low content of normative quartz (Figure 11)b, but there are no strong grounds for regarding this rock as being chemically distinct from all the other amphibolites.

Chapter 5 Inter-relations of the regionally metamorphosed rocks

Stratigraphical relations

Correlation of the units of regionally metamorphosed rocks identified within the Sheet 77 area with the lithostratigraphical units and sequences recognised in the Dalradian elsewhere is greatly handicapped by the lack of information from the immediately surrounding areas- particularly to the west, where exposures are poor and the metasediments have been extensively intruded by bodies of granite. The difficulties in correlation are made still greater by the lack of distinctive marker horizons within the Sheet 77 area, and persist even when the attempted correlations are based on associations of rock types, such as impure limestones with quartzites (Harte, 1979; Harris and Pitcher, 1975, fig. 12), rather than on specific lithologies, such as limestone. Facies change along strike, such as have been recorded elsewhere in the Dalradian (Harris and others, 1978; Harte, 1979) may also be a factor hampering correlation in the Aberdeen area, and, indeed, may explain the absence of significant bodies of limestone in the area to the east of Banchory (Read, 1955). The effects of faulting are also difficult to assess but, as a major dislocation has been identified in the Dee Valley (Chapter 12) may be significant in the map area.

Read (1928) regarded virtually all the regionally metamorphosed rocks in the Sheet 77 area ('gneisses') as being equivalent to the Queens Hill Quartzite and Mica Schist 'Group' of Middle Deeside, and on the basis of his correlation of the Deeside and Loch Tay limestones, further deduced that the Queens Hill 'Group' could be equated with the Ben Lui Schist of Perthshire. He noted that both the latter formations were extensively migmatised and contained abundant bodies of metamorphosed basic igneous rocks and, as it was known (Bailey, 1925) that the Perthshire succession was inverted, deduced that the Queens Hill 'Group' was stratigraphically below the Deeside limestone, even although it structurally overlies the latter formation. These correlations have been widely accepted, with only minor amendments being made by Harte (1979). In the more refined terminology that has been developed to describe the Dalradian succession, the Queens Hill 'Group'/Ben Lui Schist formation (i.e. the Sheet 77 gneisses') is a division of the upper part of the Argyll Group (Middle Dalradian), (Harris and Pitcher, 1975; Institute of Geological Sciences, 1977a).

The remapping of the Sheet 77 area indicates that some modification of Read's original correlations is required, nevertheless, as the original 'gneiss' is now known to consist of at least two major lithological units-the Aberdeen and Ellon formations. The prevailing NE–SW strike of the lithological sub-units in the Northern unit of the Aberdeen Formation (Figure 2) suggests a probable extrapolation south-westwards along the valley of the Dee, to link with the Middle Deeside succession (Read 1928), either as components of the Argyll Group or possibly of the lower part of the Southern Highlands Group, but only very tentative correlations of all or part of the Aberdeen Formation with lithostratigraphical units in the areas of Dalradian to the west can as yet be made.

It is possible that the Northern unit of the Aberdeen Formation is equivalent to a combined Queens Hill/Deeside Limestone unit, as has been implied by Harte (1979), and certainly such a correlation is supported by the relative abundance of calcareous rocks and amphibolites in this part of the Aberdeen Formation (Figure 2). If this suggestion is correct, and the stratigraphical succession in the Dalradian of the Aberdeen area is inverted, then the Southern unit of the Aberdeen Formation might be equivalent to the Upper Dalradian Glen Effock Schist unit recognised by Harte (1979) in Glen Esk. However, other correlations are possible, e.g. the entire Aberdeen Formation with the Queens Hill 'Group'.

The remaining component of the original 'gneiss' unit in the Sheet 77 area—the Ellon Formation—is considered to be a distinctive lithological unit. The boundary between this unit and the Aberdeen Formation is defined, in part at least, by a zone of dislocation within the Sheet 77 area (see below). It has been suggested (Sturt and others, 1977; Ramsay and Sturt, 1979) that gneiss units in NE Scotland have undergone a Precambrian metamorphism, are separated from the associated rocks by zones of discordance and cataclasis, and represent tectonically emplaced fragments of the basement to the Dalradian. Although this possibility cannot be excluded, it is noteworthy that the Ellon and Collieston formations (i.e. the suggested basement and cover respectively) show considerable structural similarities and that the differences in metamorphism between these two units could be due to a straightforward progression from low-grade (Collieston Formation) to high-grade (Ellon Formation). The two formations may therefore be parts of a continuous sedimentary sequence that has been subjected to differing grades of regional metamorphism, and there may be no need to invoke a basement-cover relationship to account for their different characteristics. If this suggestion is correct, then the Ellon Formation may be no older than Middle Dalradian (Argyll Group) as the lithological characteristics of the Collieston Formation (Read and Farquhar, 1956) show that this formation is almost certainly part of the Upper Dalradian (Southern Highland Group) succession.

Structural relations

Although the three formations of regionally metamorphosed rocks in the map area are characterised by important lithological and metamorphic differences, certain broad structural similarities can be recognised in these groups of rocks.

The rocks in all three formations contain a foliation that was developed prior to the main episode of metamorphic crystallisation and is essentially parallel to the lithological banding. Tightly appressed, rootless folds, with an axial-planar cleavage parallel to the prevailing trend of the foliation and with sub-horizontal, N–S axes can be recognised in all three formations. At many localities, particularly in the Ellon and Collieston formations, intense deformation parallel to the axial planes has disrupted these folds and has given the rocks a distinctive 'streaky' appearance. These tightly appressed folds have been affected by later folding in all three formations, and although there is no certainty that all these later structures are related in origin, they frequently have axes trending approximately E–W, and, like the early folds, are overgrown by the minerals formed during the main episode of metamorphism. Open folding about N–S axes has affected the rocks in all three formations after the climax of the metamorphism and in some areas produced structures with a wavelength of approximately 1 to 2 km. However, the zone of steep structures adjoining much of the boundary between the Aberdeen and Ellon formations is associated with shearing and mylonitisation affecting the 'Younger Basic' masses (Chapter 8) and is probably unrelated to any fold episode described above.

Metamorphism

Porteous (1973a) established that andalusite, sillimanite and garnet are characteristic minerals of the pelites in the Southern unit of the Aberdeen Formation, and concluded that the rocks in the coast section between Stonehaven and Aberdeen had been metamorphosed under conditions transitional between the metamorphic regimes prevailing in the classic areas of Barrovian metamorphism to the south-west and of Buchan metamorphism to the north. Harte (1975) later applied the name Stonehavian to the metamorphic progression visible on the Kincardine coast, and from a compilation of petrographic, mineralogical and experimental data showed how variations in the sequences and nature of the mineral assemblages in pelites from different areas in the eastern Dalradian could be related to variations in P-T conditions (Figure 14). The rocks of the Southern unit of the Aberdeen Formation can be located in this diagram by the occurrence of assemblages containing andalusite-staurolite-biotite and sillimanite-staurolite-biotite, and are further characterised by the association of andalusite and garnet, in contrast to the kyanite-garnet association in Barrovian rocks and the andalusite-cordierite association in rocks subjected to Buchan metamorphism.

More difficulty is experienced in assessing the conditions of metamorphism of the Northern unit of the Aberdeen Formation as the rocks of this unit show only limited variations in mineralogy, with sillimanite being the only form of Al2SiO5 that has been found, and with biotite being virtually the only ferromagnesian mineral, except in the limited number of rocks that also contain garnet. The study of the mineralogy of the amphibolites in the Aberdeen Formation ((Figure 8); Duncan, 1974) leads to the conclusion that, throughout both units of this formation, metamorphism occurred at lower pressures than in the Barrovian areas to the south-west, while the information on the regional distribution of kyanite and andalusite (Chinner and Heseltine, 1979; Porteous, 1973a) suggests that the physical conditions during metamorphism were appropriate for the formation of andalusite in both units. These relationships suggest the absence of andalusite in the pelites and semi-pelites of the Northern unit can probably be ascribed to chemical constraints.

The 'typical' Buchan series is characterised by the successive appearance of cordierite, andalusite and sillimanite in pelites (Figure 14) and was described by Read (1952) in a west to east traverse from low to high grade in the Ythan Valley, immediately to the north of the Sheet 77 area. The sequence in a traverse in the reverse direction into the metamorphic 'high' near Ellon from the Collieston Formation into the Ellon Formation is essentially similar in character, and although rocks with cordierite, but without andalusite, which are representative of the lower grades of metamorphism, do not occur within the Sheet 77 area in the Collieston Formation, they probably occur in the coastal section further north, near Whinnyfold [NK 080 330] (Read and Farquhar, 1956). The higher grade rocks, the Ellon Formation, are characterised by a general coarsening in grain-size, extensive migmatisation, the presence of sillimanite, andalusite and cordierite, and by the apparent absence of garnet in the pelitic rocks of this formation within the map boundaries. The metamorphic evidence that can be used to support the identification of a major discontinuity between Elton and the Collieston formations (see Sturt and others, 1977; Ramsay and Sturt, 1979) is therefore no more compelling than the structural evidence discussed above.

Sillimanite (fibrolite) occurs in a markedly sporadic fashion in the rocks of both units of the Aberdeen Formation and of the Ellon Formation. While this mineral coexists with andalusite in many specimens, there is only occasional evidence of direct replacement of andalusite by sillimanite. These relationships can also be recognised in kyanite-bearing rocks subjected to Barrovian metamorphism, and when taken with the evidence that the sillimanite isograd and the andalusite and kyanite isograds cannot be related in a straightforward fashion, led Chinner (1966) to suggest that the formation of sillimanite in the rocks of the eastern Scottish Dalradian could be ascribed to a temperature rise that post-dated the formation of andalusite and kyanite, the so-called 'sillimanite-overprint'. Although Harte and Hudson (1979) have subsequently suggested that the overprinting of andalusite and kyanite by sillimanite may have occurred during a single phase of progressive metamorphism, nevertheless, in many rocks there appears to have been a hiatus in the processes of mineralogical and textural readjustment between the stage when the entire rock was affected by these processes and andalusite, or kyanite, formed, and the stage when readjustment was localised and sillmanite formed.

Chinner (1961) concluded that the intimate association of sillimanite with biotite in these regionally metamorphosed rocks merely indicated that biotite was acting as a nucleating agent, and that no extensive breakdown of biotite was involved. However, in a number of specimens in the Sheet 77 area, biotite associated with fibrolite is decolourised and reduced to vestigial relicts, and clearly seems to have been involved in the sillimanite-forming reactions, although there is no evidence that breakdown has been accompanied by the formation of magnetite and potash feldspar as suggested, by Francis (1956). Chinner (1961) argued that when fibrolite nucleated on biotite, kyanite elsewhere dissolved in the intergranular fluid, thereby providing the Al and Si necessary for the growth of the fibrolite. It is possible that the effects of such complementary reactions may also be recognisable in some of the rocks in the Sheet 77 area, in which andalusite is commonly mantled by, and partly replaced by, biotite. Such a replacement appears unlikely to be a straightforward pro-grade reaction, and it is possible that when biotite decomposed to form fibrolite, elsewhere in the rock biotite of different composition renucleated and grew on unstable andalusite.

Chinner (1966) also suggested that the development of sillimanite could probably be related to the development of migmatitic gneissose rocks and to the appearance of potash feldspar as a prograde phase. However, the relationships between metamorphism and migmatisation are obscure in both the Aberdeen and the Ellon formations within the map area. The formation of fibrolite appears to have preceded the emplacement of the 'Younger Basic' masses (see Chapter 7), but it is possible that most of the bodies of migmatitic granite are associated with the granitic sheets that cut the 'Younger Basic' intrusions, and a clear record of any earlier (pre 'Younger Basic') migmatitic event has not been preserved. The cordierite-bearing granitic bodies that occur locally in the Ellon Formation appear to be distinctive, however, and may be the products of syn-metamorphic migmatisation associated with formation of fibrolite.

The porphyroblastic crystals of muscovite that occur widely in both the Ellon and Aberdeen formations may have formed contemporaneously with the fibrolite, as is generally thought to be the case elsewhere (e.g. Chinner 1966; Harte and Johnson 1969). However, the textural relations of the muscovite crystals are complex and in many rocks there has clearly been more than one episode of muscovite crystallisation. In some specimens crystals of muscovite which are intergrown with the other metamorphic minerals can be identified as prograde constituents, possibly formed before sillimanite. In other rocks, much of the muscovite shows replacive relationships to the other minerals, including fibrolite, and appears to have crystallised either in association with potash feldspar when the migmatitic bodies of 'granite were formed, or at a later stage in association with quartz in symplectites which replace potash feldspar. The formation of these quartz-muscovite intergrowths probably occurred in association with the development of myrmekite and the recrystallisation of quartz which are widespread features of rocks in the map area, particularly in the Ellon and Aberdeen formations. As the rocks in most of the major granitic bodies in the Aberdeen area also seem to have been affected by this last recrystallisation event (see Chapter 9) it must have occurred at a very late stage (about 400 Ma?) in the orogeny.

The main episode of recrystallisation in all three formations was later than much of the deformation and has obliterated virtually all the evidence of earlier textural and mineralogical features in the rocks. The precise date of this metamorphic event is uncertain, but it is generally assumed (e.g. Dunning, 1972) to have occurred immediately prior to the emplacement of the 'Younger Basic' instrusions (c. 490 Ma; Pankhurst, 1982).

Chapter 6 'Younger Basic' intrusions

The Belhelvie mass

The original version of Sheet 77 depicted the Belhelvie mass as a body of diabase and serpentine lying wholly within the map area, 5 to 10 km north of Aberdeen. This representation was later modified by Stewart (1947), who provided the first detailed description of this intrusion, and showed that it consists mainly of peridotitic, troctolitic, noritic and gabbroic rocks. During the remapping a widespread programme of magnetic surveying and shallow drilling (Figure 15) was carried out in the Belhelvie area, and has provided much new information leading to a further, major revision of the map of the intrusion. The following account is based on the description of the intrusion by Ashcroft and Boyd (1976) which incorporates much of the new data, supplemented by mineralogical and chemical data from Boyd (1972) and structural data from Boyd and Munro (1978).

General description of the intrusion

The Belhelvie mass is now known to occupy an area of approximately 25 km2 on land, compared to the previous delineation of about 18 km2, and aeromagnetic data (Institute of Geological Sciences, 1968) suggest that igneous rocks extend for at least 5 km offshore (Figure 16). The onshore portion of the mass is elongated NW–SE and has a length of 11 km and a maximum width of about 5 km at the coast. Exposures of the igneous rocks and of the adjoining country rocks are reasonably abundant in the north-west, but, near the coast, virtually the only direct source of information on the nature of the bedrock is provided by drill holes.

The magnetic map of the Belhelvie area (Figure 15) is characterised by extensive positive anomalies of high amplitude, generally 600 nT, and occasionally 1500 nT or more above the level of the zero anomaly. These positive anomalies are markedly linear, frequently trend NW–SE and are sharply delineated from the surrounding negative anomalies by zones of steep magnetic gradient. Information from outcrops and drill holes shows that such positive anomalies invariably coincide with the occurrence of ultramafic igneous rocks. The other igneous rocks in the Belhelvie mass have less strongly defined magnetic characteristics, but generally give rise to small positive anomalies, approximately 200 nT in amplitude, rising from a general level which is depressed by the presence nearby of the strongly magnetised ultramafic units. Troctolites, gabbros and norites all display similar magnetic characteristics and cannot be reliably distinguished in the magnetic map. The country rocks surrounding the intrusion produce a very uniform field with virtually no anomalies and the position of the contact between igneous rocks and country rocks can normally be clearly identified on magnetic profiles (Figure 15), particularly when the igneous rocks are ultramafic.

Although there are departures from this broad pattern of relationships, such as the lack of positive anomalies in certain areas of ultramafic rocks (e.g. at [NJ 907 200]), or the presence of positive anomalies in areas of hornfels (e.g. at [NJ 935 197]), these are relatively rare, and the magnetic map has been used in conjunction with the outcrop and drill hole data to produce a more accurate and detailed map of the complex than would otherwise have been possible (Figure 16).

The revised map shows that an important NE–SW discontinuity (the Belhelvie Fault) cuts through the igneous rocks approximately 1 km south of Belhelvie village (AB, (Figure 15)). There is little drill hole and outcrop data to the south of this discontinuity but the occurrence of extensive positive magnetic anomalies here, and in the area offshore (Institute of Geological Sciences, 1968), suggests that much of the southern part of the mass consists of ultramafic rocks. North of the fault, anomalies with a roughly NW–SE trend dominate the magnetic map and information from the relatively abundant outcrops and drill holes shows that lithological units elongated parallel to the length of the intrusion occur in this area. These units include an extensive strip, or septum, of hornfelsed country rocks which extends into the igneous rocks from the north-eastern contact of the intrusion and effectively divides the northern part of the mass into two sub-areas.

In the north-west a narrow (about 100 m wide) zone of noritic rocks occurs at the western boundary of the intrusion and wider (about 0.5 km) zones consisting of peridotitic, troctolitic and gabbroic/noritic rocks are successively encountered in a traverse eastwards across the igneous mass to the country rock septum. To the east, no marginal norite can be recognised adjoining the hornfelses at the eastern margin of the septum, but there is a repetition of the west to east sequence of peridotitic, troctolitic and gabbroic/noritic units. The individual lithological units in this eastern sequence are much narrower and more irregular than the comparable units west of the septum.

The zones of troctolitic and gabbroic/noritic rocks west of the septum contain subsidiary lenses or layers of ultramafic rock. These are up to 100 to 150 m thick and are elongated parallel to the length of the intrusion, in one instance, being traceable for nearly 3 km. Smaller (about 50–100 m thick) lensoid bodies of troctolitic rock can also be recognised within the major peridotitic unit east of the country rock septum.

The major lithological units show considerable variations in thickness, and, east of the septum, the zone of gabbroic/noritic rock appears to be lacking near Sparcraigs [NJ 934 197] for several hundred metres at the eastern boundary of the mass. The eastern troctolite zone cannot be traced to the north-eastern limit of the mass and appears to terminate abruptly 200–300 m south of that boundary.

The main peridotite zones in both the western and eastern sequences can be traced southwards to the Belhelvie Fault, but lack of exposures in the west and the effects of post-consolidation deformation in the east make it impossible to trace the troctolitic and gabbroic/noritic units south of Grid line 18.

The magnetic data can also be used to determine the possible three-dimensional configuration of the ultramafic units in the mass. (Figure 17) shows that the magnetic anomaly associated with the western peridotite unit in the Whitecairns area [NJ 920 180] can be explained by postulating that this ultramafic unit has steep contacts and extends to a depth of 1 km. Another profile to the south-west of Balmedie (Z–Z″, (Figure 15)) traverses a magnetic discontinuity (AC, (Figure 15)). North of this discontinuity magnetic modelling suggests that the E–W anomalies are due to units of ultramafic rocks dipping northwards at approximately 40–50°. South of this discontinuity the modelling suggests that the subsidiary NW–SE anomalies superimposed on the generally high magnetic field are due to steeply dipping bodies of ultramafic rock.

Primary structures in the igneous rocks

The Belhelvie mass resembles many other large mafic and ultramafic intrusions (Wager and Brown, 1968) in displaying layered structures defined by planar or sheet-like concentrations of ferromagnesian minerals or of plagioclase. These structures are best developed in the western troctolite and gabbro/norite units (Figure 16) but can also be recognised at some localities east of the country rock septum. In a few instances (e.g. near Hillhead of Craigie [NJ 914 202] the layers are sharply defined and show no appreciable variations in thickness, but, more typically, the layering is much less regular and in many rocks is represented merely by ill-defined lensoid aggregates of ferromagnesian minerals or of plagioclase. Layers of extreme, almost monomineralic, composition are generally no more than a few tens of millimetres thick and occur only intermittently within unlayered troctolite, gabbro or norite. At some localities in the western troctolitic unit in the Wester Craigie area [NJ 915 197] and in the western part of the western gabbroic/noritic unit on Harestone Moss [NJ 923 195], occasional layers show internal gradation from a ferromagnesian-rich western margin to a feldspathic eastern margin.

Primary lamination due to the- planar orientation of the constituent crystals can be recognised in both layered and unlayered rocks at many localities in the mass (Plate 19)g. Plagioclase feldspar is commonly the phase that is best orientated, with the large {010} faces of the crystals lying in the plane of the lamination, but pyroxene and, more rarely, olivine crystals may also display a preferred orientation. In layered rocks the lamination is invariably parallel to the layering. (Figure 16) shows that the layering and lamination in the northern parts of the mass generally dip steeply and strike approximately N–S parallel to the trend of the major lithological units, and to the direction of elongation of the intrusion. Minor variations in attitude of these structures can sometimes be observed and a more marked deviation was recorded at a temporary exposure [NJ 9355 1895] in a small troctolitic lens within the eastern ultramafic unit where the layering was seen to strike E–W. The attitude of the primary structures can be measured at only a single locality in the southern part of the mass [NJ 952 154], where layering and lamination with an E–W strike and a moderate northerly dip are visible in troctolites and picrites. The magnetic evidence from this area-the E–W magnetic anomalies (Figure 15) and the inferred attitude of the rocks producing one of these anomalies (Figure 17), suggests that this single measurement may be representative of the attitude of the primary layering and lamination in the part of the mass immediately south of the Belhelvie Fault. However, the inferred steep dip of the ultramafic rocks associated with the NW–SE anomalies south of discontinuity AC (Figure 15) suggests that there are structural complexities in the southern part of the mass.

Petrology and mineralogy of the igneous rocks

The Western marginal unit

The rocks in this unit are exposed at three localities ([NJ 904 216], [NJ 904 213], [NJ 904 208]) and have also been encountered in shallow drill holes at five localities, including a series of inclined holes near Kingseat at [NJ 905 197] which provided an almost complete section through the marginal rocks.

The majority of the specimens from this unit contain abundant orthopyroxene and lesser amounts of augite and plagioclase and are thus noritic in character (e.g. 1695* [NJ 9053 1970]). Olivine may also occur, and occasionally is the dominant constituent (e.g. 1697* [NJ 9050 1970]), while brown hornblende and biotite are commonly present in minor amounts in the form of overgrowths on the main ferromagnesian minerals. The rocks are generally rich in ferromagnesian minerals with the plagioclase content rarely exceeding 30–40 per cent, and in a number of instances are virtually pyroxenites (e.g. 1640* [NJ 9140 1787]), while in other specimens (e.g. 1699* [NJ 9049 1970]) olivine and orthopyroxene are the only major constituents, i.e. the rocks are harzburgites. Augite predominantes over orthopyroxene in some specimens, particularly near the northern limit of the mass (e.g. 1693* [NJ 9038 2178]).

Textural features, particularly the common occurrence of relatively small (1 to 3 mm) idiomorphic or subidiomorphic crystals of olivine and pyroxene within larger (> 10 mm) poikilitic crystals of plagioclase (Plate 19)e, suggest that most of the rocks are pyroxene or pyroxene-olivine cumulates (Wager and others, 1960). The more feldspathic rocks have a more even-grained texture, with all the phases being present as intergrown crystals of similar size, and have been identified as plagioclase-orthopyroxene-augite cumulates (e.g. 118 [NJ 904 213]).

The inclined series of drill holes at Kingseat shows that a narrow (< 10 m) band of harzburgitic and picritic rocks (1699*; 1700*) adjoins the country rocks in this area. Pyroxene-rich noritic rocks (1701*) occur to the east and extend for approximately 100 m across most of the zone of marginal rocks, although olivine-rich rocks occur again in the vicinity of the peridotitic rocks that lie further to east.

A drill hole near the northern limit of the mass (1661* [NJ 9045 2210]) encountered a rock displaying a steep foliation but the other specimens of marginal rocks are devoid of primary structures.

The outcrop, drill hole and magnetic data, particularly the subsidiary positive anomaly to the west of the main ultramafic unit (Figure 15), suggest that the marginal rocks extend as a continuous unit from the northern limit of the mass to the altered noritic rock (1684*) in the drill hole at [NJ 9250 1683] near Braehead. To the south of this drill hole the magnetic evidence suggests that ultramafic rocks and metasediments are in direct contact at the western margin of the mass, but the occurrence of a pyroxenite (1660*) in the drill hole near Goudieburn at [NJ 9402 1506] may indicate that the marginal facies is locally present to the south of the Belhelvie Fault.

An altered orthopyroxenite (1610*) has also been found in a drill hole at the north-eastern margin of the mass [NJ 9171 2121] and it is possible that a marginal facies may also be locally preserved at the eastern boundary of the intrusion (see comments in Chapter 8 on the evidence of deformation within and around the mass).

Ultramafic rocks

Much of the southern part of the mass consists of these rocks, while in the north ultramafic rocks occur, not only in the major units to the west and east of the country rock septum, but also as subsidiary bodies within the areas largely occupied by troctolitic, gabbroic and noritic rocks. The ultramafic rocks are dark coloured and often appear massive and structureless in hand specimen, although varieties characterised by the presence of large (10 to 20 mm), poikilitic pyroxene and plagioclase crystals are also to be found, particularly in the north-west (e.g. at [NJ 905 213]). Under the microscope it can be seen that the olivine crystals, which form 70 to 80 per cent of most of these rocks, have generally been almost completely replaced by serpentine, and that fresh olivine survives only occasionally in relict kernels (Plate 19)f. However, the former boundaries of the olivine crystals can still be recognised in many specimens, despite the widespread alteration, and it is clear that these crystals were originally xenomorphic or subidiomorphic and ranged up to 4 mm in diameter. In many rocks the original crystal boundaries are defined by thin (0.1 to 0.2 mm) overgrowths of orthopyroxene formed during crystallisation of the magma (e.g. 321 [NJ 933 190]). Larger (10 to 20 mm) crystals of orthopyroxene (e.g. 200 [NJ 929 178]) sometimes enclose the olivine crystals poikilitically, but poikilitic pyroxene is more commonly augite. Interstitial plagioclase can be identified in the thin sections of some of the ultramafic rocks, but is generally replaced by sericite or by turbid, fine-grained alteration products (see Stewart, 1947, p.470) and, in some instances (e.g. 1607* [NJ 9050 2165]) by prehnite. The textural relations show that the rocks are olivine cumulates.

Brown, picotitic spinel also occurs as small (0.2 to 0.4 mm) idiomorphic to subidiomorphic crystals which are sometimes (e.g. 144 [NJ 928 200]) partially mantled by a thin rim of pyroxene, but are generally poikilitically enclosed in pyroxene and feldspar crystals (Plate 19)f. Small (0.2 to 0.5 mm) idiomorphic to subidiomorphic crystals of opaque phases poikilitically enclosed in pyroxene and feldspar are also probable products of primary magmatic crystallisation. However, the highly magnetic character of most of the ultramafic rocks can be largely ascribed, not to these crystals, but to grains of secondary magnetite formed when olivine was serpentinised.

Plagioclase is sometimes relatively abundant in ultramafic rocks in the vicinity of the boundaries between ultramafic and mafic units. At these localities e.g. [NJ 933 190] the ultramafic (picritic) rock, may show variations in the proportions of olivine and plagioclase which define 'streaky' banding and an imperfect planar orientation of the olivine crystals.

Troctolitic rocks

The main occurrence of rocks consisting largely of plagioclase and olivine is in the zones that lie on either side of the country rock septum, but evidence from widely scattered outcrops elsewhere suggests that troctolitic rocks are present in other parts of the mass, including areas south of the Belhelvie Fault (e.g. near [NJ 950 155]). Large (4 to 5 mm), irregular grains of olivine and smaller (1 to 2 mm) laths of plagioclase form the greater part (over 90 per cent) of most of these rocks, with the proportion of feldspar generally exceeding that of olivine. Orthopyroxene, which is commonly found in association with olivine crystals as thin mantles that sometimes (e.g. 265 [NJ 904 204]) extend interstitially between the adjoining plagioclase grains, and augite, which invariably forms large (10 mm) poikilitic grains, are the only other phases to occur in appreciable amounts in many troctolites. The ortho- and clinopyroxene crystals are often patchily overgrown by brown amphibole and biotite, and small (0.1 to 0.2 mm) subidiomorphic grains of opaque constituents may also be mantled by these phases or by orthopyroxene. Grains of subidiomorphic brown spinel occur in some specimens.

The layering or 'streaky' banding displayed in many troctolite outcrops is mainly due to variations in proportions of olivine crystals giving rise to picritic or peridotitic bands or lenses, while the increases in plagioclase content, which occur rather less frequently, result in the development of anorthositic bands. These variations may be abrupt, thereby producing bands with sharply defined margins, but more commonly are gradational, even at the boundaries between the major ultramafic and troctolitic units. The plagioclase tends to become an interstitial phase (e.g. 1657* [NJ 9265 1745]) when the proportion of olivine increases, while in feldspathic rocks the olivine frequently occurs in clusters of smaller (1 to 2 mm) grains which are extensively penetrated by idiomorphic laths of plagioclase (e.g. 278 [NJ 933 196]). The conspicuous lamination displayed by many troctolites is mainly due to the planar alignment of the plagioclase laths, but, in some rocks, is also due in part to the preferred orientation of the olivine crystals.

These structural and textural features enable the troctolitic rocks to be identified as plagioclase-olivine cumulates.

There are minor differences between the western and eastern troctolitic units in the northern part of the mass, as the eastern troctolites are often less well laminated and more feldspathic, and contain smaller olivine crystals and a greater abundance of interstitial clinopyroxene. Other variations within both of these units are due to increases in the proportions of the pyroxenes which lead to the sporadic occurrence of rocks that are essentially noritic or gabbroic in character (e.g. 110 [NJ 911 219]; 160 [NJ 916 201]; 177 [NJ 930 196]). Plagioclase-rich rock also occurs within the troctolites, either as small, sharply defined, angular blocks, or as lenses of dyke-like bodies that sometimes cross-cut the structure in the surrounding rock (e.g. at [NJ 933 189]).

Widespread post-magmatic alteration has affected many of the troctolites and has led to the complete or partial replacement of olivine by serpentine or talc (e.g. 309 [NJ 931 197]), of plagioclase by sericite and fine-grained alteration products and to the development of conspicuous coronas (Stewart, 1947, p. 472) at the boundaries between olivine and plagioclase crystals. These coronas range up to 0.5 mm in width and often consist of an inner zone of colourless amphibole and an outer zone of fine symplectitic intergrowths of similar amphibole with pale-coloured spinel, the amphibole in both zones often being in optical continuity (Plate 19)h. Some coronas contain an additional, innermost zone of orthopyroxene which separates olivine from the zone of amphibole, but coronas are never developed at boundaries between olivine crystals and large separate crystals of orthopyroxene or of augite.

Spinel crystals with dark brown cores and pale green margins are also found in a number of troctolite specimens (e.g. 229 [NJ 936 181]). As the pale, hercynitic margin of these crystals is similar to the spinel in the olivine coronas, it is probable that this colour zonation reflects post-magmatic reaction and alteration, rather than compositional variations during crystallisation.

Gabbroic and noritic rocks

Rocks with abundant pyroxene are largely confined to the major zones that lie east and west of the country rock septum, but such rocks also occur at a number of localities south of the Belhelvie Fault. In many specimens both orthopyroxene and augite are important constituents, but rocks with only a single pyroxene phase are also common, and a considerable proportion (about one third) also contain appreciable amounts (> 5 per cent) of olivine. Plagioclase is usually the most abundant mineral in all of these rocks, and, in certain instances (e.g. 137 [NJ 918 204]) is present in sufficient quantity to give the rock an anorthositic character.

Gradations into feldspar-poor rocks sometimes occur, and, as noted above, at some localities olivine-rich peridotitic lenses can be recognised within the areas of gabbroic or noritic rocks (Figure 16).

No clear pattern of lithological variation can be recognised in the gabbros and norites, though olivine-bearing varieties appear to be somewhat more abundant in the vicinity of the major troctolitic units. The gabbroic and noritic rocks immediately west of the country rock septum are often coarse grained (with crystals up to 60 mm in length) and pegmatitic (e.g. 171 [NJ 927 194]).

Thin sections of the gabbroic and noritic rocks show that plagioclase usually occurs as sub-idiomorphic laths which sornetimes display a planar orientation (e.g. 153 [NJ 920 197] (Plate 19)g. Subidiomorphic laths of orthopyroxene of comparable size to the plagioclase crystals (1 to 3 mm) are present in noritic rocks (e.g. 149 [NJ 921 203]), but in other rocks the orthopyroxene is generally interstitial to plagioclase. Crystals of augite rarely show any approach to idiomorphism and often have a morphology largely determined by the boundaries of sub-idiomorphic laths of plagioclase or orthopyroxene. Nevertheless, the augite crystals in gabbroic rocks are generally intergrown with the other main phases to some extent and do not display truly interstitial relationships (e.g. 267 [NJ 916 204]; 151 [NJ 922 198], (Plate 19)h. Crystals of both clino- and orthopyroxene may display a limited degree of preferred orientation, and sometimes assist in defining the foliation in a rock. Olivine generally forms irregular crystals up to 5 to 7 mm in size, which are penetrated by subidioblastic plagioclase laths, but is sometimes found in small grains which form elongated clusters (e.g. 151 [NJ 922 198]).

The pyroxenes are often extensively mantled by red-brown amphibole and biotite, and in pegmatitic rocks are sometimes replaced almost completely (e.g. 275 [NJ 927 195]) by these minerals. Opaque grains are present in minor amounts in many gabbros and norites, and frequently consist predominantly of ilmenite (e.g. 216 [NJ 938 174]) particularly in specimens from immediately west of the country rock septum. Subidiomorphic apatite crystals are also sometimes abundant west of the septum in coarse pegmatitic rocks.

In general, the textural relations imply that most of the gabbroic and noritic rocks can be identified as plagioclase-augite-orthopyroxene cumulates, but in some specimens only a single cumulus pyroxene phase is present, while in others cumulus olivine can be recognised.

Many of the gabbroic and noritic rocks have been affected by post-magmatic alteration in a similar fashion to the troctolitic and ultramafic rocks. Plagioclase crystals are often sericitised and olivine grains are serpentinised and are surrounded by coronas, while crystals of pyroxene are replaced by clusters of amphibole laths, which sometimes form pseudomorphs of the original crystal, but in other instances extend beyond the former crystal boundaries into the adjoining feldspar grains (Stewart, 1947, pp. 477–478). This replacive amphibole is often pale green in colour, with a darker green border against feldspar, but orthopyroxene is frequently replaced by aggregates of cummingtonitic amphibole which shows polysynthetic twinning and sometimes contains granules of green spinel (e.g. 153 [NJ 920 197]).

Quartz is present as an interstitial phase in gabbros and norites at three localities immediately west of the country rock septum (171 [NJ 927 194]; 186 [NJ 929 189]; 190 [NJ 928 186]). However, as all three rocks show evidence of deformation, and as it is known that silicification has accompanied deformation at other localities in the Belhelvie mass (see Chapter 8), it is possible that the quartz in these rocks is not a primary magmatic phase.

Detailed mineralogy

Published determinations of the composition of minerals in the Belhelvie mass (Rothstein, 1962; Wadsworth and others, 1966) obtained by optical, X-ray and wet chemical methods have been greatly augmented by Boyd (1972) using microprobe and optical techniques. These determinations show that, in the northern part of the mass, the composition of cumulus olivine and orthopyroxene crystals varies systematically from the ultramafic units (Fo85–89, occasional En83) through the troctolitic units (Fo85–87, occasional Eng]) to the gabbroic and noritic units (Fo76–85, occasional En76–83), while the composition of crystals of cumulus plagioclase varies sympathetically from An76–87 in the troctolitic units to An74–79 in the gabbroic and noritic units (Figure 18). The minerals in the two series of cumulate units on either side of the country rock septum display essentially the same pattern of compositional variation (Figure 18), although some specimens of pegmatitic gabbroic and noritic rocks from immediately west of the septum differ in that they contain relatively albitic feldspar (An55).

Cumulus augite crystals also show appreciable compositional variation (e.g. from En52Of9Wo39 to En47Of8Wo45). However the variations in the proportions of Mg, Fe and Ca in these crystals do not appear to be simply related, nor is there a readily identifiable relationship between the variations in the composition of these crystals and the variations in the nature and composition of the associated cumulus phases.

The cumulus phases in the Belhelvie mass generally show no obvious zoning, even when there are considerable inter-cumulus extensions to the original grains (Wadsworth and others, 1966, p. 56). Consequently most of these rocks can be classed as adcumulates (Wager and others, 1960).

The Al2O3 content of both the orthopyroxene (1.88 to 2.29 per cent) and the augite (2.66 to 4.24 per cent) crystals is relatively high and suggests that these pyroxenes have crystallised at considerable pressures (Goode and Moore, 1975). The crystals of orthopyroxene and augite generally contain exsolved pyroxene lamellae (of augite in the orthopyroxene and vice versa) orientated parallel to {100} of the host crystal, and both pyroxenes occasionally contain fine iron-oxide lamellae orientated parallel to {100} of the host.

Mineralogical and structural relations in the intrusion

It is widely accepted (e.g. Wager and Brown, 1968) that the mineralogical, textural and structural features displayed by large, layered mafic and ultramafic intrusions such as the Belhelvie mass, indicate that crystal accumulation occurred when the magma was consolidating. In the detailed interpretation of structural relations in such intrusions, two assumptions are normally made: (i) that the lithological layering in much of the intrusion was originally horizontal or sub-horizontal, (ii) that the compositional variation from high temperature to low temperature members of solid solution series (cryptic variation) can be used to deduce an igneous 'stratigraphy', as the rocks containing the higher temperature minerals must have formed before the rocks with lower temperature minerals. The interpretation of relations in the Belhelvie mass in the light of the first of these assumptions leads to the conclusion that the northern part of the intrusion has been tilted through approximately 90° about a N–S axis. If the second assumption is valid, the cryptic variation recognised in this part of the mass suggests that a traverse from the ultramafic cumulates at the western margin through troctolitic cumulates to the gabbroic and noritic cumulates to the east, not only provides an indication of the changing phase relations in the magma as it crystallised (i.e. olivine crystallising alone, olivine crystallising with plagioclase and then with plagioclase and two pyroxenes), but also encounters rocks which formed at successively later stages and probably at higher levels in the magma reservoir. The few examples of internally graded layers in the northern part of the mass display relations in agreement with this interpretation as the ferromagnesian part of such layers (presumably the former base (Wager and Brown, 1968, p. 22)) is always in the west, the feldspathic part in the east.

The country rock septum may represent the original roof of this part of the reservoir, or, alternatively, since the cumulate sequence east of the septum repeats the pattern of phase and cryptic variation seen in the west, it may represent an originally subhorizontal, shelf-like irregularity in the wall of the magma chamber which acted as a local 'floor' for the crystallisation of the portion of the intrusion now lying to the east. However, as there is abundant evidence that many of the rocks in the septum and at the eastern margin of the mass have undergone strong, post-magmatic deformation (see Chapter 8), the original relations of the igneous rocks have probably been much disturbed. The cumulate sequences on either side of the septum may therefore consist of rocks which were formerly at comparable 'stratigraphic' levels in the intrusion.

The relationships of the rocks in the northern part of the mass to the portion of the intrusion south-east of the Belhelvie Fault cannot be assessed satisfactorily because of the paucity of exposures and the limited amount of mineralogical and petrological data from the coastal areas. The magnetic data (Figure 15) suggest that much of this part of the mass, including the portion of the intrusion that lies offshore, consists of ultramafic rocks. The cryptic variation in the northern part of the mass suggests that such rocks are probably concentrated near the former base of the intrusion, as is commonly the case in large, layered intrusions elsewhere (Wager and Brown, 1968), and most of the southeastern part of the intrusion may therefore consist of rocks from such a basal unit. However, the detailed interpretation of the magnetic data (Figure 17) has shown that there are structural complexities in the southern part of the mass, and the moderate northerly dip of the cumulate units immediately south-east of the Belhelvie Fault suggests that these rocks have been tilted in a different fashion from the cumulate units in the northern part of the mass. The Belhelvie Fault therefore appears to be a discontinuity separating two parts of the mass displaying structural as well as lithological differences.

The narrow zone of noritic rocks at the western margin of the intrusion north of the Belhelvie Fault seems likely to be a contact facies. However, as these rocks show considerable variation and sometimes display cumulate textures, it is unlikely that this zone constitutes a chilled marginal facies with the same bulk composition as the original magma.

Chemistry of the igneous rocks

Analyses of 10 igneous rocks from the Belhelvie mass have been published by Stewart (1947) and an additional 40 rocks from this mass have been analysed by Boyd (1972). The variation displayed by these 50 analyses can be largely related to obvious variations in mineralogy. Thus, a plot of weight per cent MgO against weight per cent SiO2 (Figure 19)a shows that the ultramafic rocks cluster near the composition of forsteritic olivine (Fo85) and, indeed, the correspondence would be closer if allowance were made for the high content of H2O (5 to 9 per cent) in most of these rocks, while the composition of most of the troctolitic rocks falls between the composition of this olivine and that of a bytownitic plagioclase (An81). The gabbros and norites are less closely grouped, but the composition of most of these rocks can be represented in terms of plagioclase-pyroxene mixtures. Comparable relationships can be observed in other plots of the weight percentages of major constituents in the rocks (e.g. CaO, Al2O3; (Figure 19)b, c) against weight percentages of MgO.

The widespread occurrence of orthopyroxene in the rocks of the Belhelvie mass and the presence of quartz in the norms of many of the noritic and gabbroic rocks show that the magma from which these rocks crystallised was tholeiitic.

Chemical analyses of two noritic rocks from the western marginal zone display appreciable differences in cation per cent Mg/Mg + Fe + Mn (81.2 to 85.4) and in weight per cent MgO (18.3 to 21.8), CaO (10.65 to 7.6) and SiO2 (49.7 to 52.8) and emphasise the variability of the rocks in this zone.

Many of the rocks, particularly in the eastern part of the mass are highly altered ('uralitised') and deformed, and the chemical analyses suggests that this alteration has been accompanied not only by hydration, but also by addition of K2O ((Figure 19)d) and possibly of Na2O.

The chemistry of the rocks in several of the other 'Younger Basic' intrusions in Aberdeenshire and Banffshire has also been investigated-notably in Insch (Read and others, 1961, 1965; Read and Hach 1963; Busrewil and others, 1975) and Huntly (Weedon, 1970). Analyses of 27 peridotitic, troctolitic and gabbroic cumulates from Insch (Read and others, 1961, 1965) have been plotted with the 50 Belhelvie analyses in (Figure 20). This diagram shows that while there is considerable compositional overlap between the cumulates in the two masses, the gabbroic and noritic cumulates from Belhelvie mostly plot in the compositional gap between the peridotitic/troctolitic cumulates and the gabbroic cumulates at Insch. Although these relationships support the hypothesis that all the 'Younger Basic' intrusions originally formed part of a single stratiform mass, differences in the composition of co-existing cumulus phases and in the sequence of cumulates in different masses (Table 4) suggest that each individual 'Younger Basic' mass had a different crystallisation history, and these intrusions seem more likely to have formed from separate bodies of magma.

Other 'younger basic' rocks in the sheet 77 area

The Insch mass

No rocks of this mass are exposed within the Sheet 77 area but magnetic surveys and shallow drilling have established (Ashcroft and Munro, 1978) that a small portion (c. 0.25 km2) of this intrusion extends into the map area (Figure 21).

The drill cores consist mainly of olivine-hypersthene-gabbros (plagioclase-olivine-orthopyroxene cumulates; e.g. 2443* [NJ 8212 2625]) resembling the rock exposed a little to the west of the map boundary in the old mill race at Cromlet Mill [NJ 812 268]. Peridotitic olivine cumulates are also exposed immediately west of the map boundary at [NJ 816 258] near Woodside and the magnetic survey indicates that such rocks extend into the Sheet 77 area at [NJ 821 258] near Greenford. The boundary between these rocks and the olivine-hypersthene-gabbros trends NW–SE and is probably faulted (Ashcroft and Munro, 1978, p. 74).

Bodies of 'Younger Basic' rock near Udny and Pitmedden

Gabbroic rocks are exposed in small outcrops immediately north and south of Ivy Cottage [NJ 8675 2625], approximately 1 km west of Udny (Figure 21). Hornfelsed metasediments occur in association with the igneous rocks in the southern exposure and were also temporarily exposed in a trench for a pipeline north-west of Ivy Cottage. Although the rocks in the southern exposure at Ivy Cottage (2381) are cut by narrow shear zones, thin-sections indicate that the igneous rocks from both outcrops (2381; 2353) consist largely of uralitised, but undeformed gabbro, containing augite in ophitic intergrowth with labradorite. Further to the north, similar uralitised mafic rocks (2380) occur as vein-like bodies in hornfelsed amphibolites in an outcrop at [NJ 875 278] near Cairdseat. A short distance to the north-east of this locality, small, possibly vein-like, bodies of ophitic, gabbroic rocks are found in close association with amphibolites and hornfelsed metasediments in exposures near an old lime kiln, immediately north of the map boundary at [NJ 881 281]. Between these two northern localities essentially undeformed mafic igneous rocks and hornfelses were extensively exposed in a trench c. [NJ 874 274] and it is clear that a body of 'Younger Basic' rock with dimensions of approximately 1 km occurs in this area. Specimens from the trench include picritic (2122*) and pyroxene-rich rocks (2120*) with idiomorphic or subidiomorphic olivine, augite and orthopyroxene crystals, and poikilitic plagioclase crystals up to 20 mm in dimension, which appear to be olivine-augite-orthopyroxene cumulates, but also include ophitic-gabbros (2123*) and fine-grained quartz-biotite gabbros.

To the north of this body of igneous rocks, within the Sheet 87 area, hornfelses are exposed over a considerable area south and east of Tolquhon Castle, and further outcrops of 'Younger Basic' rocks occur at [NJ 883 291] and [NJ 885 288] near East Newseat of Tolquhon. The igneous rocks at this last locality have never been described although they are depicted on Sheet 9 of the 1:253 440 geological map of Scotland. East of Udny and Pitmedden, in the area of complexity near the boundary between the Aberdeen and Ellon formations, a drill hole at [NJ 9291 2767] near Hillhead of Mosstown encountered a quartz-biotite norite (1875*; (Plate 20)a, while uralitised, but undeformed, mafic igneous rock (1817*) was obtained in another drill hole at [NJ 9382 2532] near Auchindarg. No other specimens of undoubted 'Younger Basic' rocks were obtained from this area, but it is possible that some of the amphibolitic rocks that occur nearby and occasionally show relict igneous textural features are sheared, silicified, 'Younger Basic' rocks (e.g. 1876* [NJ 9343 2750]; 1879* [NJ 9373 2645]). However, other possible source rocks for these deformed specimens occur in the area, including the dioritic rocks that sometimes occur in intimate association with granitic rocks (e.g. on Hill of Minnes [NJ 947 237] and the migmatised 'regional' amphibolites that are interstratified with the metasediments (e.g. 1336 [NJ 930 289]), immediately north of the boundary of the map), and these deformed rocks cannot be identified with certainty as being members of the 'Younger Basic' suite.

Drill holes at [NJ 941 254] south-east of Auchindarg show that a prominent magnetic anomaly in this area (Figure 4a) is due to the presence of 'xenolithic gneisses' (1823*, 4*, 5*) (Chapter 7), while another anomaly to the north at [NJ 933 277]

Chapter 7 Thermal metamorphism

Poorly-fissile rocks showing evidence of static recrystallisation post-dating regional metamorphism (Plate 21) and displaying distinctive mineralogical and textural features in thin section (see below) are identified as being the products of thermal metamorphism. Such rocks occur at a number of localities in the vicinity of the Belhelvie 'Younger Basic' intrusion (Figure 21), and are also extensively developed in two areas where small, isolated bodies of 'Younger Basic' rock occur-namely, in the zone of complexity adjoining the N–S-trending portion of the boundary between the Ellon and Aberdeen formations, and in the Cairdseat-Cairnfechel area approximately 1 km west of Udny and Pitmedden (Figure 21). In these latter areas the extent of the thermal metamorphism seems disproportionately large in view of the small size of the bodies of 'Younger Basic' rock, but the close spatial relationship between the hornfelses and the 'Younger Basic' rocks is marked. Limited evidence of thermal metamorphism is found near the Crathes granitic mass (Chapter 9), but, in general, the 'Newer Granite' masses in the map area do not have identifiable thermal aureoles, and it is concluded, therefore, that all the extensive areas of hornfelses within the Sheet 77 area were formed when the 'Younger Basic' masses were emplaced. This interpretation is also supported by the relationships in the Sheet 87 area, where the hornfelses adjoining the boundary between the Ellon and Aberdeen formations can be traced without a break towards the Arnage-Haddo 'Younger Basic' masses, and where it can be seen that the hornfelses within the Sheet 77 area near Udny and Pitmedden represent the southern portion of the thermal aureole adjoining bodies of basic rock with dimensions of 1 km or more north-west of Pitmedden (Figure 21). Within the Sheet 77 area the hornfelses appear to be derived largely from the Aberdeen Formation, but some specimens from the boundary zone between the Aberdeen and Ellon formations have characteristics that suggest the parent rocks were members of the Ellon Formation.

Distribution of hornfelsed rocks

In the Belhelvie area the septum of country rocks in the northern part of the 'Younger Basic' mass consists entirely of hornfelsed metasediments and amphibolites (Plate 21), and similar rocks are extensively exposed at the north-eastern margin of the mass near Sparcraigs and Shiels (Figure 21) and a kilometre or more to the north near Catcraig Wood [NJ 921 203]. Evidence of thermal metamorphism was also found at the eastern margin of the mass in drill holes at [NJ 9380 1958] near Shiels, East Ardo Lodge [NJ 9332 2065] and [NJ 9183 2122] near East Cannahars, at the north-western limit in three drill holes near Skelly Bridge at [NJ 9076 2234], [NJ 9041 2210] and [NJ 9033 2182], and at the western margin in a drill hole on Red Moss [NJ 9131 1786] (Figure 21). However, other drill holes in the vicinity of the intrusion penetrated metasediments devoid of any obvious evidence of thermal metamorphism at [NJ 9520 1843], [NJ 9432 1905], [NJ 9117 1784] and [NJ 9317 1583], and similar, apparently unhornfelsed, rocks were temporarily exposed in a trench north of Belhelvie village at c. [NJ 948 185]. At some localities (e.g. near Shiels at [NJ 938 196]; on Red Moss at [NJ 912 178]) drill holes show that hornfelsed and unhornfelsed rocks occur in close proximity. Although the evidence is limited and fragmentary, it is clear that the Belhelvie intrusion is not surrounded by a uniform and continuous envelope of hornfelsed rocks.

A zone of thermally metamorphosed rocks can be traced northwards from the exposures of hornfelses at the northeastern margin of the Belhelvie mass to the northern margin of the map area (and beyond into the Sheet 87 area). This zone is relatively narrow (c. 1 km wide) in the south, but becomes wider (c. 3 km) north of Grid line 23 (Figure 21). Virtually all of the rocks of the Aberdeen and Ellon formations in this zone show evidence of hornfelsing, although difficulties in interpretation arise because readjustment to thermal metamorphism appears to have been incomplete in some specimens, and in metasedimentary specimens from the Ellon Formation and in amphibolites has not been accompanied by the development of distinctive 'thermal' mineral assemblages.

In the south, temporary exposures in a trench enable the hornfelses to be traced continuously for more than a kilometre north of the Belhelvie mass (to near [NJ 936 217]). Poorly fissile, partially hornfelsed metasediments are exposed further north near Tillyfour [NJ 929 231] and were also encountered nearby in shallow pits at Darrahill [NJ 935 221] and Craibadona [NJ 934 230]. Hornfelsed rocks occur still further north in outcrops at [NJ 935 243] near Hill of Fiddes and in the farmyard at Fiddesbeg [NJ 943 244] and were also found in a trench at Hill of Minnes [NJ 949 234] and in two nearby drill holes ([NJ 9344 2413], [NJ 9434 2436]). Towards the northern margin of the map area, hornfelses are visible in outcrops at Tillyfar [NJ 912 270], in the railway cutting at Mains of Mosstown [NJ 926 275], and, until recently, at [NJ 927 271], and were also encountered in a drill hole at [NJ 940 263] and in a trench extending northwards from [NJ 911 259] near Mains of Orchardtown to the northern margin of the map (and beyond into the Sheet 87 area).

The temporary exposures in the trench at Mains of Orchardtown showed that the transition from regionally metamorphosed rocks to rocks with hornfelsic characteristics occurs within a comparatively short distance (c. 50 to 100 m) at this locality. A similar rapid transition into hornfelsed rocks was also observed in a trench at [NJ 906 300] near Mill of Drumbeck within the Sheet 87 area, but the boundaries of the zone of hornfelses are exposed at no other localities within the Sheet 77 area. However, some indication of the limits of this zone is provided in the west by the occurrence of rocks devoid of any evidence of thermal metamorphism in drill holes at [NJ 9211 2129] and [NJ 9271 2130] and in outcrops at [NJ 926 227], [NJ 918 235], [NJ 915 236], [NJ 912 253] and [NJ 905 261]. To the north of Grid line 23 the eastern boundary of the hornfelses appears to coincide with the western limit of the Ellon Formation (see (Figure 4c)) and south of this Grid line this boundary must lie to the west of the unhornfelsed metasediments found in a trench near [NJ 945 229] and in outcrop at Dubbystyle [NJ 948 212].

A wide area containing regionally metamorphosed rocks typical of the Northern unit of the Aberdeen Formation separates this zone of hornfelses from the occurrences of hornfelsed rocks to the west of Udny and Pitmedden. These hornfelses are exposed at only one locality at [NJ 875 278] near Cairdseat within the map area, but outcrop more extensively immediately to the north within the Sheet 87 area (e.g. at [NJ 872 281], [NJ 878 287], [NJ 882 285]), while the rocks exposed almost continuously in a trench between [NJ 866 264] and [NJ 877 279] were found to consist largely of metasediments showing evidence of thermal metamorphism.

The country rocks in the vicinity of the small portion of the Insch mass that lies within the Sheet 77 area are virtually unexposed. However, metasediments devoid of any evidence of thermal metamorphism outcrop at one locality [NJ 824 261] within 200 to 300m of the eastern limit of the intrusion and, further west, within the Sheet 76 area, drill holes have shown that the rocks in the vicinity of the intrusion frequently display little or no evidence of thermal metamorphism (Ashcroft and Munro, 1978).

Petrography

Metasediments

The dominant minerals in hornfelsed metasediments are biotite and plagioclase, with cordierite, sillimanite, garnet and potash feldspar being common associates in the more pelitic rocks. Most of the hornfelses contain quartz, and although this mineral is confined to post-hornfelsing veins in a number of speciments, silica-deficient hornfelses appear to be uncommon. Many of the rocks are finely-banded, making the recognition of stable assemblages of co-existing minerals a matter of difficulty in some instances.

Pelites and semi-pelites.

Typical assemblages in these rocks include biotite-plagioclase t cordierite ± garnet ± sillmanite ± potash feldspar ± quartz. Quartz and feldspar are more widespread and abundant in semi-pelites than in pelites. Many hornfelsed pelites and semi-pelites contain andalusite, but, as is described below, the textural relations of this mineral are ambiguous, and often suggest that it is a relict from a regional metamorphic assemblage.

Silica-deficient assemblages are generally characterised by the presence of spinel and include:

biotite-garnet-cordierite-spinel; biotite-cordierite-plagioclase-sillimanite-spinel; cordierite-garnet-potash feldspar-spinel; biotite-orthopyroxene-plagioclase-spinel.

Some narrow bands consist almost entirely of orthopyroxene and spinel (Plate 20)g or cordierite and spinel.

These silica-deficient assemblages appear to be relatively abundant in the Belhelvie aureole where it has been found (Stewart, 1947, p.494) that corundum or anthophyllite sometimes occur as additional phases in the pelitic hornfelses.

Psammites

The majority of these rocks do not contain mineral assemblages that can be reliably ascribed to thermal metamorphism. Common assemblages include:

biotite-garnet-plagioclase-quartz; biotite-garnet-plagioclase-potash feldspar-quartz-sillimanite.

Calcareous rocks

In the field, calc-silicate rocks are more conspicuous in areas of hornfels than in areas of regionally metamorphosed rocks, and in the Belhelvie aureole such rocks are possibly more abundant than elsewhere. They contain a great diversity of mineral assemblages, including:

zoisite/clinozoisite-diopside-wollastonite-quartz; epidote-diopside-wollastonite-plagioclase-biotite; diopside-wollastonite-plagioclase; diopside-amphibole-potash feldspar.

Accessory minerals

The common accessory minerals in the hornfelses are zircon and apatite in pelites, semi-pelites and psammites and sphene in talc-silicates, as in the equivalent regionally metamorphosed rocks, and may be phases which have been largely inherited from the latter. Opaque constituents may also have been constituents of pre-hornfelsing assemblages, but seem to be largely the products of thermal metamorphism in certain pelites in which they are particularly abundant (c. 10 to 15 per cent). In many instances the main opaque phase must be magnetite as such rocks produce considerable magnetic anomalies (e.g. near Sparcraigs [NJ 934 195], (Figure 15)). Green or yellow tourmaline occurs sporadically, being relatively abundant at some localities (e.g. Catcraig Wood [NJ 921 203]; at [NJ 931 265] near Cloisterseat) and lacking elsewhere.

Textural and mineralogical details

Some of the specimens of hornfelsed metasediment appear to have recrystallised completely during thermal metamorphism and now display decussate textures, with randomly orientated crystals of phases such as sillimanite (e.g. 81 [NJ 943 244] (Plate 10)b; 338 [NJ 921 203]) or biotite (e.g. 2378 [NJ 943 244]) dominating the fabric of the rock. However, in the majority of the hornfels specimens textural relations are complex because recrystallisation has only partially destroyed the earlier regional metamorphic fabric, and crystals of new, 'thermal' phases have developed in a highly irregular and sporadic fashion. The original preferred orientation of the biotite crystals has often been retained to some extent, and the rocks still contain the biotitic melanosomes and quartzofeldspathic leucosomes (Plate 20)c typical of many of the regionally metamorphosed pelites and semi-pelites. Aggregates of fibrolite, which are intimately associated with biotite in many of these rocks, are also probably relicts of regional metamorphism, being overgrown by, or replaced by crystals of prismatic 'thermal' sillimanite in some instances (e.g. 2407 [NJ 935 221], (Plate 20)c). Crystals of corundum in micaceous specimens (1867* [NJ 9248 2406]) showing only limited evidence of hornfelsing are also regarded as having formed during regional metamorphism.

Some hornfelsed pelites and semi-pelites contain crystals of andalusite that are generally extensively mantled by, and apparently partially replaced by biotite, and are occasionally partially replaced by sillimanite (e.g. 301 [NJ 929 189]; 2407* [NJ 935 221], (Plate 20)d) or fibrolite (e.g. 341 [NJ 937 195]) and may also be relicts of regional metamorphism. These crystals range up to 2 to 3 mm in dimensions and are generally subsidiary (< 5 per cent) constituents although, in some specimens (e.g. 335 [NJ 921 203]), the andalusite content approaches 25 per cent. The andalusite-bearing pelites occur widely in the aureole of the Belhelvie mass, and in the zone of hornfelses extending northwards from that intrusion, but there is no obvious relationship between the distribution of these rocks and that of the igneous rocks, and andalusite is found in specimens from the contact areas of the Belhelvie mass (e.g. 341 [NJ 937 195]) as well as in areas further removed from the intrusion. These relationships suggest that the andalusite in these hornfelses may have been derived from rocks resembling the andalusite-bearing metasediments in the Brig O' Balgownie area (e.g. 2415 [NJ 9415 0965]), or, in the case of some of the specimens from the zone of hornfelses north of the Belhelvie mass, from rocks of the Ellon Formation. This mineral has not been found in the area of hornfelses west of Udny and Pitmedden, and it may be significant that the regionally metamorphosed rocks surrounding these hornfelses are also devoid of andalusite.

Certain hornfelses are distinctive because they contain minerals that are lacking in any of the equivalent regionally metamorphosed rocks. Such minerals include spinel (e.g. 247 [NJ 934 197] (Plate 20)e; 2114* [NJ 867 266]; 2375 [NJ 926 276]) and orthopyroxene (e.g. 252 [NJ 929 194]) in pelites, and wollastonite (e.g. 212 [NJ 938 174]) in calc-silicates. In other hornfelses which can be identified with reasonable certainty as being thermally metamorphosed rocks of the Aberdeen Formation (e.g. 2107* [NJ 871 271]) the presence of cordierite is a distinctive 'thermal' feature as this mineral is unknown as a 'regional' phase. Although corundum is not confined to thermally metamorphosed rocks, its occurrence in mica-poor, silica-deficient hornfelses (Stewart, 1947) contrasts with its association with abundant mica in relatively siliceous regionally metamorphosed rocks (e.g. 2338 [NJ 852 116], (Table 1)).

In other thermally metamorphosed rocks, the mineralogy generally resembles that of comparable regionally metamorphosed specimens, but certain phases in the two types of rock display compositional differences (see below), and often show marked differences in size and relative abundance. Thus cordierite can be identified as the product of thermal metamorphism, even when there is a possibility that the parent rock is a member of the Ellon Formation that contained 'regional' cordierite (e.g. 1827* [NJ 944 244]; 2376 [NJ 926 276]), as it generally occurs in larger, crystals (up to 4 mm) and in greater abundance (up to 60 to 80 per cent) in hornfelses, than in any of the regionally metamorphosed rocks (Plate 20)f. Garnet is also more abundant (up to 20 per cent, e.g. 339 [NJ 921 203]) in the hornfelsed metasediments than in the regionally metamorphosed rocks and occurs in xenoblastic crystals that range up to 10 mm in size (e.g. 301 [NJ 929 189]) and are generally larger (c. 1 to 2 mm) than the garnets in the regionally metamorphosed rocks (c. 0.5 to 1 mm) (Plate 20)e. The crystals of sillimanite in the hornfelses occur as idioblastic prisms that range up to 7 mm in length (Plate 20)b, which are invariably larger than, and sometimes replace (Plate 20)c, the aggregates of fibrolite in the regionally metamorphosed rocks. Unlike the fibrolite, the sillimanite is sometimes an important (10 to 15 per cent) rock-forming constituent (e.g. 81 [NJ 943 244]).

Cordierite, garnet and sillimanite are often concentrated in the melanosomes in intimate association with biotite, and, in general, as the proportions of cordierite and garnet increase, the proportion of biotite decreases. Extreme examples of this relationship are provided by silica-deficient rocks in the Belhelvie aureole in which there is no mica and the main ferromagnesian minerals are cordierite, garnet, spinel and opaque constituents (e.g. 301 [NJ 929 189]).

Ferromagnesian and aluminous minerals are closely associated throughout the hornfelses and crystals of cordierite (e.g. 325* [NJ 932 190]), garnet (e.g. 302 [NJ 929 189]) and sillimanite (e.g. 2378 [NJ 943 244]) are sometimes crowded with small (c. 0.5 mm) opaque grains or with subidioblastic or irregular crystals of green spinel.

Plagioclase is a constituent of most hornfelses, generally as round or subidioblastic crystals ranging up to 3 mm in diameter. Potash feldspar is also found in many of the thermally metamorphosed metasediments, but in most instances is confined to quartz-rich, vein-like leucosomes which show no evidence of thermal metamorphism and are probably associated with the widely distributed bodies of migmatitic granite already described (see Chapter 2). In some specimens, however, the crystals of potash feldspar are integral components of the intergrown hornfelsic fabric of the rock, often being found in close association with biotite, and sometimes forming large (c. 2 to 3 mm) grains that enclose laths of sillimanite (e.g. 193 [NJ 930 182], (Plate 20)f; 1606* [NJ 908 223]).

In some hornfelses quartz is confined to the migmatitic veins which occur even within silica-deficient hornfelses (e.g. 302 [NJ 929 189]) and in other specimens the effects of post-hornfelsing recrystallisation make it difficult to determine whether quartz was formed during thermal metamorphism. Nevertheless, in many other hornfelsed rocks, including semi-pelites as well as psammites, quartz is intimately intergrown with the other minerals and appears to be an integral component of the 'thermal' assemblage. Posthornfelsing muscovite is frequently associated with recrystallised quartz, but also occurs replacing fibrolite (e.g. 1923* [NJ 949 235]) or biotite (e.g. 324 [NJ 932 189]) and in the migmatitic leucosomes.

The common constituents in calc-silicate hornfelses are colourless epidote, identified as zoisite in one specimen (197 [NJ 936 193]) and as clinozoisite in another (2108* [NJ 871 271]), diopsidic pyroxene, relatively anorthitic plagioclase (An55–65) and quartz. Pale green amphibole often occurs as a subsidiary constituent. Wollastonite is an important constituent in a number of specimens from the Belhelvie aureole (e.g. 212 [NJ 938 174]), in which it occurs in relatively large (1 to 2 mm) crystals set in a finer matrix of diopside, plagioclase, opaque ore and sphene. Scapolite is a matrix constituent in some of these wollastonite-bearing rocks (e.g. 323* [NJ 932 189]).

Data on mineral compositions

The biotite crystals in the hornfelses are generally red-brown, and in three specimens from the Belhelvie aureole (238 [NJ 921 203]; 247 [NJ 934 197]; Stewart (1942)) biotite and host-rock compositions have been determined (Boyd, 1972). The host rock's M/M + Fratios (100 Mg/Mg + Fetot as cations) vary from 33 (Stewart) to 53.8 (238) and the composition of the biotites shows some sympathetic variation. The limited evidence available suggests that, in rocks with the same M/M + F ratios, the biotites in hornfelsed specimens are richer in Mg than the biotites in regionally metamorphosed rocks of the Aberdeen Formation. This contrasts with the relations observed in the only well-described aureole in the Grampian region, that of the Lochnagar granite (Chinner, 1962), where the 'thermal' biotites are more ferriferous than the 'regional' biotites.

The compositions of the garnets in these three hornfels specimens, and in a fourth Belhelvie specimen (180 [NJ 934 197]) have also been determined (Boyd, 1972). These analyses again show (Figure 6) that there is relationship between whole rock and garnet M/M + F ratios. However, whilst the composition of the Belhelvie and Lochnagar garnets shows similarities (Figure 6), once again there is a contrast in relations as, unlike Lochnagar, the 'thermal' garnets at Belhelvie are generally more Mg-rich than the 'regional' garnets. The hornfels garnets at Belhelvie are also appreciably poorer in Mn and Ca than the garnets in the regionally metamorphosed rocks of the Aberdeen Formation (Figure 6).

The composition of the plagioclase crystals in the hornfelses generally lies in the range An25 to An35, but more albitic crystals (An10–25) are sometimes found in psammites (e.g. 1613* [NJ 9266 2065]), while silica deficient (e.g. 2375 [NJ 926 276]) and calcareous rocks (e.g. 213 [NJ 938 174]) often contain more anorthitic (An50–65) feldspars.

Amphibolites

Most of the amphibolites associated with the hornfelsed metasediments show close mineralogical and textural similarities to amphibolites from areas of the Aberdeen and Ellon formations where there is no evidence of thermal metamorphism. Some of the amphibolites in the hornfels area have a more even-grained, granoblastic texture than the 'regional' rocks, and may therefore, have been recrystallised (e.g. 2352 [NJ 926 262]). In the Cairdseat area (Figure 21), other specimens that contain spongy crystals of cummingtonitic amphibole (e.g. 2379 [NJ 875 278]) or orthopyroxene (e.g. 2124* [NJ 873 275]) possibly display evidence of mineralogical adjustments in response to thermal metamorphism. However, it is possible that these unusual amphibolites are modified derivatives of the 'Younger Basic' rocks which also outcrop in this area (Chapter 6).

General assessment of the hornfelsed rocks

The interpretation of relations in the hornfelsed rocks is rarely straightforward because readjustment to thermal metamorphism was incomplete and sporadic and relict 'regional' textural and mineralogical features have survived in many rocks. It has not been possible to subdivide the hornfelses into zones that can be related to proximity to igneous contacts, even in the relatively extensive areas of thermally metamorphosed rocks at the north-eastern contact of the Belhelvie mass (Figure 21), and the transition from hornfelsed rocks to rocks devoid of evidence of thermal metamorphism often appears to be abrupt. As is discussed in Chapter 8, these anomalous field relationships are probably due to disruption and shearing after hornfelsing had occurred.

In many of the hornfelsed pelites and semi-pelites it is clear that sillimanite, cordierite and garnet are the main products of thermal metamorphism, with potash feldspar, spinel and opaque ores often occurring as additional phases. The association of sillimanite with potash feldspar in many of the hornfelses is particularly marked, and in certain rocks sillimanite crystals are virtually isolated within areas of potash feldspar (Plate 20)f. As there is generally evidence that the muscovite in the hornfelses is replacive and post-metamorphic in origin, it is probable that thermal metamorphism has involved the reaction (Evans and Guidotti, 1966)—muscovite + quartz → potash feldspar + sillimanite + H2O. Biotite is the dominant ferromagnesian mineral in the regionally metamorphosed rocks of the area, particularly in the Northern unit of the Aberdeen Formation, and the formation of new ferromagnesian phases during thermal metamorphism has probably involved its breakdown. Although the reactions are likely to have been complex, and to have involved the crystallisation of new 'thermal' biotite (cf. Chinner and Heseltine, 1979), an important reaction may have been (Holdaway and Lee, 1977)-Al2SiO5 + biotite + quartz → cordierite + garnet (andalusite or sillimanite). Alternatively, possibly at slightly lower temperatures (Schreyer, 1976), biotite, muscovite and quartz may have reacted to yield cordierite and potash feldspar. The production of spinel possibly involved the breakdown of regional garnet (Chinner, 1962) or staurolite, particularly in rocks with a low potassium content (e.g. 247 [NJ 934 197]), but the occurrence of spinel in more potassic rocks (e.g. 193 [NJ 930 182]) probably reflects the breakdown of biotite in rocks that are relatively poor in silica (Rutherford, 1973).

From these proposed reactions it can be inferred that the temperature in the country rocks in the immediate vicinity of the 'Younger Basic' masses during thermal metamorphism probably exceeded the maximum temperature attained anywhere in the map area during the earlier regional metamorphic episode. The absence of convincing evidence in the regionally metamorphosed rocks that muscovite and quartz broke down to give sillimanite and potash feldspar is particularly significant in this context (Figure 14). The maximum pressure in the thermal aureole may have been comparable to that prevailing in the Ellon Formation during regional metamorphism, but is likely to have been less than that attained in the regional metamorphism of the Aberdeen Formation (see Chapter 5).

The 8 analysed hornfelses from the Belhelvie areas include 7 pelites and semi-pelites and there are obvious relationships (Figure 3) between mineralogy and chemistry in these rocks, with biotite, sillimanite and quartz being more abundant in rocks rich in potash, alumina and silica (e.g. 150 [NJ 922 205]), while rocks rich in cordierite, garnet, spinel and orthopyroxene are poorer in silica and alumina and richer in ferromagnesian oxides (e.g. 148 [NJ 921 202]). These 7 analyses show considerable similarities to the analyses of the regionally metamorphosed pelitic rocks although some of the hornfelses are richer in MgO, others in iron oxides (Figure 3)b. Two specimens, in particular (Stewart's specimen and 247), display exceptional chemical characteristics, not only having a high content of ferromagnesian oxides, (particularly Fe oxides), but also being poor in SiO2 and rich in Al2O3, and cannot be correlated with any of the analysed regionally metamorphosed rocks (Table 1). It is also difficult to match these hornfelses with any likely parent sedimentary rock-the nearest equivalents being lateritic rocks (e.g. Patterson, 1967, (Table 4)) that would be out of context in a Dalradian sedimentary environment (Harris and others, 1978). These unusual compositional features suggest that thermal metamorphism was accompanied by compositional changes.

'Xenolithic gneisses'

These rocks occur in close spatial association with hornfelses and 'Younger Basic' rocks in the zone of complexity between the Aberdeen and Ellon formations (Figure 21). Similar xenolithic rocks have been found elsewhere in Aberdeenshire and Banffshire (Gribble, 1970) in intimate association with 'Younger Basic' rocks and it has been suggested (Gribble and O'Hara, 1967) that they are the products of the partial melting of metasedimentary country rocks in the vicinity of the 'Younger Basic' intrusions.

The 'xenolithic gneisses' are well exposed in an area extending from [NJ 934 260] near Redheugh to [NJ 937 254] near Auchindarg and were also encountered in drill holes a few hundred metres to the east [c. 941 254]. In these rocks, bodies of fine-grained (c. 1 mm) metasediments with a hornfelsic appearance are enclosed within a more abundant, medium-grained (1 to 3 mm), foliated matrix. The inclusions range up to 0.3 m in dimensions, often have a roughly lensoid shape and are frequently aligned parallel to the foliation in the matrix. The majority have sharply defined boundaries, but examples with indistinct margins can also be observed, and wispy, fine-grained areas in the matrix are probably relicts of inclusions that have disintegrated almost completely.

Thin sections show that the matrix in most of the xenolithic rocks contains approximately 30 per cent ferromagnesian minerals, mainly biotite and cordierite, although garnet is also found in a number of specimens (e.g. 80 [NJ 934 260]; 84 [NJ 936 255]). Spongy crystals of andalusite are present in many specimens, and are sometimes mantled by biotite and overgrown by fibrolite (e.g. 85 [NJ 937 254] (Plate 20)h). Quartz and plagioclase (An25–30) are the main leucocratic constituents in the matrix, but potash feldspar is an additional phase in several specimens (e.g. 85). Muscovite generally appears to have formed by replacement of other minerals and is often found in intimate association with quartz in rocks that also contain myrmekitic intergrowths (e.g. 84). Lensoid aggregates of the ferromagnesian or the leucocratic phases can sometimes be discerned, but in most instances the texture of the matrix is essentially granoblastic, and the constituent minerals are evenly distributed.

The xenoliths are predominantly metasedimentary and range from virtually monomineralic quartzites to silica-deficient pelites containing assemblages such as biotite-cordierite-plagioclase-potash feldspar-andalusite-sillimanite-spinel (e.g. 1820* [NJ 941 254]). Calcareous xenoliths consist of phases such as amphibole-garnet-plagioclase (An65)-sphene-quartz (e.g. 82 [NJ 934 260]). The texture of these rocks is also granoblastic in most instances, although concentrations of ferromagnesian and leucocratic constituents may give the specimen a banded, gneissose appearance.

The chemical analysis (Duncan, 1974) of the matrix of one of these rocks (80 [NJ 934 260]) is similar to the analyses of the matrices in specimens of xenolithic rocks from outside the map area (Gribble, 1968; 1970). However, none of the specimens within the Sheet 77 area displays igneous textural features which would support the view (Gribble and O'Hara, 1967) that these rocks are the products of partial melting. It may also be significant that, while the hornfelses of unusual composition in the Belhelvie aureole (see above) are capable of being interpreted as the refractory residues of partial melting (Gribble and O'Hara, 1967), no likely 'partial melt' rocks, such as the 'xenolithic gneisses', are known from the Belhelvie aureole.

The matrices of many of the 'xenolithic gneisses' in the map area show marked mineralogical and textural similarities to gneissose specimens of the Ellon Formation and many of the features in rocks from the RedheughAuchindarg area can be matched in the leucocratic 'xenolithic' rock (2348) which occurs in the Ellon Formation near Newburgh at [NJ 996 254] in an area where 'Younger Basic' rocks are unknown. The possibility remains, therefore, that the 'xenolithic gneisses' in the map area are the products of regional metamorphism and not of partial melting in the vicinity of the 'Younger Basic' intrusions.

Chapter 8 Late (post 'Younger Basic') deformation

Evidence of shearing and mylonitisation with accompanying recrystallisation can be recognised in many of the 'Younger Basic' intrusions of Aberdeenshire and Banffshire (Ashcroft and Munro, 1978; Boyd and Munro, 1978). Within the map area, exposures of rocks showing the effects of post-consolidation deformation are widespread near Balmedie Quarry [NJ 944 181] but rocks affected by similar deformation can also be recognised elsewhere in the vicinity of the Belhelvie mass (Figure 22). Deformed 'Younger Basic' rocks have also been exposed temporarily near Pitmedden at c. [NJ 877 279], and at localities a short distance to the north c. [NJ 884 287] within the Sheet 87 area. An account of the deformation in the Belhelvie mass has already been published (Boyd and Munro, 1978), and has been used as the basis for much of the description that follows.

The Belhelvie mass

Deformed igneous rocks in the Balmedie Quarry area are mainly gabbroic or noritic cumulates displaying a megascopic foliation defined largely by the parallel orientation of crystals of plagioclase and of elongated clusters of the ferromagnesian minerals. Thin-sections show that in slightly deformed, virtually unfoliated rocks the original mineralogy is largely preserved and the igneous texture is modified only by local bending and marginal granulation of the crystals (227; (Plate 22)a). In more strongly deformed rocks the plagioclase crystals have extensive granulated margins and the original ferromagnesian constituents have been largely replaced by amphibole with almost complete obliteration of the original texture (226). Still stronger deformation has produced well foliated rocks in which relict plagioclase crystals (1 to 2 mm) and lensoid aggregates of amphibole, biotite and opaque constituents are set in a fine-grained (0.5 mm) often mafic matrix (343; (Plate 22)b). The relict crystals in these strongly deformed rocks are sometimes lensoid, but are more commonly irregular in outline, and, in some rocks (e.g. 342), consist, not only of plagioclase, but also of augite mantled by amphibole.

Most of the rocks in the quarry are foliated 'flaser gabbros' in which deformation has not progressed beyond this stage of disruption. However, more intensely deformed, mylonitised rocks also occur in narrow (under 50 mm wide) anastomising zones which traverse the foliated rocks and isolate lensoid bodies of 'flaser gabbro' ranging up to a metre in width. These zones are frequently ill-defined, and when they are parallel to the foliation in the surrounding rock appear to have been produced merely by a local intensification of that structure. Zones with sharper boundaries also occur, however, which often cut across and deflect the foliation in the surrounding rock. Many of the zones, particularly the discordant examples, contain leucocratic and melanocratic bands which define an internal lamination parallel to the zone boundaries.

Thin sections of specimens from these zones (e.g. 344) show that they are composed of mylonitised rocks in which frayed, bent relict grains, are set in a fine-grained (c. 0.1 to 0.2 mm), essentially granular matrix. The relict grains consist mainly of plagioclase crystals, but also include crystals of amphibole, which sometimes contain cores of ortho- or clinopyroxene, and fragments of mafic igneous rock still displaying the original texture and mineralogy (346). The groundmass in many mylonites consists largely of amphibole, biotite and opaque minerals showing little or no preferred orientation, but occasionally contains abundant plagioclase. In laminated specimens (e.g. 246) feldspathic and ferromagnesian rich bands, 5 to 10 mm wide, can be recognised in the groundmass.

Some of the relict feldspar crystals have turbid cores of similar composition to the plagioclase crystals in undeformed rocks in this part of the Belhelvie mass (c. An75–80), and sharply-defined, clear margins that are appreciably more albitic (c. An40–45). The small groundmass grains generally have a similar composition to the margins of these relict grains, but rocks with more anorthitic groundmass crystals occur (297).

Many mylonitised rocks contain quartz, either as relatively large (1 to 2 mm), porphyroblastic crystals, or as aggregates of fine-grained (0.1 to 0.2 mm) crystals which are often concentrated in leucocratic bands in the matrix of the rock (344). The boundaries between the quartz crystals and the crystals of the other phases are often highly complex and suggest that the quartz has formed by replacement, and as the mylonite zones occur within igneous rocks which are generally devoid of quartz (Chapter 6), it is probable that extensive silicification has occurred on many of these zones.

The attitude of the foliation in the rocks of the quarry is often variable, and the mylonite zones are generally curving and sinuous, but the prevailing trend of both sets of structures is similar, with N–S strike and steep dip being typical (Figure 22). Some of the very sinuous mylonite zones have been flexed into folds which generally have wavelengths of less than 0.5 m and steeply plunging axes. A steeply plunging lineation defined by orientated clusters of ferromagnesian minerals and by elongate grains of plagioclase was also observed at one locality on the west face of the quarry.

The sense of relative displacement on a mylonite zone can sometimes be inferred from the distortion of the foliation in the surrounding rock. However, the limited number of observations show no consistency: for example, the horizontal displacements on zones of similar orientation are sinistral in some instances, dextral in others.

(Figure 22) Simplified map showing shearing affecting the 'Younger Basic' intrusions. The broad structural relations of the 'Younger Basic' masses and the hornfelses are also shown.

Strongly deformed mafic igneous rocks are also exposed extensively to the north and south of Balmedie Quarry (Figure 22), and it is clear that a zone of strong deformation approximately 1 km wide extends northwards through the intrusion from south of the quarry for approximately 2 km to the north-eastern contact with the country rocks. The deformed rocks display similar characteristics throughout this zone, but the mean trend of the structures varies, with the N–S strike and steep dip at the quarry giving way to a NW–SE strike and a moderate south-westerly clip both north and south of the quarry. The structures in the foliated, mylonitised igneous rocks are therefore approximately parallel to the outer boundary of the intrusion north of the quarry.

Essentially undeformed igneous rocks and hornfelses form the bulk of the exposures elsewhere within and around the mass, but evidence of deformation ranging from minor granulation (e.g. 138 [NJ 919 204]; 216 [NJ 938 174]) to intense mylonitisation (e.g. 1611* [NJ 917 212]; 1662* [NJ 905 2211; 1684* [NJ 925 168]; 303 [NJ 929 189]) can be recognised locally, especially in specimens from peripheral areas of the intrusion.

Two inclined drill holes that penetrated the boundary between the igneous rocks and the country rocks have provided particularly valuable information on the nature of the outer contacts of the intrusion. In one instance, at Sparcraigs on the north-eastern contact of the mass (1675* [NJ 9342 1963]), the hole was drilled eastwards at 45° from within the intrusion towards the country rocks. Essentially undeformed olivine norite occurs 5 m from the contact, but in the next 4 m of core the texture of the rock has been affected by general granulation and the ferromagnesian minerals are extensively replaced by amphibole. Core recovery is incomplete in the contact zone, but well-foliated, fine-grained amphibolite closely resembling the mylonitised rocks in Balmedie Quarry forms the contact facies of the igneous rock. As the foliation in this amphibolite is approximately perpendicular to the axis of the drill core, it can be inferred that this structure has a N–S strike and a moderate (c. 45°) westerly dip in this area. The general trend of the contact of the igneous mass at Sparcraigs is also N–S, and it is probable that the outer boundary of the intrusion is defined here by a mylonite zone dipping at moderate angles to the west. The hornfels adjoining the igneous rocks also displays evidence of local deformation for at least 2 m from the contact.

A body of coarse-grained (10 mm) granite was also encountered in this inclined drill hole, immediately west of the contact amphibolite. This granite appears to be a vein-like body cutting the mafic rocks, and is devoid of any evidence of deformation, except at its eastern margin where it becomes fine-grained and foliated and is probably interdigitated with the adjoining, strongly-deformed amphibolite.

The other inclined drill hole through the outer margin of the intrusion, was drilled at 60° westwards from the intrusion into the country rocks at the western contact of the mass in the Kingseat area at [NJ 905 197]. Serpentinised, but apparently undeformed, olivine-rich cumulates occur within 3 m of the contact in this drill hole (1700*), but, as the contact is approached, give way to deformed, amphibolitised, more feldspathic rock with a steeply dipping foliation which is probably a modified norite. A metre-wide zone of poor core recovery at the actual contact can be related to the presence of a highly fissile and micaceous contact facies of the igneous rock. The adjoining country rocks are fine-grained (c. 2 mm) and granitic, and are traversed by innumberable thin zones in which the rock has been granulated and silicified.

Evidence of deformation can also be recognised locally in the hornfelses adjoining the intrusion (Figure 22), particularly in the septum of country rocks in the northern part of the mass. This deformation is generally confined to zones in which the hornfelsic fabric of the rocks has been destroyed and in which, original minerals (plagioclase, garnet, cordierite, sillimanite, orthopyroxene) have survived only as relict grains in a fine-grained, mylonitised, matrix (e.g. 187 [NJ 929 189] (Plate 22)c; 337 [NJ 930 193]). This matrix may be highly micaceous (187), or contain appreciable quartz and feldspar, and in many specimens (e.g. 302, 303 [NJ 929 189]) it appears that the fine-grained material in the mylonite zones has been progressively silicified until only the larger relict grains remain within a matrix of elongated, strained quartz crystals.

The recognition of the effect of comparable (post 'Younger Basic') deformation in the regionally metamorphosed rocks adjoining the Belhelvie mass has been handicapped by poor exposures, and by the obvious difficulties in identifying evidence of mylonitisation in foliated rocks which sometimes contain a record of several earlier periods of deformation. However, unhornfelsed metasediments from near Balmedie Quarry (temporarily exposed between [NJ 948 184] and [NJ 948 187]) appear to have undergone an episode of widespread deformation and retrogressive alteration that post-dates the main period of metamorphic recrystallisation, and may correspond to the episode of shearing and mylonitisation in the nearby igneous rocks.

Other 'younger basic' masses within the sheet 77 area

Ashcroft and Munro (1978) established that a major zone of mylonitisation trending ENE–WSW cuts through the eastern part of the Insch 'Younger Basic' mass, and concluded that the southern boundary of this mass was probably defined by a similar shear zone. Although these zones must extend into the Sheet 77 area near Oldmeldrum, no evidence of extensive late deformation has been identified in the igneous rocks of the Insch mass, or in the nearby metamorphic rocks in the north-western part of the map area. Further to the east, in the Udny-Pitmedden area, there are no outcrops of deformed 'Younger Basic' rocks, but boudinaged basic igneous rocks traversed by mylonite zones were observed in temporary exposures west of Pitmedden [NJ 877 279]. These mylonite zones range up to 0.5 m in thickness, generally strike N–S or NW–SE and dip E or NE at moderate angles (c. 40° to 50°).

North of the Belhelvie mass in the zone of hornfelses adjoining the Ellon Formation, two drill cores of undoubted 'Younger Basic' rock (1875* [NJ 9291 2767]; 1817* [NJ 9382 2532]) show no evidence of deformation. However, other drill holes in this area (1876* [NJ 9343 2750]; 1879* [NJ 9373 2645], and from immediately north of the sheet boundary at [NJ 9307 2972]; [NJ 9272 2945]; [NJ 9300 2902]), encountered 'dioritic' rocks similar to the deformed, amphibolitised rocks from Balmedie Quarry, which contain augen of andesine set in a finer foliated matrix of biotite, amphibole, plagioclase and quartz. Evidence of strong deformation is also visible at the eastern margin of the granite quarry [NJ 9475 2385] on the Hill of Minnes, where there is a rapid transition from the equigranular granite with a moderate foliation which predominates in the quarry, into highly deformed rock. The latter resembles an augen gneiss and contains relict crystals (2 to 5 mm in dimensions) of feldspar and quartz showing evidence of marginal granulation and recrystallisation (1034; 2361) in a fine-grained (0.5 mm) foliated matrix of 'flattened' quartz, biotite and feldspar showing good preferred orientation. The strong foliation in this rock trends N–S and dips steeply, and is orientated at a considerable angle to the less well-defined foliation in the undeformed granitic rock exposed elsewhere in the quarry. Similar, strongly deformed granitic rock with a steeply dipping, N–S foliation was formerly exposed near Littlemill of Esslemont at [NJ 927 292], immediately to the north of the map boundary, 4 km north of the Hill of Minnes occurrence.

Evidence of the age of this deformation episode has been found in the Belhelvie mass at Balmedie Quarry, where sheet- and dyke-like bodies of granitic rock that cut through the deformed basic rocks have been isotopically dated (462 ± 5 Ma; Rb-Sr, van Breemen and Boyd, 1972; Pankhurst, 1982). These granitic intrusions are generally medium-grained, but locally become very coarse-grained (50 to 100 mm) and pegmatitic and sometimes contain large crystals of black tourmaline. Thegranitic rocks appear undeformed in most of the exposures, and in a number of instances can be seen to cross-cut the foliation in the surrounding 'flaser gabbros'. Biotite crystals showing no preferred orientation, which overgrow and partially obliterate the foliation, are often abundant in the deformed mafic rocks at the granite contacts. Xenoliths of mafic rock within the granite frequently show evidence of even more extensive modification and are sometimes reduced to ill-defined, wispy relicts rich in mica.

Although these relationships demonstrate that the bodies of granite were emplaced after much of the deformation in the surrounding mafic rocks has occurred, the relationships at some of the boundaries of the granitic rocks are more ambiguous. Many of the masses of granite have a highly irregular form and a contorted, fragmented appearance, and although these irregularities sometimes appear to be associated with replacement of the mafic rock by leucocratic material, they are often displayed by granitic bodies lying within mylonite zones. Many of the thin sections of these irregular granitic bodies show that the rock has been strongly deformed (e.g. 2449) and zones of crushing and shearing up to 100 mm wide can be recognised locally within the massive bodies of pegmatitic granite. The contact of the bodies of pegmatitic granite also show a tendency to dip steeply and to strike N–S in approximate parallelism to the prevailing trend of the mylonite zones, suggesting that these contacts may sometimes be defined by zones of deformation. Although it is possible that there is more than one suite of granitic intrusions in the quarry, as many of the highly irregular bodies lack the potash feldspar that is a dominant constituent in the more massive bodies, nevertheless, it is clear that deformation continued, on at least a reduced scale, until after the emplacement of the bodies of pegmatitic granite, i.e. until 30 to 40 Ma after the 'Younger Basic' complexes had formed (c. 490 Ma; Pankhurst, 1982).

Significance of the deformation of the 'younger basic' masses

The widespread development of the sheared and mylonitised rocks near Balmedie Quarry is a clear indication that the Belhelvie mass was affected by a major episode of post-consolidation deformation. Although only limited evidence of similar deformation has been found elsewhere in this mass, the highly irregular distribution of rocks showing evidence of thermal metamorphism in the vicinity of the intrusion and the occurrence of deformed rocks at many of the outer contacts, suggests that original field relations have been disturbed and the igneous rocks have been largely detached from the aureole of hornfelses which, presumably, originally surrounded the intrusion. The boundaries between igneous rocks and country rocks may therefore be largely determined by shear zones. As sheared rocks also occur within the intrusion (Figure 22) the original intermal relationships may also have been disturbed, but the consistency of the cryptic layering over extensive areas in the north of the mass suggests that large parts of the intrusion were displaced as coherent units and that the main movements were localised in narrow zones. In these northern areas rotation on the marginal shear zones possibly has led to the general parallelism of the internal lithological boundaries and primary igneous structures to the outer contacts of the intrusion.

North of the Belhelvie mass the presence of only small bodies of 'Younger Basic' rock within extensive areas of hornfelses, and the occurrence of probable enclaves of hornfelsed rocks of the Ellon Formation (e.g. 1333 [NJ 928 276]) within the more widespread hornfelses attributed to the Aberdeen Formation, suggests that original relationships have been much disturbed. It is possible that the hornfelses are a detached portion of the Belhelvie aureole and the associated 'Younger Basic' rocks are fragments of the Belhelvie intrusion. If these inferences are correct, then a major zone of deformation must extend northwards from the Belhelvie mass towards the occurrences of 'Younger Basic' rocks in the Arnage-Haddo area within Sheet 87.

It has been noted (Chapter 2, (Figure 5)d, k) that a zone of metamorphic rocks with steep, approximately N–S foliation, extends from the Belhelvie mass to the northern margin of the map area and for several kilometres further north into the Sheet 87 area. This foliation trend is markedly different from the trends prevailing in the unhornfelsed rocks of the Aberdeen Formation in the north-west of the Sheet 77 area and in the rocks of the Ellon Formation (Figure 5), (Figure 22), and the zone as a whole is clearly discordant to the trend of the major lithological sub-units in the Aberdeen Formation to the west and cuts the boundary between the Aberdeen and Ellon formations in the east. In the northern part of the map area (north of Grid line 25) the western limit of this zone lies at, or near, the western margin of the area of hornfelses (near Grid line 91). The eastern limit lies within the Ellon

Formation a short distance (c. 1 km) east of the boundary between the Ellon and Aberdeen formations and is approximately parallel to that boundary and to the elongated magnetic anomalies in this area (Figure 4a).

Relations are more complex to the south of Grid line 25 (Figure 22). In the west, near Monkshill [NJ 913 257], the western limit of the zone of steep structures continues to the SSW towards the exposures west of the Belhelvie mass in the railway cuttings at [NJ 903 231]; [NJ 893 209]; [NJ 897 185] and is no longer coincident with the western margin of the hornfelses which now trends NW–SE. In the east, difficulties arise because the magnetic lineament at the boundary between the Aberdeen and Ellon formations cannot be traced into the area south of Fiddesbeg [NJ 946 246] where the interpretation of relations on the magnetic map is complicated by the presence of a quartz-dolerite dyke and by the occurrence of the composite granite-diorite mass on the Hill of Minnes [NJ 947 237] (Figure 4a),  (Figure 4c). However, temporary exposures in a trench have shown that, to the west of Gateside [NJ 942 224], the foliation in the metasediments of the Aberdeen Formation generally strikes N–S and dips steeply, while a foliation with an E–W or NE–SW strike and a moderate dip characterises the rocks to the east of this locality. The zone of steep structures must therefore diverge from the boundary between the Aberdeen and Ellon formations in the vicinity of the Hill of Minnes, and although there is virtually no information from the area between Gateside and the Belhelvie mass, it seems reasonable to extrapolate the eastern limit of this zone southwards from Gateside to link with the eastern limit of the zone of mylonitised rocks recognised within the Belhelvie mass in the Balmedie Quarry area (i.e. to [NJ 950 183]). If this extrapolation is correct, the eastern boundary of the zone passes through the Belhelvie mass, and thus differs from the western boundary which is approximately parallel to, and lies some distance west of the western contact of the igneous mass (Figure 22).

Many of the exposures of metamorphic rock south and south-west of the Belhelvie mass contain a steeply-dipping foliation with a N–S strike, and it is probable that the zone of steep structures extends southwards from the western contact areas of the intrusion to the Brig O' Balgownie area ([NJ 942 097], (Figure 5)p), and is bounded by the Aberdeen granite in the west.

These relationships suggest that the development of the zone of steep discordant structures in the metamorphic rocks is related in origin to the episode of shearing and mylonitisation that has affected the Belhelvie mass. The sinuosity of the major shear-zone in the Balmedie Quarry area (Figure 22), and the sense of folding of some of the mylonite zones in the quarry can be explained by invoking sub-horizontal, N–S dextral displacements. The sinuosity of the zone of steep structures in the vicinity of Grid line 24 can also be explained in this fashion, although the possibility cannot be overlooked that E–W displacements occurred on the fault which probably controlled the emplacement of the quartz dolerite dyke in this area (Figure 4a)(Figure 4b),(Figure 4c); Chapter 11). Sub-horizontal dextral movements in the zone of steep structures would account also for the sense of curvature of the amphibolite horizons to the east of this zone near Hillhead of Ardo [NJ 954 213], and possibly also for the distribution of ultramafic and corundum-bearing rocks near the western margin of the zone (Figure 22).

Psammitic metasediments of the Aberdeen Formation near Hillhead of Ardo [NJ 956 211] display evidence of strong E–W shearing, and as these rocks lie near the portion of the boundary between the Aberdeen and Ellon formations which trends E–W towards the sea near Grid line 21 (Figure 22), it is possible that the two formations are also separated by a zone of deformation in this area. If such a zone exists it may either be an offshoot from the main N–S zone of shearing, or it may be an earlier structure which has been cut and deflected by the latter.

Elsewhere in the map area there is no convincing evidence of the presence of major discordant shear zones. The occurrence of mylonitised 'Younger Basic' rocks west of Udny and Pitmedden accompanied by a disproportionally large area of hornfelses (Figure 22) suggests that some disruption of the original relations of the mafic igneous rocks has taken place here, but no major discordant structures can be recognised in the surrounding country rocks. Likewise, there is no evidence of late shearing at other localities in the Aberdeen Formation where it has been noted (Chapter 2) that the foliation locally strikes N–S and dips steeply (e.g. Cran Hill [NJ 910 005]).

Chapter 9 Major granitic intrusions

The Clinterty mass

This mass has been named after one of the more important of the quarries (Little Clinterty Quarry [NJ 835 122]) that were formerly worked on a large scale near Blackburn and Tyrebagger. This is the only area where the intrusion is reasonably well exposed but sufficient information is available from elsewhere, including the data from trenches at [NJ 810 110], [NJ 832 114], [NJ 849 0981 and [NJ 845 087], to show that the granitic rocks form a roughly circular body with a diameter of approximately 4 km (Figure 23). The contact between the granitic rocks and the surrounding rocks of the Aberdeen Formation is nowhere exposed, but was temporarily visible in an excavation immediately to the west of the Sheet 77 area at Auchronie Hill [NJ 807 0931, which showed that typical, coarse (4 to 5 mm) granodiorite of the mass cuts across the foliation in the adjoining migmatitic gneisses. Elsewhere within the map area there are exposures near the southern margin of the intrusion at [NJ 823 084] near Souterhill, where coarse granodiorite forms the northern slopes of the hill and veins of slightly finer-grained and more biotitic granitic rock cut the metasediments on the hilltop a few tens of metres to the south, and in the Tyrebagger area at [NJ 8495 1170], where massive psammites exposed in a small quarry on a forestry track near the eastern margin of the intrusion, are cut by veins of coarse granitic rock. These exposures suggest that the contacts are transgressive and that intrusion was accompanied by small-scale veining. There are also sufficient outcrops in the Tyrebagger area (e.g. [NJ 844 124]; [NJ 848 108]; [NJ 847 103]; [NJ 8495 1000]) to show that the boundary between the granitic rocks and the metasediments is highly irregular and that the major psammite sub-unit on Tyrebagger Hill is truncated by the intrusion (Figure 23). No evidence of thermal metamorphism can be recognised at any of the localities where the country rocks are exposed in the immediate vicinity of the granitic mass.

The rocks of the Clinterty mass are generally pink and often contain large (5 to 7 mm, occasionally up to 15 mm) crystals of potash feldspar which are enclosed within a finer-grained (2 to 4 mm) groundmass. The relatively coarse grain and paucity of ferromagnesian minerals handicaps the recognition of primary igneous structures, but in most exposures the rock is structureless, except in Clinterty Quarry, where mafic xenoliths show some degree of alignment, and in an outcrop at [NJ 846 101] on Elrick Hill, where the feldspar magacrysts display a weak preferred orientation.

Modal analyses (Figure 26)). Data from Porteous, 1973b and Walsworth-Bell, 1974." data-name="images/P999791.jpg">(Figure 24)a show that typical rocks contain 25 to 30 per cent quartz, 40 to 50 per cent oligoclase and 20 to 25 per cent potash feldspar, and can thus be classified as granodiorites (Streckeisen, 1967). Biotite is the only important ferromagnesian constituent, forming 4 to 8 per cent of the rocks, opaque constituents are generally present in minor amounts, and sphene, apatite and zircon are the main accessory minerals.

The crystals of plagioclase are often lath-like and subidiomorphic and show complex oscillatory zoning superimposed on normal zoning in the range An22 to An8. The biotite crystals also show some tendency to develop a subidiomorphic, lath-like form, range up to 1 to 1.5 mm in size, and are sometimes intergrown with the plagioclase crystals. The biotite is often greyish-brown and has Mg/Mg + Fe values (expressed as cations with total iron as Fe2) ranging from 49.6 to 54.4 (Figure 26)). Fe2O3 is expressed as FeO and included in 'FeO total'. Field of biotites associated with muscovite, topaz etc. from Albuquerque, 1973. Data from Walsworth-Bell, 1974. " data-name="images/P999792.jpg">(Figure 25). The crystals of potash feldspar are invariably microperthitic and although they often display microcline twinning, X-ray diffraction generally shows that parts of these crystals are monoclinic. In hand-specimen, the large potash feldspar crystals often appear to have a rectangular form, but in thin section it can be seen that these megacrysts have highly irregular margins and never show any tendency to develop crystal faces. Small (c. 0.5 mm), subidiomorphic crystals of biotite, plagioclase and quartz are frequently enclosed within these megacrysts, and, in some instances, very small (c. 0.1 mm), included grains of quartz are concentrated in a zone of limited width within the potash feldspar (e.g. 1076 [NJ 817 108]).

Quartz is generally interstitial to plagioclase and biotite, but often forms intergrowths with potash feldspar which have relatively simple, almost linear boundaries (e.g. 1075 [NJ 816 109]). However, in many of the specimens the quartz crystals are strained, show internal polygonisation, and have complex, sutured boundaries against all the other phases in the rock. In some instances, particularly in rocks in which the quartz crystals are concentrated in lensoid or vein-like aggregates, the quartz appears to have a replacive relationship to the other minerals (e.g. 1084 [NJ 838 121]). Muscovite occurs as a minor constituent, sometimes being intergrown with biotite, in other instances occurring as small grains within the feldspar crystals. In general, the textural relations suggest that the muscovite is a phase of secondary (post-magmatic) origin.

The Crathes mass

Temporary exposures in a trench at [NJ 813 018] and [NJ 812 022] show that a grey, medium-grained (c. 2 mm) granitic rock occurs widely near Anguston at the western margin of the map. This rock is often relatively rich in biotite, generally displays a weak foliation, and sometimes contains abundant xenoliths of metasediments and amphibolite. Similar rock is exposed in an outcrop at [NO 817 998] near Coalford, a kilometre to the south, but the highly shattered nature of specimens from this locality (e.g. 1055) handicaps the recognition of original petrographic features. The trench exposures show that the relatively fine-grained, and biotite-rich granitic rock within the Sheet 77 area grades westwards into coarser grained (3 to 4 mm) granodiorite with potash Metasediments in the vicinity of the contact of this intrusion were temporarily exposed at [NJ 814 018] near Anguston and include poorly fissile rocks (e.g. 2444*) with a granoblastic texture, abundant cordierite and sillimanite, which are undoubted hornfelses (Plate 22)d. However, as typical schistose and gneissose rocks of the Aberdeen Formation occur immediately to the east, the effects of thermal metamorphism appear to have been confined to a zone only a few metres wide.

The Aberdeen mass

Feldspar megacrysts (up to 20 to 30 mm) that forms the greater part of the Crathes mass in the Sheet 76 area (Walsworth-Bell, 1974). The rocks within the Sheet 77 area therefore appear to be a marginal facies of the Crathes mass (Figure 26), probably forming a zone approximately a kilometre wide. The medium-grained, foliated granitic rocks exposed in Anguston Quarry [NJ 806 020], immediately west of Sheet 77, probably lie within the inner part of this zone as they contain occasional potash feldspar megacrysts (up to 10 mm).

Modal analyses of a specimen of medium-grained rock from Anguston Quarry shows that it is a granodiorite with 24 per cent quartz, 59 per cent oligoclase, 5 per cent potash feldspar and 11 per cent biotite, differing from the typical megacryst granodiorites from within the Crathes mass to the west in being poorer in potash feldspar and richer in plagioclase and biotite ((Figure 26)). Data from Porteous, 1973b and Walsworth-Bell, 1974." data-name="images/P999791.jpg">(Figure 24)a).

This body of granitic rocks underlies much of the city of Aberdeen, and was extensively quarried in the past, (Figure 23); (Plate 23), formerly providing the basis for one of the main industries of the area. The most northerly exposures of this mass occur in Swailend Wood [NJ 881 169], and the occurrence of metasediments and granitic rocks in close proximity on the north bank of the Don at [NJ 876 156] near Cothall and at Dyce Quarries [NJ 865 135] enables the western contact to be traced for 4 km to the south-west, truncating the major Tyrebagger psammitic sub-unit in the area immediately to the south of the Don [c. 875 150]. Exposures are scarce further south, but drill holes and temporary exposures ([NJ 865 114]; [NJ 871 088]; [NJ 897 037]) and outcrops ([NJ 886 063; 893 046]) in migmatitic metasediments, and drill holes and temporary exposures ([NJ 873 111]; [NJ 877 095]; [NJ 885 070]) and outcrops ([NJ 902 044]; [NJ 899 035]) in granite show that the western contact of the granitic rocks trends SSE from near Dyce Quarries towards the Dee near [NJ 900 030]. Metasediments occur in the southern suburbs of Aberdeen to the south of the Dee, notably near the Wellington Bridge [c. 943 048] whilst granitic rocks clearly underlie Ferryhill area of the city just north of the river (e.g. at [NJ 934 056]). The southern boundary of the granitic mass must therefore lie near the river, probably being defined by the major post-Devonian fault that appears to determine the general trend of the lower Dee valley (Chapter 12).

The eastern boundary of the Aberdeen mass is also poorly exposed, but has been drawn south-east from Swailend Wood [NJ 881 169] for c. 8 km between the metasediments found in drill holes and temporary exposures ([NJ 895 163]; [NJ 908 154]; [NJ 911 144]; [NJ 912 119]) and outcrop [NJ 917 111], and the granitic rocks found in drill holes ([NJ 889 163]; [NJ 906 118]; [NJ 906 128]; [NJ 918 106]) and outcrop [NJ 903 151]. To the south, the boundary of the granitic rocks must lie between the exposures of metasediments on the banks of the Don at Braes O' Don [NJ 932 096] and of granite in the railway cutting at [NJ 932 083]. Still further south, between this last locality and the Dee, the eastern boundary of the granitic rocks is apparently entirely overlain by Old Red Sandstone sedimentary rocks (Figure 27).

The Aberdeen mass thus consists of an elongated body of granitic rocks which extends for 16 to 17 km NNW from Aberdeen and is approximately 6 km wide. Metasediments are widely exposed in the vicinity of the granitic rocks on the ridge of high ground north-west of Dyce Quarries c. [NJ 863 137] but the contact relations of the mass are nowhere visible in this area. However, bodies of granitic rock showing a marked petrographic similarity to the typical rock of the Aberdeen mass are abundant in the metasediments on this ridge, in many cases forming flat-lying sheets, 10 to 15 m thick, which are approximately concordant to the lithological banding in the adjoining metasediments. If similar relationships prevail near the other contacts of this mass, the relatively coherent core of this body could be surrounded by a zone in which the country rocks are cut by a plexus of sub-concordant veins and sheet-like offshoots from the main mass. However, when viewed on a larger scale (Figure 23) the Aberdeen mass clearly cuts across the structures and lithological units in the surrounding rocks of the Aberdeen Formation. No evidence of thermal metamorphism has been found in any of the country rocks adjoining the mass.

The rocks of this mass are typically grey, but a pink or red colouration may be developed in highly altered or crushed rocks, particularly in the vicinity of joint planes (e.g. in Dancing Cairns Quarry [NJ 904 092]). Considerable variations in grain size and mineral proporations can be observed, but most specimens are medium-grained (1 to 3 mm), equigranular and devoid of any regular persistent banding. Much of the variation in the granite is due to the presence of coarse-grained masses consisting either of quartz or of quartz and feldspar that range up to 150 mm or more in dimensions, of biotitic lenses or schlieren that are over 150 mm in length at some localities, and of irregularly distributed feldspar megacrysts up to 15 to 20 mm in dimensions (e.g. in Sclattie Quarry [NJ 893 099]). Bands of finer-grained, somewhat more mafic rock are present locally within the typical granite (e.g. in the Persley Quarries [NJ 905 107]), while coarse pegmatitic granite occurs in some outcrops in veins or in more irregular, ill-defined areas (e.g. in the Bucks Burn c. [NJ 892 092]). Xenoliths of metasedimentary rock are abundant in places (e.g. in parts of the Persley Quarries) and are generally highly migmatised and modified. In a number of instances a progression can be traced from a gneissose migmatitic xenolith into a granitic rock which has an imperfect banding defined by variations in grain size and biotite content, and it seems likely that much of the variation in the granite has arisen as the result of the incorporation of fragments of country rock.

The foliation displayed by most specimens of the Aberdeen mass is mainly defined by the preferred orientation of crystals of mica and feldspar, but schlieren and xenoliths displaying preferred orientation also occur at some localities (e.g. Sclattie Quarry). Cameron (1945) studied the relations between the foliation and jointing in the granitic rocks of Lower Persley Quarry [NJ 909 100] and identified a set of cross joints striking perpendicular to the foliation, and a set of longitudinal joints, parallel to the foliation that correspond respectively with the 'easy way' and the 'hard way' in the rocks.

The foliation is often weak and ill-defined but generally strikes parallel to the length of the mass (NNW-SSE) and dips at moderate angles to the ENE (Figure 23). Local variations can be discerned, with steeper dips being more common in the eastern part of the mass and a NE–SW strike prevailing at a number of localities, mainly in the northern part of the mass. In some instances where the rocks adjoin a NE–SW-trending contact with the country rocks (e.g. Dyce Quarries [NJ 865 135]) it is probable that the unusual strike of the foliation reflects the influence of this boundary. As other localities in the northern part of the mass where the NE–SW trend of the foliation is oblique to that of the contact (e.g. Corsehill [NJ 903 151]) it has already been noted (Chapter 8) that the present boundary of the intrusion may be defined by a dislocation and original relations may have been disturbed.

When viewed on a large scale (Figure 23) the foliation of the Aberdeen mass shows a discordant relationship to the country-rock structures and, as the igneous rocks show only limited evidence of post-consolidation deformation (see below), it is concluded that the foliation is a primary structure produced whilst the magma was consolidating.

In thin-section, it can be seen that the original texture, and possibly the mineralogy, of the granitic rocks have frequently been modified by post-consolidation recrystallisation, but rocks virtually devoid of evidence of recrystallisation can also be recognised (e.g. 738 [NJ 906 106]; 710, 715 [NJ 893 099]). In the latter, the texture is dominated by subidiomorphic laths of plagioclase, 2 to 3 mm in length, and by rather smaller (1 to 2 mm) laths of biotite, which frequently occur in clusters showing preferred orientation. Quartz and alkali feldspar are generally xenomorphic in these rocks, although the crystals of alkali feldspar are often considerably larger than the other constituents (e.g. 10 mm in 710) and are visible as megacrysts in many outcrops. The megacrysts often appear to be rectangular and idiomorphic in hand specimen, but thin sections show that their margins are highly irregular and many are penetrated by crystals of biotite and plagioclase. Small (0.1 to 0.5 mm), subidiomorphic crystals of biotite, plagioclase and quartz occur as inclusions throughout the megacrysts and, in some instances, small (0.1 mm) quartz grains are largely concentrated in a zone of inclusions parallel to the outer boundary of the potash feldspar (Plate 22)e, as in the megacrysts of the Clinterty mass. Round quartz inclusions are also common in the plagioclase crystals but most of the quartz in these rocks occurs as relatively small (2 to 3 mm), interstitial grains that have simple boundaries and are intergrown with potash feldspar. Muscovite generally occurs as a minor constituent (c. 2 to 3 per cent) in close association with biotite, sometimes as medium-grained (1 to 2 mm) laths which have idiomorphic boundaries against the biotite.

In most of the specimens of the Aberdeen mass evidence of post-consolidation recrystallisation is more widespread and the original igneous texture has been much modified. Quartz, in particular, has recrystallised extensively and the appearance of the rock in thin section is generally dominated by the highly irregular boundaries between quartz and the other phases. Individual quartz crystals are often strained, and many specimens contain quartz aggregates ranging up to 10 mm in dimensions (e.g. 702 [NJ 920 083]; (Plate 22)f, which frequently have intricate, and apparently replacive, boundaries against the other phases. Much of the muscovite in these rocks consists of small (< 0.5 mm), irregular grains, which in some instances have peripheral, possibly replacive, relations to clusters of biotite crystals, and in others are wholly enclosed within crystals of feldspar (e.g. 1064 [NJ 865 136]; 708 [NJ 903 044]). Larger (3 to 4 mm) muscovite crystals in other rocks (e.g. 1092 [NJ 903 044]) form spongy intergrowths with quartz and are generally more abundant where there is widespread evidence of the recrystallisation of quartz (e.g. 701 [NJ 920 083]), and myrmekitic intergrowths are common (e.g. 708 [NJ 903 044]; 717 [NJ 896 093]).

Recrystallisation has not occurred uniformly throughout the mass, however, and modified and unmodified igneous textures can sometimes be recognised in different specimens from the same outcrop, and in some instances, are even visible in different parts of the same thin section (e.g. 738 [NJ 906 106]). In some rocks (e.g. 756 [NJ 917 059]) in which elongated, lensoid aggregates of strained quartz crystals have a sub-parallel orientation, recrystallisation may have been accompanied by deformation. Most of the specimens lack any evidence of recrystallisation under stress however, and Cameron (1945) has shown that the quartz crystals in a granite specimen from Lower Persley Quarry display no preferred orientation. Indeed, the thin sections lend support to the view that the foliation in most of the rocks is a primary igneous structure (see above), as it is apparent under the microscope that this structure is mainly defined by the planar orientation of crystals of primary igneous phases -biotite, plagioclase, potash feldspar megacrysts. The idiomorphic crystals of muscovite associated with biotite are also probably of magmatic origin as they are often aligned parallel to the foliation, but the irregular grains and large spongy crystals of this mineral which show no preferred orientation, are probably post-magmatic.

Modal analyses of three specimens emphasise the high quartz content of the rocks of the Aberdeen mass (33.5 to 35.5 per cent), and show that plagioclase (32 to 35.5 per cent) predominates over potash feldspar (20.5 to 23 per cent) ((Figure 26)). Data from Porteous, 1973b and Walsworth-Bell, 1974." data-name="images/P999791.jpg">(Figure 24)a). Micas are the other main constituents (6.5 to 11 per cent), with biotite always predominating over muscovite. These rocks can be classified as granites (Streckeisen, 1967), but lie close to the boundary between granite and granodiorite. The relatively melanocratic, fine-grained bands and zones within the normal granitic rock differ in being enriched in biotite and plagioclase and generally have a granodioritic composition.

Chemical analyses of ten rocks show that there is considerable compositional variation in the mass (e.g. SiO2 69.4 to 75.7 per cent; Al2O3 12.0 to 16.4 per cent; K2O 3.43 to 5.92 per cent) but there are no obvious relationships between the chemistry and the degree of recrystallisation in any of these rocks. This suggests that although recrystallisation and replacement occurred widely in the rocks of the Aberdeen mass, any associated compositional changes were of minor significance. Much of the compositional variation displayed by the granitic rocks can probably be ascribed to the assimilation of the country rocks as it is clear that, were the granite contains abundant xenoliths, it generally shows marked variations in grain size and in mineral proportions.

The plagioclase generally displays oscillatory zoning superimposed on normal zoning from cores of c. An27 (occasionally as anorthitic as An31), to margins of c. An19 (occasionally as albitic as An7). The plagioclase crystals in many rocks are crowded with turbid alteration products, or with irregular flakes and grains of muscovite, but highly albitic margins (e.g. 701 [NJ 920 083]) are often clear and unaltered.

The majority of the crystals of potash feldspar are microperthitic and show microcline cross-hatch twinning. X-ray diffraction studies show that these crystals display a considerable range of obliquities (generally greater than 0.80), however, and many of the crystals of potash feldspar are partly monoclinic.

The biotite crystals are generally reddish-brown, but grey-brown crystals occur in some specimens, notably from Dyce (e.g. 1061 [NJ 866 135] and Hilton Quarries (e.g. 701 [NJ 920 083]). Microprobe analyses of a red-brown (737 [NJ 906 106]) and a grey-brown biotite (1061) show that these colour differences are probably due to differences in composition, as the M/M + F values are respectively 26 and 27.5, and the contents of octahedral Al per unit formula (based on 22 oxygen ions) are respectively 0.191 and 0.438 (Walsworth-Bell, 1974).

Apatite and zircon are the common accessory minerals, with the apatite sometimes being relatively abundant and occurring in idiomorphic laths up to 0.5 mm in diameter (e.g. 1061 [NJ 866 135]; 743 [NJ 906 106]). Mackie (1926) found that monazite was more abundant than zircon in granite from Rubislaw Quarry [NJ 912 055] and also mentions the occurrence of andalusite and sillimanite as accessory minerals in the Aberdeen mass. Opaque constituents are much less common in the Aberdeen mass than in the Clinterty mass.

Comparison of the Aberdeen mass and the migmatitic vein complex

The rocks of the Aberdeen mass and of the migmatitic vein complex in the Aberdeen Formation (Chapter 2) show general similarities in texture, mineralogy (Figure 26)). Fe2O3 is expressed as FeO and included in 'FeO total'. Field of biotites associated with muscovite, topaz etc. from Albuquerque, 1973. Data from Walsworth-Bell, 1974. " data-name="images/P999792.jpg">(Figure 25) and chemistry, and, in many instances, specimens of the major intrusion cannot be distinguished from specimens of the migmatitic bodies, either megascopically or in thin section. The chemical similarities are clear, not only when comparisons are made on the basis of relatively simple variables (Figure 26)). Data from Porteous, 1973b and Walsworth-Bell, 1974." data-name="images/P999791.jpg">(Figure 24), but also when the data are processed in a more sophisticated fashion using the techniques of cluster and discriminate analyses (Walsworth-Bell, 1974). These technique have been applied to the large number (c. 150) of chemical analyses of granitic rocks of all types from the Aberdeen area and show that the muscovite-biotite granites of the migmatitic vein complex and the Aberdeen mass (and the Kemnay mass west of Sheet 77-see below) form a coherent group geochemically that can be discriminated from the other granitic rocks at a confidence level of nearly 100 per cent.

The main difference between the Aberdeen mass and the vein complex is that, in the former, essentially uniform granitic rock occurs in a large coherent intrusion, in the latter, similar, but more variable rock occurs in a large number of relatively small intrusions. However, the portions of the Aberdeen mass which display local inhomogeneities, particularly the rocks with ill-defined banding, closely resemble the 'transitional' rocks at the gradational boundaries of migmatitic veins and sheets, and it is significant that these inhomogeneities are often more conspicuous at localities where partly digested country-rock xenoliths can be recognised (e.g. Persley Quarry [NJ 905 107]). Furthermore, the exposures near Dyce Quarries at c. [NJ 864 137] suggest that the coherent body of granite forming the mass is surrounded by a marginal zone of vein and sheet-like apophyses resembling the migmatitic complex.

There are thus good grounds for regarding the origin of the Aberdeen mass as being related to that of the migmatitic vein complex. The Aberdeen mass may have developed in an area where the supply of magma was particularly copious, in a fashion similar to that envisaged for the formation for the smaller granitic bodies at Cove and Hill of Crimond (see Chapter 2), and may even have been a major feeder of the system of migmatitic veins and sheets. The prevailing trend of the foliation in the intrusion (Figure 23) suggests magma ascended in the east and spread upwards and outwards to the WSW.

The Hill of Minnes intrusion

The medium-grained (2 to 3 mm), weakly foliated granitic rocks (1034, 2361) exposed in the quarry near the summit of the Hill of Minnes at [NJ 947 238] (Figure 23) consist mainly of potash feldspar (often as megacrysts), oligoclase, quartz and biotite. They show similarities to the rocks of the Aberdeen mass and of the migmatitic vein complex, but generally have a higher content of ferromagnesian minerals than the other granites, as is reflected in the relatively high CaO, Fe O and MgO content and relatively low SiO2 content of an analysed specimen (1034). They appear to grade westwards into the coarse-grained (4 to 7 mm), foliated dioritic rocks exposed near the summit of the Hill c. [NJ 947 237]. Similar dioritic and granitic rocks are known to occur in close association to the south near Hill of Minnes farm c. [NJ 949 234] where they were temporarily exposed in a trench, and it is probable that a composite granite/diorite mass c. 0.5 km in diameter underlies the hilltop. More extensive bodies of diorite occur in association with some of the major granitic intrusions in the area to the west of Sheet 77 (e.g. at Gask Quarry, near Skene [NJ 794. 065]; Sundayswells, near Torphins, [NJ 614 032]) and it is more likely that the Hill of Minnes rocks have affinities with these occurrences rather than with the Aberdeen mass and the vein complex.

Dioritic rocks also occur in a small exposure, approximately a kilometre to the north of the Hill of Minnes [NJ 942 249], and it is possible that some of the amphibole bearing rocks within the area of hornfelses further to the north (1876* [NJ 9343 2750]; 1879* [NJ 9373 2645]) are deformed diorites (see Chapter 8).

General review of the granitic rocks in the Aberdeen area

A large undifferentiated mass of granitic rocks is depicted as extending for more than 50 km to the west of Aberdeen in the older geological maps of north-east Scotland (e.g. Sheet 9, 1:253 440 map). However, Bisset (1934a) showed that this 'mass' actually consists of a considerable number of different intrusions, which in some instances have overlapping boundaries, and in others, are separated by areas of country rocks. He subdivided these intrusions into two groups-the diorites and porphyritic and grey granites forming the Skene complex, and a later series of red granites. The map of these igneous rocks was further revised by Walsworth-Bell (1974), and it is now known that at least seven major bodies of granitic rock occur between the Dee and the Don in the area which extends for 40 km to the west of Aberdeen (Figure 26). The term Skene complex has been discarded as the rocks grouped by Bisset in this unit cannot be regarded as forming a distinctive, genetically related assemblage.

The individual masses have been distinguished by using field relations and petrological and mineralogical characteristics. Five of these bodies show only limited variation and can each be reagarded as consisting essentially of a single lithological unit: Aberdeen, muscovite-biotitegranite; Kemnay, muscovite-biotite-granite; Clinterty, granodiorite; Hill of Fare, biotite-granite; Torphins, diorite-appinite. The Crathes and Tillyfourie masses show greater lithological diversity and, if exposures were more extensive, it is possible that these bodies would be divisible into a number of lithological sub-units. The southern part of the Crathes mass consists largely of granodiorite containing potash feldspar megacrysts, but much of the northern and western parts consist of non-porphritic, possibly slightly younger, granodiorite, and dioritic rocks occur locally near the eastern boundary of the intrusion near Gask [NJ 794 065]. The Tillyfourie mass is highly variable, and whilst granodioritic rocks predominate, diorites are also widespread.

The elucidation of the age relationships of the granitic masses is handicapped by the lack of exposures of the contact relations between the rocks of the different intrusions. However, the areal distribution of the rocks characteristic of each intrusion (Figure 26) suggests that cross-cutting relations often prevail and that the sequence of intrusion was Kemnay, Tillyfourie and Torphins, Crathes, Hill of Fare. The Aberdeen and Clinterty masses do not have overlapping boundaries with any of the other intrusions but marked petrological and geochemical similarities (Figure 26)). Data from Porteous, 1973b and Walsworth-Bell, 1974." data-name="images/P999791.jpg">(Figure 24) suggest that there are genetic affinities between the Aberdeen and Kemnay masses and between the Clinterty and Crathes masses. The relatively widespread evidence of post-magmatic recrystallisation in the Aberdeen and Kemnay masses, and the similarities that the rocks of these two masses display to the granite in the migmatitic vein complex in the Aberdeen Formation, are additional features suggesting that they were emplaced at an early stage in the igneous sequence. Isotope studies (Halliday and others, 1979) also show that the rocks of the Aberdeen mass display features, such as high 87Sr/86Sr initial ratios and the presence of inherited zircons, that are thought to be characteristic of the earlier (pre-414 Ma) granitic intrusions in the Scottish Highlands. The inferred relatively youthful age of the Hill of Fare mass is also supported by the lack of any evidence that it is cut by the suite of felsitic and lamprophyric dykes and sheet which intrudes all the other granitic rocks (Bisset, 1934a).

The chemical analyses of the rocks in the different masses display broad patterns of variation which can be largely related to differences in mineralogy, with, for example, the granodiorites and diorites being richer in CaO, MgO and Fe-oxides, and poorer in alkalies than the granites, and with the muscovite-biotite-granites having higher values of normative corundum than the other rocks. However, the application of the techniques of cluster and discriminate analyses to the large body (c. 150 analyses) of chemical data, has not only shown that the muscovite-biotite granites have a distinctive geochemistry (see above), but has also established that four other groups of rocks with different geochemical characteristics can be identified-the Hill of Fare mass, the Crathes and Clinterty masses, the Tillyfourie mass and the Torphins mass (Walsworth-Bell, 1974). It seems probable that these subtle geochemical differences are a reflection of petrogenetic differences between the different groups.

It is concluded, therefore, that the sequence of events during the formation of the major granitic intrusions in the Aberdeen area commenced with the emplacement of the early muscovite-biotite granites which form the Aberdeen and Kemnay masses and the migmatitic vein complex, followed by an episode, or episodes, when the hornblendic rocks of the Torphins and Tillyfourie masses were emplaced. An episode of granodiorite emplacement leading to the formation of the Crathes and Clinterty masses was then followed by the intrusion of the biotite granite of the Hill of Fare mass.

In general, the granitic rocks of the Aberdeen area do not display any truly distinctive mineralogical and geochemical characteristics (Figure 26)). Data from Porteous, 1973b and Walsworth-Bell, 1974." data-name="images/P999791.jpg">(Figure 24), (Figure 26)). Fe2O3 is expressed as FeO and included in 'FeO total'. Field of biotites associated with muscovite, topaz etc. from Albuquerque, 1973. Data from Walsworth-Bell, 1974. " data-name="images/P999792.jpg">(Figure 25) and are essentially similar to the granitic rocks that occur elsewhere in the Caledonides of Scotland and Ireland. The similarities extend to the general sequence of intrusive events, particularly the sequence from relatively early hornblendic to late granite intrusions, which can be matched with the sequence recognised in many other Caledonian complexes (e.g. Garabal Hill; Nockolds, 1941).

Subsidiary igneous activity associated with the major granitic intrusion

The rocks of the large granitic masses are cut by veins of aplite and pegmatite, which generally do not appear to extend beyond the boundaries of the instrusions, often have ill-defined gradational margins, and probably represent the products of crystallisations of the final residues of the granitic magmas. Sheets and dykes of felsite and lamprophyre which cut many of the granitic rocks have less obvious genetic relationships to the major intrusions as they also occur widely in the country rocks (Figure 23). However the felsites and lamprophyres were probably emplaced prior to the final episode of granitic activity in the Aberdeen area, as they are not known to cut the Hill of Fare mass (Bisset, 1934a) and it is concluded that they are related in origin to the major intrusions, as is the case in many of the other Caledonian granitic complexes in the Scottish Highlands (e.g. Ben Nevis and Glencoe; Bailey, 1960).

A body of breccia on the Kincardine coast at Souter Head [NJ 962 018] includes fragments of lamprophyre (Plate 25), but is cut by intrusions of felsite, and is probably also a product of the episode of the igneous activity that produced the suite of felsitic and lamprophyric intrusions.

Lamprophyric intrusions

Altered, generally dark-coloured igneous rocks occur at a number of localities in the map area, mainly as dykes with trends varying between NW–SE and NE–SW (Figure 23). Most of these rocks contain phenocrysts of plagioclase and of hightly altered ferromagnesian minerals. The chemical analyses of a specimen from within the map area (790 [NJ 836 122]) and of two specimens from the area immediately to the west (Bisset, 1934, analyses I and II) are closely similar, and show that all three rocks are relatively siliceous (c. 61 to 63 per cent SiO2). This chemical characteristic, when taken in conjunction with the presence of plagioclase phenocrysts in these rocks, could be interpreted as indicating that they are porphyrites rather than lamprophyres (cf. Rock, 1977). However, the three analysed specimens are somewhat richer in Fe-oxides, MgO and CaO, and poorer in alkalies than the porphyrites in other Caledonian igneous suites in the Scottish Highlands (e.g. Groome and Hall, 1974), and some specimens display pan-idiomorphic texture. It is concluded, therefore, that this group of intrusions has stronger affinities with lamprophyres than with porphyrites.

The rocks in this group are well exposed at only two localities in the map area; at the entrace to Little Clinterty Quarry [NJ 836 122] and on the foreshore in the bay south of Cove Harbour [NJ 9545 0055] (Plate 7). At Little Clinterty fine-grained, reddish brown rock containing feldspar phenocrysts and aggregates of ferromagnesian minerals occurs as a NE–SW dyke which is several metres wide and has dark, very fine-grained, chilled margins (1073) against the granodiorite of the Clinterty mass. The typical rock from the interior of this dyke (790, the specimen analysed chemically) has pan-idiomorphic texture, with plagioclase (c. An25–30) laths ranging from 2 mm to 0.5 mm in dimensions, predominating over altered ferromagnesian phenocrysts. The latter were probably amphibole originally and form approximately 15 to 20 per cent of the rock. Dark olive-brown biotite is abundant, occurring in small (0.5 mm) crystals which, in part, overgrow the ferromagnesian phenocrysts, while clusters of grains of opaque constituents are also conspicuous.

On the Cove foreshore the lamprophyric rock occurs in an intrusion that is up to 2 m thick and has a markedly irregular form, being dyke-like and discordant to the flat-lying lithological banding in the surrounding metasediments at some localities, sill-like and concordant at others (Plate 7). This rock is fine-grained (c. 1 mm) and often weathers in an irregular fashion to yield a pitted surface. Thin sections (637) show that ferromagnesian phenocrysts have been completely replaced by aggregates of pale amphibole and chlorite and are set in a groundmass of highly irregular crystals of feldspar, which sometimes show an approach to radiating growth and are often highly altered, and elongate biotite laths.

Other specimens with lamprophyric affinities include a relatively coarse-grained (2 to 3 mm), hornblende-plagioclase rock (694) found in Craigingles Quarry [NO 880 993], and a decomposed mafic rock exposed as a narrow (c. 1 m), NW–SE dyke in a small quarry north of Brimmond Hill at [NJ 853 098]. Narrow (c. 1 m) dykes of similar appearance were also encountered in trenches at two localities, having a NW–SE trend in one instance ([NJ 811 127], immediately west of the map area) and a N–S trend in the other [NJ 931 109], while a drill hole near Pitmedden (1882* [NJ 913 277]) encountered a body of lamprophyric rock of unknown configuration (Figure 23).

Felsites

Fine-grained, leucocratic rocks, which are generally pink and sometimes contain only a few small phenocrysts of quartz and feldspar, occur widely within the Sheet 77 area (Figure 23). They are most abundant in the areas south of the Dee, and west of Grid line 85 near the Clinterty mass, but have not been recorded from the areas of the Ellon and Collieston formations lying within the map boundaries. The felsites generally occur in subhorizontal, sill-like intrusions, hut, are dyke-like at some localities (e.g. Dancing Cairns Quarry [NJ 901 091]; Tyrebagger Hill [NJ 844 124]). A particularly well-exposed example of a sub-horizontal intrusion occurs on the Kincardine coast at the Cave of Red Rocks [NJ 965 028] (Plate 24), where the felsite forms a sheet that is c. 5 m thick and is roughly concordant with the flat-lying lithological banding in the surrounding metasediments. This body can be traced for 300 m in the cliffs, thinning to a wedge-like termination in the south, and descending to a lower level by means of a dyke-like apophysis before terminating abruptly after a few tens of metres in the southern cliffs of Long Slough [NJ 965 029] in the north. Banding defined by colour variations occurs in narrow (c. 0.2 to 0.3 m) zones at the upper and lower contacts of the felsite. This banding is generally orientated parallel to the prevailing trend of these contacts but displays folds and convolutions near irregularities in the boundary of the intrusion. A similar, relatively thick (c. 10 m) sill-like intrusion with flow-banded marginal zones occurs in a quarry near Parkhead [NO 852 992], and a series of thin (c. 2 to 3 m), flat-lying lenses of felsite can be recognised in the south-western part of Craigingles Wood c. [NO 877 995]. The felsites are seen to intrude metamorphic rocks in most of the exposures, but cut the granitic rocks of the Aberdeen mass at Dancing Cairns Quarry [NJ 901 091] and of the Clinterty mass at Tertowie [NJ 818 101].

Thin sections show that the rocks are composed largely of quartz, oligoclase and potash feldspar, with all three minerals generally being present as idiomorphic to subidiomorphic phenocrysts ranging up to 3 mm in size, and also as constituents of the granular groundmass (Plate 22)g. In one specimen (642 [NJ 960 016]) the quartz and feldspar phenocrysts are mantled by granophyric overgrowths. The phenocrysts are often sparsely distributed (e.g. 2340 [NJ 852 116]) but sometimes form 40 per cent or more of the rock (e.g. 644 [NJ 961 017]). The term felsite seems appropriate for the aphyric speciments, but the more porphyritic examples are possibly better described as quartz porphyries.

Ferromagnesian minerals are virtually lacking in most of these rocks but biotite is a minor constituent in some specimens (e.g. 2340; 691 [NO 879 995]) and occasional small, spongy crystals of garnet occur in others (e.g. 642 [NJ 960 016]. Muscovite has been found as a phenocryst in one rock (644) (Plate 22)g. None of the felsite specimens from the Sheet 77 area has been analysed chemically, but the analyses of similar felsites from near Midmar (Walsworth-Bell, 1974, BC125) to the west of the Sheet 77 area, and Crossley to the south of that area (Guppy and Sabine, 1956, 628) show that these rocks are highly siliceous (c. 76 to 77 per cent SiO2) and rich in Na2O and K2O (c. 4 per cent of each oxide).

Bisset (1934b, p. 145) recorded two instances where felsite are cut by lamprophyric rocks in the area to the west of Sheet 77, but the relations at Souter Head (see below), where a breccia containing lamprophyre clasts is cut by felsite, show that some felsites post-date lamprophyres. It is possible, therefore, that there were several episodes of felsite intrusion, and possibly also of lamprophyre intrusion, in the Aberdeen area.

The Souter Head breccia

A breccia consisting largely of blocks of metasediment, amphibolite and migniatitic granite which outcrops (Figure 23) over a distance of c. 500 m on the Kincardine coast near Souter Head [NJ 962 018] has been described by Porteous (1973c). The clasts in this breccia show close similarities to the undisturbed rocks of the Aberdeen Formation that outcrop on the coast to the north and south, and generally consist of angular blocks ranging in size from a few tens of centimetres to several tens of metres (Plate 25). These blocks show no preferred orientation, and although they often have a complex morphology, they form a close-packed, interlocking aggregate with little or no interstitial matrix. Finer, apparently comminuted material occasionally can be recognised between the blocks, but interpretation of relations is handicapped by the recrystallisation and alteration that has affected this interstitial material. The breccia contains lamprophyric clasts (640, 681) resembling the rock in the intrusion on the Cove foreshore (637), and is cut by highly irregular intrusions of felsite, and breccia formation must have occurred in the interval between the emplacement of this particular suite of lamprophyres and the intrusion of these felsites.

A large coherent mass of foliated muscovite-biotite granite in the centre of the area of brecciated rock was identified as an intrusion into the breccia by Porteous (1973c), who considered that the isolated blocks of breccia within the marginal areas of this granite mass were xenoliths. However, the trend of the foliation in this granite mass is virtually constant throughout the outcrops, and at some localities this structure intersects contacts with the breccia at a large angle. Detailed relations at these contacts are often ambiguous, but in many instances suggest that the coherent granite mass is fringed by a zone of brecciation in which granitic clasts predominate and the clasts of other rocks (the 'xenoliths' of Porteous) are relatively subsidiary.

These features, and the similarities that this granite displays to the rocks in the migmatitic vein complex which are cut by the breccia, suggest that this granite mass is of prebreccia age. It is possible that the massive character of this granite body made it resistant to disruption, and led to its survival as a large, coherent body within the more easily fragmented metasediments and migmatitic gneisses.

The occurrence of unbrecciated metasediments approximately 200 m from the coast shows that the breccia does not extend far inland, and it is possible that the total area of brecciated rocks on shore does not greatly exceed the present day exposures. Restricted areas of breccia are known to occur in association with minor intrusions of Caledonian age elsewhere in the Highlands (e.g. Wright and Bowes, 1979) and the lamprophyre clasts and intrusive felsites suggest that the Souter Head rocks belong to a similar association. The formation of the breccia can probably be ascribed to the escape of magmatic gases at high pressure, and the former presence of this fluid may be reflected in the recrystallisation and alteration of the sparse matrix of the breccia.

Chapter 10 The Old Red Sandstone

Red, unmetamorphosed sedimentary rocks underlie the eastern part of the city of Aberdeen and are also found for 7 to 8 km to the north of the city at several localities near the coast, (Figure 27). No fossils have been found in these rocks but they show a marked similarity to the Old Red Sandstone rocks of proven Devonian age that occur at a number of isolated localities elsewhere in the Grampian area (Johnstone, 1966).

The only exposures of rocks of this facies within the city boundary are on the southern bank of the Don between the Brig O' Balgownie and the Bridge of Don (c. [NJ 941 095] to [NJ 943 094]), but similar lithologies have been found in a considerable number of temporary exposures and drill holes in the city area. The account by Milne (1902) provides details of a number of the occurrences in the city area. Only two exposures have been found in the coastal area to the north of the city; at Millden [NJ 963 163], where the outcrop is now obscured, and at Skelly Rock [NJ 967 145], but additional information has again been obtained from drill holes in this area.

Aberdeen city

At the Brig O' Balgownie the sedimentary rocks can be seen unconformably overlying the steeply dipping metasediments of the Aberdeen Formation. Coarse breccia with metasedimentary clasts up to 0.3 m in dimension set in a red, silty or argillaceous matrix, forms the basal deposits in some places, but the majority of the exposures in this area, including a cliff face c. 5 m high, are of conglomerate containing well rounded clasts. At most localities the clasts consist predominatly of psammitic metasediments, but clasts of highly decomposed granitic rocks showing a close similarity to the typical rock of the Aberdeen mass (Chapter 9) are also common, and are sometimes more abundant than those of any other rock type. The clasts commonly show a considerable range in size, and include blocks up to 1 m in diameter, but the majority are much smaller, with maximum dimensions of no more than 0.3 m. Many of the clasts, particularly the fragments of metasedimentary rock, are discoid and lie with their longest dimensions subhorizontal. In most outcrops the clasts are supported by an arenaceous matrix, which often has a highly feldspathic, 'granitic' appearance and shows patchy colour variations from red to grey, but occasional horizons have an abundant (30 to 40 per cent of the rock) matrix of red silt or clay.

Ill-defined arenaceous layers or lenses up to 1 m thick occur within the conglomerates and, in turn, sometimes contain impersistent argillaceous horizons up to 100 mm thick. These layers dip at gentle angles (c. 10 to 20°) to the south, in parallelism with the irregular, flaky cleavage or fissility that is often visible in the more argillaceous parts of the matrix of the conglomerates.

Conglomerates are usually the most abundant rock type, but there are occasional records of arenaceous and argillaceous horizons (e.g. in a drill hole in the Upper Denburn [NJ 9356 0642]) while a drill hole at Innes Street [NJ 9404 0676] was almost entirely in sandstone at depths between 45.5 and 99 m below the surface. The conglomerates temporarily exposed in Fountainhall Road c. [NJ 922 063] to [NJ 923 060] and in the former railway goods yard at Kittybrewster [NJ 931 080] are of interest as the clasts in these rocks are set in a coarse (5 to 10 mm) calcite cement, suggesting that at these localities the sediment was well-washed and highly porous prior to burial and lithification.

The position of the western limit of the area of sedimentary rocks between the Dee and the Don can be defined only approximately because of the paucity of exposures. However a sinuous, roughtly N–S boundary can be located on the map (Figure 27) by the exposures of sedimentary rocks at Brig O' Balgownie [NJ 941 095]; by the temporary exposures and drill holes in these rocks at Kittybrewster [NJ 931 080]; Hutcheon Street, railway tunnel [NJ 9343 0695]; Upper Denburn;[NJ 9356 0646]; [NJ 9362 0641]; Denburn [NJ 9392 0612]; The Green [NJ 9404 0613]; Bon Accord Crescent [NJ 936 056]; (10 m sandstones and conglomerates overlying granite); Wellington Suspension Bridge [NJ 9426 0501], [NJ 9427 0504]; by the exposures of muscovite-biotite granite in a railway cutting [NJ 932 083]; and by temporary exposures and drill holes in granitic rocks at Berryden Road [NJ 9323 0730]; [NJ 9319 0721]; Golden Square [NJ 9374 0612]; Station Hotel [NJ 9411 0602], Salisbury Place [NJ 9254 0491]. The conglomerates west of this boundary in Fountainhall Road c. [NJ 923 060] probably form only a small outlier as granitic rocks occur to the east in Victoria Park [NJ 928 067] and Chapel Street [NJ 934 060].

The drill holes in conglomerate at the Wellington Bridge probably lie very near the southern margin of the main area of sedimentary rocks as three drill holes within and to the east of the river [NJ 9432 0504]; [NJ 9436 0504]; [NJ 9436 0500] encountered granitic and gneissose rocks, and migmatitic metasediments are exposed on both banks of the Dee, immediately to the south [NJ 9428 0475]; [NJ 9435 0485]. Further to the east, conglomerates were found in drill holes at the harbour entrance [NJ 9594 0576]; [NJ 9596 0574] immediately to the north of a drill hole [NJ 9603 0564] in migmatitic metasediments, and it is concluded that the southern boundary of the sedimentary rocks underlying the city probably trends ENE–WSW, close to the present course of the lower reaches of the river. As discussed below (Chapter 12) it seems likely that this boundary is defined by a fault.

In the north, near the Brig O' Balgownie, the varying elevations of the exposures at the base of the sedimentary rocks in the southern bank of the Don show that the sediments were originally deposited on a surface with topographic irregularities at least several metres in depth.

The absence of sedimentary rocks in the northern bank of the Don at this locality, even at levels appreciably higher than the exposures of the unconformity on the southern bank, may be evidence of even larger scale irregularities in the pre-Old Red Sandstone surface. Further to the south additional evidence of major irregularities in the basal surface is possibly provided by the occurrence of the outlier of sedimentary rocks in the Fountainhall Road area [NJ 923 060], and by a drill hole at Charles Court [NJ 9411 0643] which encountered only granite, even though this locality lies several hundred metres to the east of the presumed western boundary of the sedimentary rocks (Figure 27).

Most of the information on the Old Red Sandstone rocks underlying Aberdeen city has been obtained from localities near their western margin because there is a thick cover of Quaternary deposits near the coast. However, a drill hole under the eastern part of the city near Queens Links [NJ 9517 0655] which failed to reach the base of a sequence of conglomerates at a depth of 192 m, possibly provides evidence of an eastward thickening of these rocks.

The area to the north of Aberdeen

Regionally metamorphosed rocks were formerly exposed on the north bank of the Don immediately east of the Bridge of Don [NJ 947 095] and psammitic metasediments are occasionally visible on the shore near the low water mark approximately 3 km further north [NJ 961 123]. However, as a drill hole between these localities [NJ 946 110] encountered 16 m of conglomerates overlying metasediments, and Old Red Sandstone rocks occur even further north on the foreshore at Skelly Rock [NJ 967 145], the western boundary of the sedimentary rocks in this area must be irregular and lie in part on land, in part offshore (Figure 27).

To the north of Skelly Rock there is evidence that sedimentary rocks underlie a considerable area near the coast, as conglomerates were formerly exposed at Millden [NJ 963 163] and two shallow drill holes nearby encountered red sandstones, which in one instance are coarse-grained [NJ 9584 1683] and in the other, are micaceous with a patchy calcareous cement [NJ 9587 1684]. A seismic investigation at Eigie Links [NJ 972 172] (Ashcroft and Boyd, 1976) also shows that the overburden is underlain by a layer at least 100 m thick with a seismic velocity (3.9 km/s) similar to that of Old Red Sandstone sedimentary rocks. In this area magnetic anomalies associated with the rocks of the Belhelvie mass are 'smoothed' (i.e. show smaller horizontal gradients than elsewhere), a phenomenon ascribed by Ashcroft and Boyd (1976) to the burial of the igneous rocks under Old Red Sandstone sedimentary rocks. Ashcroft and Boyd (1976) concluded that the north-western boundary of this area of sedimentary rocks lies close to the important NE–SW discontinuity that has been identified cutting through the Belhelvie mass (the Belhelvie Fault).

Investigations offshore (Institute of Geological Sciences, 1977a) have shown that extensive areas of sedimentary rocks adjoin this part of the Aberdeenshire coast and the onshore occurrences in the map area probably represent the most westerly representatives of these rocks.

The outcrop at Skelly Rock is the only exposure of Old Red Sandstone sedimentary rocks that is now visible north of Aberdeen in the map area, and even at this locality the rocks are frequently obscured by present-day beach deposits. The rocks here are poorly sorted and contain rounded and subangular clasts, ranging in size from 0.5 m to 3 mm (or less), set in a silty or argillaceous matrix, which is so abundant (50 to 60 per cent of the rock) that Bremner (1939) was led to believe that this was an exposure of till. The clasts consist of a great variety of rock types, including muscovite-biotite granite resembling the rock of the Aberdeen mass, deformed 'Younger Basic' rocks, a variety of metasediments and small fragments of unmetamorphosed silty sedimentary rock. The matrix is reddish brown and, in places, contains calcareous veins and patches. An impression of imperfect subhorizontal bedding is conveyed by indistinct colour variations in the matrix and by the preferred orientation of discoidal clasts.

The characteristics of this rock suggest that it may have been formed as a mud flow deposit. In contrast, the other Old Red Sandstone rocks in the Aberdeen area are essentially similar to the rocks in the other Old Red Sandstone occurrences in the Grampian area (e.g. Turriff, Rhynie; Read, 1923b) and have probably formed as high-energy, aqueous deposits during the rapid burial of a land surface with considerable topographic relief.

Chapter 11 Quartz dolerite dykes

Dolerite dykes with an ENE–WSW trend have been recorded from a number of localities in Aberdeenshire (Read, 1923b; Buchan, 1932), and have been identified (Buchan, 1932; Walker, 1935) as members of the suite of late Carboniferous tholeiitic intrusions that occurs widely in central Scotland and north-east England. Two of these dykes are recorded in the early version of Sheet 77, at Bridge of One Hair on the Kincardineshire coast [NJ 970 039] and 7 km inland near Townhead at [NJ 903 007] (Figure 28), but only the first of these occurrences is shown in the 1:253 440 geological map (Sheet 9) of the area.

The remapping of Sheet 77 has established that dolerite dykes of this suite are exposed at a number of other localities on the Kincardineshire coast, (Blowup Nose [NO 947 987]; Clashrodney [NO 946 992]; Cove Harbour [NJ 954 005], [NJ 957 008]; Souter Head [NJ 962 019]; Seals Hole [NJ 963 022]), and also inland in Kincorth Quarry [NJ 935 025] and in an exposure on the southern slopes of Tyrebagger Hill [NJ 846 119] (Figure 28). In addition, it has been found that linear magnetic anomalies ranging up to 1500 nT in size, which are outstanding features of the magnetic map of the northern part of Sheet 77 (Figure 4a); (Figure 15), top left), are due to the occurrence of unexposed dolerite dykes. The presence of dolerite has been confirmed by drilling at some localities ([NJ 9419 2070], [NJ 8632 2137], [NJ 9189 2509], [NJ 9189 2507], [NJ 9941 2665], [NJ 9941 2664], [NJ 9870 2514], [NJ 9904 2534]) and at others by trenching ([NJ 849 255], [NJ 839 203]) (Figure 28). Large, negative magnetic anomalies that occur at the southern margins of these dykes show that the polarity of the igneous rock is reversed (Figure 4a).

The dykes seen in natural exposures are generally between 1 and 10 m wide, but the dyke at the Bridge of One Hair [NJ 970 039] is approximately 15 m wide (Plate 26) and drill holes near Monkshill ([NJ 9189 2507], [NJ 9189 2509]) show that these intrusions can have widths of more than 20 m. On the Kincardineshire coast the dykes often strike NE–SW and since this is also the trend of the coastline, it is possible that several of the occurrences between Cove and Seals Hole are actually of a single intrusion. However the dykes in this area sometimes have an E–W trend (e.g. at Souter Head [NJ 962 017]-see Porteous, 1973c, fig. 1) and at Cove a 10m wide dyke with a NE–SW trend north of the harbour [NJ 957 007] becomes thinner and follows an E–W trend further to the south [NJ 954 005]. Even more marked irregularities and sinuosities are shown by thin (1 m) portions of the dykes exposed at Blowup Nose [NO 947 987] and Seals Hole [NJ 963 022] where the intrusions locally follow gently inclined structures in the surrounding metamorphic rocks. The dyke exposed near Townhead at [NJ 903 007] also strikes approximately E–W, and although the magnetic mapping shows that the dykes in the northern part of the map area trend predominantly ENE–WSW (Figure 28), curvature of some of the anomalies shows that the trend of individual intrusions occasionally varies from E–W to NE–SW (e.g. near Udny Station [NJ 910 246] and Straloch [NJ 861 212]).

The magnetic mapping also shows that the 'dykes' in the north are not continuous bodies, but occur as a series of relatively short (generally 1 to 6 km) intrusions, which are sometimes co-linear, but are often arranged en échelon (Figure 28). Adjoining intrusions may show considerable lateral overlap when arranged en échelon (e.g. near Pitmillan at [NJ 976 247]), but in a number of instances the successive members in a series of intrusions are separated along strike by areas devoid of dykes (e.g. from [NJ 950 260] to [NJ 923 251]). However, in most instances the dolerites occur in closely grouped sets of intrusions and appear to have been emplaced on single fracture or on a series of closely-spaced, subparallel fractures. At least three series of intrusions can be distinguished in the area north of the Belhelvie 'Younger Basic' mass, but the two southernmost converge westwards and eventually appear to coalesce near Rashiebottom at [NJ 834 205] (Figure 28).

The magnetic surveys were too limited for the extent and number of the dolerites to be determined in the southern and central parts of the map area. However, these surveys have provided no evidence of dolerite dykes cutting the Belhelvie intrusion or occurring in the area immediately to the west of this mass. The magnetic evidence also shows that the dyke exposed at Bridge of One Hair [NJ 970 039] terminates abruptly approximately 1 km south-west of the clifftop exposures.

From the magnetic maps it can be seen that the dykes are often markedly transgressive to magnetic anomalies in the surrounding metamorphic rocks, and that at some localities (e.g. near the Hill of Minnes at [NJ 945 241] and near Ythan Lodge at [NJ 998 267]) the country rock magnetic anomalies are truncated or displaced at the dykes (Figure 4a), (Figure 28). These features suggest that the dykes have been emplaced along NE–SW-trending faults. This view is supported by field evidence at the Bridge of One Hair [NJ 970 039], where the unaltered dyke stands out as a resistant mass, flanked on either side by zones of erosion in shattered and altered metasediments (Plate 26).

The dykes consist predominantly of relatively fine-grained (1 to 2 mm) dolerite, with still finer-grained, chilled rock at contacts with the country rocks (e.g. Cove [NJ 957 007] and Townhead [NJ 903 007]). No evidence of thermal metamorphism has been found in the country rocks adjoining the dykes. The dyke rocks are non-porphyritic and devoid of flow structure, but sometimes contain vesicles up to 2 to 3 mm in size (e.g. at Townhead [NJ 903 007]). Idiomorphic laths of labradoritic plagioclase up to 2 mm in length, often intergrown in a subophitic fashion with smaller (c. 1 mm), irregular crystals of the pyroxenes, dominate the microscopic texture (Plate 22)h. The pyroxenes are generally pale brown to colourless and in many rocks (e.g. 1396*, [NJ 9189 2509]; 2130*, [NJ 839 203]) consist predominantly of pigeonite with only subsidiary augite. One specimen (2447* [NJ 849 255]) is distinctive in containing, not only augite and pigeonite, but also crystals of orthopyroxene which are partly mantled by the clinopyroxenes. Crystals of opaque ore are conspicuous constituents of the dolerites, sometimes form 10 per cent or more of the rock (e.g. 1396*) and frequently have a complex skeletal morphology. The crystals of the main rock-forming minerals are set in a mesostasis that is dark and glassy in the finer grained rocks (e.g. 2447*), but is often turbid and highly altered in other specimens (e.g. 2130*). However, in some less altered rocks (e.g. 1396*) the matrix consists of micrographic intergrowths of quartz and alkali feldspar. Needles of apatite are conspicuous in the groundmass of many rocks and interstitial crystals of red-brown amphibole occur in one specimen (1396*). The extensive alteration of many rocks has often resulted in the almost complete replacement of the lime-poor pyroxenes by chloritic alteration products, while the plagioclase crystals are frequently crowded with very fine-grained secondary phases such as sericite and calcite.

These mineralogical features enable the dolerites to be classified as tholeiites, and chemical analyses of three of these rocks show that either hypersthene and quartz (2441, Cove [NJ 957 007]; 1396* [NJ 9189 2509]), or hypersthene (2130* [NJ 839 203]), are present in the norm. These three analysed specimens from within the Sheet 77 area are all very similar to each other, but differ slightly from an analysed quartz dolerite from the Insch area to the north-west (Read, 1923b, p. 164), in being poorer in silica (c. 49 per cent) and richer in alumina (c. 14 per cent) than the latter (SiO2 50.78 per cent; Al2O3 11.37 per cent).

The dolerites in the Sheet 77 area are clearly similar to the tholeiitic intrusions of late Carboniferous age in central and southern Scotland and north-east England, in mineralogy and chemistry (Figure 29), (Walker, 1935; 1965; Macdonald and others, 1981). The dykes in these southern areas generally strike E–W but it is noteworthy that they follow faults at some localities, and occasionally occur en échelon (Walker, 1935; Francis, 1967). Isotopic age determinations (Fitch and others, 1970) show that these southern tholeiitic intrusions were emplaced in a relatively short interval during the Stephanian (c. 295 Ma). Two K-Ar determinations (by Dr R. M. Macintyre of the Scottish Reactor Centre, East Kilbride) on dykes in the Aberdeen area give somewhat younger ages of 281 ± 6 Ma in one instance (near Muirtack, north of Sheet 77 [NJ 995 388]), and 267 ± 6 Ma in the other (2448* Loanhead [NJ 9904 2534]). However, as both specimens are slightly altered, these should be regarded as minimum ages, and it is probable that the dykes in the Aberdeen area are members of the regional suite of Stephanian tholeiitic intrusions.

Chapter 12 Faulting

In areas of good exposure, such as the coastal strip south of Aberdeen, the rocks are often seen to be traversed by zones of shattering and crushing which have been preferentially eroded, and where distinctive lithological units are present it can sometimes be deduced that appreciable (10 to 20 m or more) displacements have occurred on these shatter zones (e.g. in The Kettle [NJ 953 004] at the north-eastern margin of the Cove granite, and possibly also in Long Slough [NJ 965 029] at the northern limit of the felsite intrusion of the Cave of Red Rocks (Figure 28)). The shatter zones have a variety of orientations, and this is reflected to some extent by the diversity of the trends of linear erosional features in the coastal areas (NE–SW, E–W, SE–NW). However, a steeply dipping NNW–SSE zone of silicified and brecciated rock, 0.5 m wide, which cuts through the Cove granite immediately north of Clashrodney [NO 947 992] has not been selectively eroded, and the erosional features possibly provide only incomplete evidence of the orientation of the faults in the map area.

There is also circumstantial evidence that large scale faulting has occurred in the map area (Figure 28). Thus, a number of features suggest that there is a major dislocation in the lower Dee valley, notably, the lineament on the gravity map of the Aberdeen area (Institute of Geological Sciences, 1977b), the relatively linear trace of the valley between Banchory and Aberdeen, and the abrupt truncation of the Crathes granitic mass in the vicinity of the river (Figure 23). Within the Sheet 77 area this major dislocation probably passes near Coalford, where shattered granitic rocks are exposed at [NO 817 998] (Figure 23), and can be extrapolated eastwards with little deviation in trend between the localities at the Wellington Suspension Bridge [NJ 943 050] and the harbour mouth [NJ 960 057] where it is known from drill holes that Old Red Sandstone rocks and metamorphic rocks occur in close proximity (Chapter 10, (Figure 27)). The relations of the Old Red Sandstone rocks at these localities suggest that the displacements on the fault have included downthrow to the north.

The NE–SW discontinuity-the Belhelvie Fault, recognised within the Belhelvie 'Younger Basic' mass (Ashcroft and Boyd, 1976) separates steeply dipping lithological units forming the northern part of the mass from more gently dipping lithological units in the south. If it is assumed that these lithological units were originally sub-horizontal (see Chapter 6), then movements on this dislocation would seem at first sight to have included rotation of the northern part of the mass with respect to the southern part about a N–S axis. However, it is difficult to explain all the relations on the fault in this fashion as the east and west boundaries of the intrusion have been affected in different ways, with the eastern boundary showing the expected deviation in trend (Figure 15), (Figure 16) on the fault, and the western boundary showing no appreciable inflexion on this dislocation. It seems more likely that the rotational movements occurred during the episode of shearing and mylonitisation that occurred within 40 Ma of the intrusion of the Belhelvie mass (Chapter 8), and that faulting subsequently brought together a northern part of the intrusion, which had been strongly affected by this episode, and a southern part, in which the effects of shearing were more limited.

The relations in the west suggest that in both parts of the intrusion the western boundary is defined by a major shear zone, and it is concluded that this boundary is apparently unaffected by the fault because it is steeply dipping and strikes perpendicular to the fault, and because movements on the fault were essentially vertical. The Old Red Sandstone rocks in the Belhelvie area terminate near the Belhelvie Fault (Chapter 10) suggesting that the sedimentary outcrops are truncated by this dislocation and that downthrow was to the south-east.

A prominent linear anomaly can be recognised on the aeromagnetic map of the area (Institute of Geological Sciences, 1968; Ashcroft and Boyd, 1976, fig. 5) extending north-eastwards from Belhelvie immediately east of the Buchan coast. The trend of this anomaly is similar to that of the Belhelvie Fault, and it is probable that the latter dislocation extends for c. 40 km to the north-east of Belhelvie and determines the linear trend of the Buchan coast between the mouth of the Ythan and Peterhead.

The Belhelvie and Deeside faults may be related in origin as these dislocations have similar trends (c. 050° and c. 070° respectively), and other faults of similar orientation may have controlled the emplacement of some of the quartz dolerite dykes (see Chapter 11). Faults with a north-westerly trend were recognised by Ashcroft and Munro (1978, fig. 6) in the portion of the Insch mass that extends into the Sheet 77 area, and it was inferred that such a fault defines the north-eastern boundary of this intrusion at Old Meldrum.

Chapter 13 Devonian-Quaternary events

Pre-glacial erosional features

The geological history of the map area between the Devonian and the Pleistocene cannot be determined in detail as virtually no record of the events in this interval has survived, apart from the evidence of post-Devonian faulting (Chapter 12) and of late-Carboniferous dyke intrusion (Chapter 11). Most palaeogeographical reconstructions (e.g. in Anderton and others, 1979) depict the area that is now northern Scotland as being a landmass for the greater part of this time, but it is probable that much of Britain, including the eastern part of Scotland, was submerged by the late Cretaceous (Chalk) Sea (Hancock, 1975). However, at the end of the Mesozoic much of the British Isles-Irish Sea area became land again, and it has been suggested (e.g. Linton, 1951; Walton, 1963) that a record of periodic uplift during the Tertiary is preserved in north-east Scotland as a series of erosional surfaces (plateaux) that developed at successively lower levels. It has been proposed (e.g. Gemmell, 1975) that one of these surfaces (the Buchan plateau) has an elevation of approximately 130 m, and it is possible that much of the gently undulating landscape between Aberdeen and the Moray Firth, including virtually all the map area apart from the higher ground in the northwest corner and near Tyrebagger and Brimmond hills, represents the dissected remnants of this plateau ((Figure 1); (Plate 1).

Deeply weathered rock

Considerable thickness of highly altered rocks are known to lie immediately beneath the Pleistocene deposits at many localities in Scotland (Fitzpatrick, 1963), and a number of examples of such deep weathering have been previously described from north-east Scotland (e.g. Phemister and Simpson, 1949; Fitzpatrick, 1963; Basham, 1968). During the remapping of the Sheet 77 area it was found that the highly weathered rocks are widely distributed, and, with much new data being available from trenches and from drill holes, a clearer impression has been obtained of the scale of the development of this phenomenon in the Aberdeen area than was possible in the past.

The altered rocks generally occur within restricted areas, but no consistent relationships between the degree of weathering and the lithology and topography can be detected, although psammitic sediments usually show little evidence of weathering, relatively fresh rocks often occur on hilltops (e.g. Hill of Minnes [NJ 947 237], Brimmond Hill [NJ 857 091]) and weathered rocks are sometimes more widespread in low-lying area (e.g. near Kinellar at [NJ 825 120] (Figure 30)). The trench sections have shown that rapid transitions from fresh to weathered rock often take place without any change in the nature of the bedrock or elevation of the rockhead. The trench exposures also show that the zone of weathered rocks is often more than 2.5 m thick, and, at a number of localities (e.g. at [NJ 8212 2625], [NJ 8687 2154], [NJ 8432 0890], [NJ 8847 0694]) drill holes have established that weathered rock sometimes extends to depths of 18 to 20 m or more, even when fresh rock is visible at the surface in nearby outcrops (e.g. at [NJ 824 261]). Many of the extensively weathered rocks still retain original structural and textural features, even although such specimens are often so decomposed that they can be crumbled manually.

Specimens from a weathered profile in a drill core at the eastern limit of the Insch mass (2450* [NJ 8200 2617]) were studied by McLean (1977) who found that serpentine, chlorite and a hydrobiotite/vermiculite complex become more abundant as relatively fresh anorthositic gabbro (at 20 m) gives way to more altered rock (at 18 m), while highly decomposed rock (from 6 m) contains much vermiculite and degraded chlorite. Basham (1968) observed similar changes in the main part of the Insch mass to the west and concluded that the development of these deeply weathered rocks required a warm, humid climate. Appropriate conditions may have prevailed in certain interglacial periods (Coope, 1975; Godwin, 1977) but it seems unlikely that the maximum of 15 to 20 m of weathered rock could have formed in these relatively brief intervals (c. 10 000 to 20 000 years). The deeply weathered rocks probably developed prior to the climatic deterioration that commenced in late Miocene times, therefore, possibly under tropical or subtropical conditions (West, 1977), and the present-day occurrences of these rocks may be the remnants of a formerly thick, and virtually continuous layer which has largely been removed by glacial erosion.

Quaternary

A layer of till overlies bedrock in much of the map area (Figure 30), while evidence of glacial erosion is visible at many localities (Figure 31), and it is concluded that the entire map area was glaciated during the Pleistocene. Deposits associated with glaciation include extensive bodies of sands and gravels laid down by meltwater and more localised occurrences of glacio-lacustrine and possibly of glacio-marine clays and silts (Figure 30), as well as till.

Evidence of glacial erosion

The widespread cover of superficial deposits in the map area handicaps any attempt to assess the scale on which glacial erosion has occurred. Nevertheless, there is abundant evidence that wide areas within the boundaries of the map have been subjected to glacial erosion. Thus, knolls of fresh rock with a smoothed appearance and elongate streamline form are present within areas of 2 to 4 km2 or more at certain localities (e.g. near Banchory–Devenick at [NJ 915 005]; near Belhelvie at [NJ 930 190]; near Ythan Lodge at [NJ 995 270]—see (Figure 31)). These knolls ('rock drumlins'), vary in height from 3 to 8 m, have length to breadth ratios ranging from 2:1 to 5:1 and usually have a single direction of elongation at a particular locality. They include knolls with irregular northern terminations near Doonie's Hill at [NJ 960 030] which can be identified as roches moutonees. Similar smoothed knolls also occur beneath a thin cover of till at a number of localities (e.g. near South Lasts at [NJ 830 038] and near Newmachar at [NJ 880 180]).

On a larger scale, much of the area between the valleys of the Dee and Don is characterised by 'streamlined' topography with the hills and spurs often being elongated approximately W-E, and it is also clear from trench exposures that bedrock profiles are often smooth and gently rounded over horizontal distances of several hundred metres. Shallow, poorly drained depressions with dimensions ranging up to 1 to 2 km (e.g. Harestone Moss [NJ 930 196], see (Figure 31)) which are underlain by fresh rock beneath only a thin layer of superficial deposits, appear to be rock basins produced by glacial erosion. Seismic traverses in the Ythan Valley show that bedrock is c. 10 m below present sea level at Waterside Bridge [NK 002 269] and that a wide (c. 500 m channel extending c. 40 m below present sea level occurs in the bedrock 1 km upstream (c. [NK 002 282], within the Sheet 87 area) and this overdeepening is probably due to glacial erosion. Overdeepening can also be recognised near the mouths of the Dee and Don, but, as is described below, probably developed in association with the formation of relatively narrow, steep-sided channels, which are unlikely to have been produced by ice action.

Evidence of abrasion accompanying the movement of ice is provided by the striae ranging up to 0.3 m in length and 3 mm in depth that are visible on many fresh bedrock surfaces. These striae are commonly aligned W-E in much of the map area (Figure 31), although there are examples orientated NW–SE in the north and SW-NE near the coast, particularly south of Aberdeen. Jamieson (1882) noted that striae trending S-N were superimposed on striae trending WSW–ENE in a outcrop near Cove at [NJ 948 005] (now buried as the result of the infilling of a quarry), but no other evidence of the relative ages of striae with different trends has been obtained.

The presence of clasts of andalusite schist in till in the north-west part of the map area, which must have been transported from outcrops lying further to the north-west, and of clasts of Old Red Sandstone rocks in till in the coastal zone, which have probably been derived from the south or south-east (Institute of Geological Sciences, 1977a) provides evidence of derivation of material from sources outside the map area (Figure 31). However, there is also abundant evidence that most of the material incorporated in the till in the map area was derived from the nearby rocks, with, for example, clasts of distinctive mafic and ultramafic igneous rocks being particularly abundant in till near the Insch and Belhelvie 'Younger Basic' masses. Trails of large clasts from these intrusions can be traced outwards for distances of 2 to 3 km, (Figure 31), and the survey of the soils of the Aberdeen area (Glentworth and Muir, 1963) has established that distinctive soils derived from the debris of basic and ultrabasic rocks (The Leslie and Insch associations) occur within the Sheet 77 area only in the vicinity of the Belhelvie and Insch Younger Basic masses. It is probable, therefore, that much of the till in the map area consists largely of locally derived material and that the formation of this deposit was accompanied by erosion of the underlying rocks.

Till

The layer of till overlying bedrock in much of the map area is generally 1.5 to 4 m thick, but is often thinner on hill tops and may be appreciably thicker in low-lying areas, particularly near the coast where till thicknesses may be 10 to 15 m or more (e.g. in the exposures in Nigg Bay [NJ 965 045], in drill holes in Aberdeen city at the Victoria Bridge [NJ 947 055] and in the Denburn area [NJ 936 064]). It was appreciated even in the earliest investigations (e.g. Jamieson, 1865) that the deposits of till in NE Scotland show considerable diversity, and the evidence of variation in the till was an important factor in leading Jamieson (1906) and Bremner (1934) to suggest that at least three separate glacial events had affected the area. However, whilst the remapping has confirmed that the till in the Sheet 77 area shows marked variations, the many new exposures, particularly the lengthy trench sections, show that there are no strong grounds for invoking a multiglacial hypothesis to explain the nature and inter-relations of the different types of till.

(a) Argillaceous basal till. The sections temporarily exposed in a trench between Mains of Kirkhill [NJ 864 121] and Broomhillock [NJ 923 200] showed that the widespread grey/brown, relatively arenaceous till described below (unit (b)) is locally underlain by a compact, more argillaceous deposit (Figure 32). This lower till is generally dark brown (Munsell colour chart number 10 YR 3/4) but may be blue-grey (5 PB 5/2) and gleyed, and often occurs in bedrock hollows as a basal layer up to 1 m thick. The boundary between the two tills is often sharp, and while lenses of sand and gravel may occur at this contact, or at the bottom of the till sequence against bedrock, arenaceous lenses or areas are generally lacking elsewhere within the argillaceous till unit.

The mineralogy of the sand-grade and clay-grade material and the shape and lithology of the clasts in the dark basal till and in the grey/brown till, are generally essentially similar, but, the basal till invariably contains a higher proportion of clay-grade material (c. 24 to 31 per cent). Fabric analyses show that the clasts in the basal till unit generally have a good preferred orientation (Figure 31) with many of the long axes of the pebbles plunging at low angles either to the SE (110° to 115°) or to the NW (290° to 335°).

(b) Grey/brown till. This is the most abundant and widespread type of till in the Sheet 77 area, being virtually the sole till unit in the west, and underlying the red till (unit (c), see below) at most localities in the eastern part of the map area (Figure 32). The grey/brown till shows considerable variations in the proportions of sand, gravel and clay-grade material over distances of no more than a few centimetres, both horizontally and vertically and has a markedly heterogeneous appearance as a result. The sandy areas are generally well-drained and yellow-brown (10 YR 5/5), and the areas richer in clay-grade material are generally dark brown (7.5 YR 4/5) or dark grey, particularly when the till is gleyed immediately above bedrock.

Temporary exposures near Westhill at [NJ 836 067], showed that areas of weathered rock are often overlain by, and appear to grade into, sandy till, and that areas of fresher rock are commonly sharply separated from relatively argillaceous till. This suggests that in part at least, variations in this till unit are related to variations in the degree of weathering of the bedrock. At some localities (e.g. near Dyce Airport at [NJ 864 117]), most of this till unit consists of unbedded, sand-grade material, devoid of large clasts. Lenses and impersistent layers of silt, sand and gravel, up to 1 m thick and several tens of metres long, also occur widely in this till unit, but are probably meltwater sediments as they are well-bedded and have sharply defined margins against the surrounding till.

This till unit generally contains 9 to 19 per cent clay-grade material and 33 to 52 per cent of material coarser than sand grade. The clay-grade material consists largely of quartz and feldspar ('rock flour'), with kaolinite the main clay mineral and biotite, muscovite and illite common. Chlorite is usually a subsidiary constituent but is abundant in tills overlying mafic igneous rock. Quartz predominates in the sand fraction which generally consists largely (80 to 90 per cent) of quartz and feldspar (Hart, 1941), with biotite, hornblende, garnet, iron oxides, and muscovite forming the remainder. The rock clasts generally range up to 1.5 m in diameter, in the majority of cases (80 to 85 per cent) being angular to sub-angular, but faceted pebbles also occur. Granitic rocks and psammitic and semi-pelitic metasediments, i.e. the dominant lithologies in the map area, are the dominant (80 to 100 per cent) clast lithologies, although clasts of mafic and ultramafic igneous rocks, and of andalusite schist are locally abundant (see above).

Analysis of pebble orientation shows that this till generally has a weak, variable fabric, although the long axes of the clasts often plunge at low angles (10 to 20°) to the west.

(c) Red till. This till occurs extensively in the coastal areas north of the Don, sometimes extending as much as 8 to 10 km inland (Figure 30), and is also found at a number of isolated localities on the coast south of Aberdeen. The red till has occasionally been seen directly overlying bedrock in trenches (e.g. near Tipperty at [NJ 969 275], (Figure 37)b) but in the north-east (e.g. in the continuous trench section between [NJ 941 222] near Darrahill and [NJ 980 275] near Mains of Tarty) it is generally present as a surface layer of fairly constant thickness (1.0 ± 0.4 m), underlain by a layer of grey/brown till (unit (b)) resting on bedrock (Figure 32). The western limit of the extensive area of red till in this north-east area is highly irregular, and ill-defined isolated areas of red till up to 1.5 m in diameter are visible within grey/brown till west of this boundary (e.g. near Ardo House at [NJ 929 208]) and near Tipperty at [NJ 969 277] isolated patches of each of these till units (i.e. grey/brown or red) can be found within the other.

The boundary between the two types of till is generally regular and sharp where red till overlies grey/brown till as a definite layer. However, where these two deposits have complex field relationships the contacts are often ill-defined and gradational. This is particularly evident at the boundaries of the isolated areas of red till on the coast south of Aberdeen (e.g. in Nigg Bay [NJ 965 045]) where the two tills merge into each other over several metres.

The red till shows some colour variation from its typical strong red-brown hue (5 YR 4/4), often being pale brown near the surface where it has been weathered, or being dark coloured and gleyed in the vicinity of bedrock. Though sand-grade material locally predominantes (e.g. in Nigg Bay) this till is normally rich in clay and silt-grade material (36 to 100 per cent) and generally contains fewer clasts than the other tills in the map area. This last characteristic is often reflected in the lack of drystone dykes at field boundaries in areas underlain by this deposit (e.g. in the vicinity of Newburgh).

The clay-grade material in this till contains a relatively high proportion of iron oxides (notably haematite) and hydroxides, but otherwise the clay, silt and sand-grade fractions are essentially similar mineralogically to the material of comparable grain size in the other tills (Hart, 1941). The majority of the clasts consist of angular and sub-angular blocks of metasediments and granite resembling the dominant rocks in the map area, but a small proportion (2 to 5 per cent) consists or rocks of an entirely different provenance. These include rounded, red-stained quartzites displaying chatter marks, which are indistinguishable from the cobbles in Old Red Sandstone conglomerates, as well as occasional fragments of mafic lava, sandstone and shale that are also likely to have been derived from Old Red Sandstone parent rocks. Jasper and chert fragments resemble material in the Highland Border Series adjoining the Highland Boundary Fault near Stonehaven, while occasional flints and the fragments of dolomitic limestone and chalk recorded from Blackdog [NJ 963 139] (Bremner, 1915) are probably Mesozoic erratics. Occasional small, pale-coloured calcareous fragments are thought to be comminuted shells. Fabric analyses (Figure 31) indicate that the long axes of clasts in the red till are often subhorizontal and aligned approximately N–S (345° to 015°).

The red till is rarely arenaceous and contains only occasional small lenses of bedded silts, sands and gravels, nevertheless, it is commonly found in close association with bodies of meltwater sediments (Figure 30). Thus, many of the sand and gravel mounds in the coastal area north of the Don have a thin capping of red till (see below), while the extensive body of lacustrine clays at Tipperty [NJ 970 268] is also overlain, and in part underlain, by red till ((Figure 37)b).

(d) 'Indigo boulder clay'. This deposit was originally identified by Jamieson (1906) in the Ellon area where he found that a dark coloured till containing fragments of the shells of boreal species of molluscs and erratics of Mesozoic shale was locally present beneath the widespread grey till of that area. The descriptions provided by Bremner (1934) of till found during the construction of South Anderson Drive in Aberdeen and by Hart (1941) of a till temporarily exposed at the Bridge of Dee [NJ 927 036] suggest that a till similar to indigo till may occur locally within the map area but no examples of such a deposit were observed during the remapping. Bremner described this deposit as interdigitating with the overlying grey till in the Anderson Drive occurrence, but the age relations of this 'indigo till' were not clearly established.

Meltwater channels

Evidence of erosion by meltwater is provided by channels which occur throughout the map area and which display features such as arched longitudinal profiles (Sissons, 1961; Price, 1963), initiation in cols without catchment areas, abrupt downstream termination (Sissons, 1958; Price, 1960), and close association with mounded and sheet-like deposits of sand and gravel. All of these channels are 'dry' or are occupied by 'misfit' streams. Twenty-one major channels or channel systems have been recognised in the map area ((Figure 33); (Table 5), as well as a large number of smaller channels. Almost all of the major channels have been cut largely in bedrock to depths of 5 to 10 m or more, and have steep sides (e.g. the Leuchar Valley, (Figure 33)-2, (Plate 27), and, in most instances, can be traced for distances of 1 to 8 km.

Four of the major channels or channel systems (Figure 33)-9, 13, 14, 17 are discordant to the topography and cross major divides, having arched profiles as they cross the ridges. This group includes the system (Figure 33)-9 in the Elrick Hill area c. [NJ855 100] which consists of a main channel in Tulloch col (Figure 34)a with an arched longitudinal profile, and a series of tributary valleys. A similar system which originates in Tyrebagger col c. [NJ 850 105] amalgamates with the Tulloch system on the lower ground to the east, and the combined system leads into a deep valley that joins the valley of the Don near Stoneywood [NJ 896 103]. Another system (Figure 33)-14 with an arched profile at [NJ 850 200] near Straloch develops a complex anastomosing pattern where it crosses a ridge, having the appearance of a series of channels flowing between rock knolls ((Figure 34)b).

Three other channel systems (Figure 33)-7, 8, 10 are discordant to the topography but do not have arched profiles, including the system (10) on the northern spur of the Hill of Marcus c. [NJ 850 140] where five tributaries amalgamate into two main channels incised into cols.

Other major channels (Figure 33)-16, 18 to 21 are concordant to the topography but are identified as meltwater channels because they are incised into the landscape, have 'misfit' streams or are 'dry', and have associated meltwater deposits. They include the channel of the Blackdog Burn (18), which can be traced at right angles to the coast for 4 km, is up to 50 m wide, is locally cut into the bedrock to a depth of 8 m, and contains meltwater deposits 1 km south of Potterton c. [NJ 950 145]. Essentially concordant channels can also be recognised in the valleys of the Dee and the Don (e.g. (Figure 33)-3,4,11), which often show a tendency to be subparallel to the rivers initially, but eventually turn and trend obliquely into the major valleys. The channel (3) near Murtle is typical, originating near the area of meltwater deposits at Westfield [NJ 852 038] and eventually joining the Dee valley where it terminates on the terrace of sand and gravel on the north side of the valley.

Two channels (Figure 33)-6, 11 have distinctive features as they are outlets from large, basin-shaped topographic depressions which contain bedded arenaceous and argillaceous sediments that are possibly of lacustrine origin ((Figure 30)). In one instance (Kinellar [NJ 825 120]) the channel (11) extends northwards for 2.5 km to the Don, in the other (Westhill [NJ 845 065]) the channel (6) can be traced to the south for 1.5 km.

The majority of the channels, including all the discordant channel systems, trend approximately W-E, though, as noted above, more northerly or southerly trends are followed by several of the channels that lie near the valleys of the Dee and the Don. Northerly trends are also developed by channels in the vicinity of the coast (Figure 33) particularly in the area south of the Dee (e.g. on Doonies Hill c. [NJ 966 040]). Many of the channels are partly cut in till and some also cut across the 'grain' of elongated mounds of meltwater sediments in the area north of the Don (e.g. (Figure 33)-19, 20 [NJ 956 150], [NJ 953 160]). The channels cut in bedrock are often virtually devoid of infilling material, apart from local meltwater deposits, such as are found in the Blackdog channel (18) near Potterton at [NJ 950 145].

In the valley of the Don a drill hole at Boat of Hatton [NJ 837 159] failed to reach bedrock 20 m below the present level of the river (Peacock and others, 1977), and downstream from Stoneywood [NJ 899 110] much of the valley is cut in bedrock and is relatively steep-sided. These features suggest that the present course of the Don in the map area is to some extent determined by the presence of deeply incised channels which were probably eroded by meltwater. However, the relations between the present course of the river and pre-existing channels are complex near Seaton Park [NJ 940 090] close to the mouth of the river. Here the Don abandons a W-E valley to follow a meltwater channel cut as a gorge through a ridge of metamorphic rocks towards the Brig O' Balgownie [NJ 940 097], even although the W-E valley can be traced as a well-defined feature across Seaton Park to the east into an area where drill holes have shown that rockhead is well below sea level (Figure 35).

Relations near the mouth of the Dee are also complex. Drill hole data show that bedrock lies more than 15 m below sea level at Bridge of Dee [NJ 928 036] (Peacock and others, 1977) and that a relatively narrow (c. 150 m wide) channel has been incised into bedrock to a depth of 10 to 15 m below sea-level near the Wellington Suspension Bridge [NJ 943 050] (Figure 35). These features suggest that large volumes of meltwater followed the lower Dee valley during the last deglaciation. However, it has also been established (Simpson, 1948) that a drift-filled channel diverges from the present valley of the river near the Duthie Park [NJ 940 040] and runs eastwards through the valley between Torry and Tullos Hill. Geophysical data (Law, 1962) suggest that this channel is cut in bedrock and is probably narrow and gorge-like. This channel can be traced to the east, where a drill hole on the shore of Nigg Bay [NJ 9650 0457], encountered bedrock 40.5 m below sea level, even although outcrops are visible only a short distance to the north and to the south, while offshore geophysical surveys have shown that a steep-sided channel in bedrock extends for at least a kilometre on the floor of the Bay (Figure 35). The channel infilling in Nigg Bay was shown to consist largely (c. 30 m) of till by the drilling, and it is possible that this channel is earlier than other meltwater channels in the map area and was formed subglacially at an earlier stage of the deglaciation, or is even a pre-Devensian feature.

The overdeepened channel under Seaton Park [NJ 940 090] near the mouth of the Don, and the steep-sided channel which drill holes have shown (Figure 35) to be cut in bedrock under the valley of the Denburn in the centre of Aberdeen [NJ 936 064], may also be early meltwater features as both channels are largely filled with till.

Meltwater deposits

Coarser-grained deposits (predominantly sand and gravel)

It has been noted that small lenses of bedded silts, sands and gravels are found in close association with till in the map area, being particularly common in the widespread grey/brown till unit (unit (b)). There are also larger, more coherent bodies of similar sediments throughout this area, in many instances of a size sufficient to be shown on the 1:50 000 map (Figure 30). These deposits frequently display features indicative of meltwater origin (Allen, 1971; Flint, 1971), and, in particular, are often overlain by, or interbedded with, till and show evidence of widespread post-depositional faulting, slumping and folding. The deposits are often well-sorted and are characterised by rapid horizontal and vertical variations in grain size with individual 'beds' having a lensoid rather than a layer-like form. The most extensive occurrences of these deposits are on the lower slopes of the Dee and Don valleys and in the coastal area (Figure 30), but there are also large areas of sands and gravels at c. [NJ 840 060] near Westhill and at c. [NJ 875 200] near Newmachar (see also Merritt, 1981).

The Don Valley

Meltwater deposits are widespread west of Parkhill [NJ 892 139] where terraces bounded by steep fronts can be recognised on both sides of the valley (Figure 33). A trench section through one of these deposits at a locality immediately west of the map boundary [NJ 786 178] shows that it is formed of variable sands and gravels with subhorizontal bedding disturbed by faulting and collapse near the front of the terrace. The sands and gravels wedge out above till on the side of the valley, but are overlain in turn by alluvium where they extend below the terrace front onto the valley floor. These features suggest that this is an ice-contact deposit.

A formerly extensive deposit (c. 0.5 km2) of sands and gravels near the Chapel of St. Fergus [NJ 875 150], has been largely removed by quarrying. This deposit originally formed a gently undulating plateau with elongated, upstanding hummocks which merged with the terrace to the east, and terminated abruptly in a steep slope above the river in the west. Exposures in the gravel pit show that at least 6 m of sands with large scale cross-bedding are overlain by 6 to 8 m of well-bedded sands and gravels with complex, collapse structures, which are in turn overlain by fine sand, with sub-horizontal bedding, filling depressions in the underlying sediments.

At one locality [NJ 8713 1496] in the pit, a plant bed some 320 mm thick was found at the contact between the uppermost sand/silt unit and the underlying gravel. Material from this plant bed has yielded radiocarbon ages of 11 550 ± 80 and 11640 ± 70 years B.P. (SRR-762 and 763, Harkness and Wilson, 1979, p. 254) but the deposit has now been quarried away. Contortions in this plant bed suggest that it was laid across buried glacier ice.

Other deposits on the flanks of the Don valley in the western part of the map area are generally associated with meltwater channels (e.g. near Newlands at [NJ 830 175]), and at Kinaldie [NJ 832 154] an alluvial fan can be recognised at the termination of the channel (Figure 33)-11 that drains the depression at Kinellar [NJ 825 120] (see above).

Downstream from Parkhill [NJ 892 139], meltwater deposits occur on only a limited scale in the lower valley of the Don (Figure 30). However, at Parkhill a wide (c. 1 km) zone of meltwater deposits can be traced eastwards up the side of the valley for c. 3 km to near Corby Loch [NJ 925 145] (Figure 36) where it merges with an extensive area of similar deposits near the coast. This zone appears to be the easterly extension of the extensive deposits in the valley west of Parkhill (Figure 30).

The Dee Valley

Terraces of sand and gravel occur extensively at similar elevations (c. 35 m) on both the northern and southern slopes of the valley. Outlying bodies of sands and gravels are present locally on the valley floor, and include the ridge that connects two separate sections of terrace near Templars Park [NJ 850 000], and the isolated mound near Milltimber at [NJ 853 008] that rises to the same elevation as the nearby terrace. A section (Simpson, 1955) through a terrace on the northern slopes of the valley near Garthdee c. [NJ 925 035] resembles the section in the terrace deposits at [NJ 786 178] in the Don Valley described above. Apart from these terraces, the meltwater deposits near the Dee valley are generally associated with tributary valleys and meltwater channels and include the extensive area of hummocky deposits near Coalford, at [NJ 830 000] and the wide train of mounds and eskers extending from near North Lasts at [NJ 839 042] through Murtle Den to the terrace near Milton of Murtle [NJ 872 020] (Figure 30).

Other inland occurrences

Mappable bodies of predominantly coarse-grained meltwater deposits outwith the valleys of the Dee and the Don occur near West Brimmondside at [NJ 851 078], where sands and gravels with folded and faulted bedding form steep-sided mounds, and west of Newmachar at c. [NJ 875 200], where hummocky topography is partly due to the presence of mounds of sand and gravel and partly to the presence of till mounds and bedrock knolls (Figure 30).

Coastal zone

Hummocky deposits cover much of the headland south of the Dee (Balnagask Golf Course c. [NJ 965 055]), and, further to the south extend south-southwest from near Nigg [NJ 948 027] to beyond the southern margin of the map area. These hummocks are up to 10 m high (e.g. Charlestown [NJ 941 007]), often have steep eastern margins and consist of highly variable silts, sands and gravels. Some of the mounds have a thin covering of red (e.g. Marywell [NO 934 992]) or brown till (till unit b) (e.g. Charlestown). In this same general area near the coast there are also patches of sand and gravel with no marked topographic expression, such as the deposits associated with the probable lacustrine sediments near Middleton Farm [NJ 952 038] (Simpson, 1948; see below) and the 5 m of sands and gravels which were found to overlie till in a drill hole in Nigg Bay [NJ 9650 0457]. Some of these gravels (e.g. at Clashfarquhar, just south of the map area [NO 920 952]) contain a relatively high proportion (c. 10 per cent) of clasts of Old Red Sandstone rocks.

A layer of sands and gravels c. 5 m thick was also encountered in the Nigg Bay drill hole between the base of the till and the bedrock and may be the equivalent of the bed of sand and gravel which was formerly exposed at the base of the cliff section on the south-western shore of Nigg Bay (see Simpson, 1948, fig. 12) before it was obscured by dumped material. This cliff-base deposit is of particular interest as it contains clasts of 'Younger Basic' rocks, presumably derived from the north-west, and rare clasts of rhomb porphyry (Bremner, 1939) (and probably also of laurvikite (Bremner, 1934)), that appear to be of Scandinavian origin. It seems unlikely that these sands and gravels were formed during the melting of the ice sheet which deposited the till units exposed at present in the Nigg Bay area as the latter contain virtually no 'Younger Basic' and Scandinavian erractics (Bremner, 1934). It is possible therefore, that this sand and gravel horizon is the product of the wastage of an older (preDevensian) ice sheet.

The eastern part of the city of Aberdeen appears to be entirely underlain by varied silts, sands and gravels which form a layer often exceeding 5 m in thickness and overlie till. The deposits in this layer are often coarse-grained and poorly sorted, and finely bedded sands, silts and clays are very subsidiary.

Mounds and ridges of similar sediments are locally superimposed on this layer, and, despite the modification of the landscape by urban development, can still be recognised in the area of varied relief between the upper harbour [NJ 945 062] and Seaton Park [NJ 940 090]. Old maps of the city, such as Gordon's 1661 map, show clearly that this originally was a region of typical 'kame and kettlehole' topography. A major esker-like feature can still be traced from Marischal College through the Gallowgate to the Spital and the old maps show the former presence of kettlehole lakes, such as the Loch of Aberdeen [NJ 938 068] and the Loch of Aberdon c. [NJ 935 085].

North of the Don a narrow strip of meltwater deposits extends northwards from near Westfield c. [NJ 935 105] to near Shielhill at c. [NJ 940 125] where it merges with the extensive area (15 to 20 km2) of similar deposits stretching from the Don near Parkhill c. [NJ 890 140] to the coast near Blackdog c. [NJ 960 140] (Figure 30). Still further to the north the meltwater sediments can be traced in a narrow zone which extends from Potterton [NJ 945 165] to near Newburgh [NK 000 250], and then continues northwards to the map boundary near Waterside [NK 005 275] and beyond (Merritt, 1981).

Within these areas hummocky 'kame and kettlehole' topography is widely developed, with steep-sided mounds and ridges of gravel, sand and silt standing above a gently undulating landscape. The mounds may be 400 m or more long, 200 m wide and 25 m high (e.g. Bishops Loch [NJ 912 144]) while the ridges may be 8 to 10 m high and extend for 1 to 2 km (e.g. Drums [NJ 987 226]). The ridges and elongate mounds generally have a N–S or NNE-SSW alignment, but an E–W trend prevails in the area between the Don c. [NJ 890 140] and Corby Loch [NJ 924 145] (Figure 33). In the area immediately to the east and north-east of Corby Loch the shape of the mounds and ridges is complex and may reflect the simultaneous imposition of two trends of alignment (Figure 36).

The sediments forming the mounds and ridges have been extracted commercially on a large scale, producing extensive exposures which show that coarse sands and gravels normally predominate over finer sands and silts in these deposits. Some mounds contain bands of interbedded red clay (e.g. Milton of Potterton [NJ 944 162]) and occasionally (e.g. Campla Hill [NJ 931 150]) also contain red sands and silts. The deposits are generally well bedded and often display a variety of sedimentary structures, such as cross-bedding and ripple lamination, and in some mounds there are horizons containing armoured clay balls (e.g, Tramaud [NJ 946 133]). The bedding is generally horizontal in the core of the mounds but may show a gentle anticlinal warping (e.g. Belhelvie [NJ 949 175]) or be extensively folded and faulted near the margins (e.g. Bishops Loch [NJ 912 144]). Occasional beds show complex slumped structures (e.g. at Campla Hill [NJ 931 150]). Similar structural features can also be seen in the ridges (e.g. Drums [NJ 987 226]).

The mounds and ridges are sometimes completely covered with a layer of till (Plate 28) which may be 0.5 m or more thick (e.g. Pettens [NJ 971 199]) but other mounds are only partially covered, or are completely devoid of a till capping (e.g. Strabathie [NJ 958 134]). In the area north of the Don the nature of the till capping on the mounds and ridges changes as the boundary between the red and grey/brown tills is crossed. Clasts of Old Red Sandstone rocks occur occasionally in the coarser meltwater deposits, but only in the area where the red till unit also occurs.

The low ground between the mounds is sometimes underlain by till but more commonly by sands and gravels while red clays are found occasionally in these low lying areas towards the eastern margin of the meltwater deposits (e.g. near Fife Hill [NJ 958 147]). At two localities (Bishops Loch [NJ 912 144]; Shielhill [NJ 938 130]) mounds of sand and gravel rest directly on bedrock.

Predominantly fine-grained deposits

Clays and silts occur within the map area, not only as subsidiary units within the predominantly arenaceous and rudaceous meltwater deposits, but also in bodies that consist almost entirely of fine-grained sediments and are sometimes of sufficient size (c. 0.5 km2) to be shown on the 1:50 000 map (see (Figure 30)). These argillaceous deposits are often closely associated with till and/or coarser-grained meltwater sediments, sometimes contain isolated large clasts, and are often finely laminated.

Low-level coastal deposits (below 15 m above OD).

Extensive bodies of argillaceous sediments have been found at three localities within the boundaries of Aberdeen city (Torry [NJ 951 052]; Clayhills [NJ 941 055]; Seaton [NJ 946 087]) and at a fourth locality near Blackdog [NJ 962 139] on the coast north of Aberdeen. All of these deposits lie near present sea level and were formerly well exposed when they were being exploited as sources of brick clay, but the workings are now abandoned and the sections through the sediments are completely overgrown. However, the descriptions provided by Jamieson (1858; 1906) indicate that horizons consisting predominantly of clay occur between overlying and underlying sand and gravel units at all four localities. At Torry a 7 m thick argillaceous horizon consisting largely of red clay overlying grey clay was recorded; at Clayhills 6 m of brown clay with laminae of fine sand grades downwards into a 5 m thick unit of red clay overlying laminated grey clay with sand layers; nearly 5 m of brown clay with sand partings was found at Seaton; 3.5 m of green and red clay with partings and lenses of silt and sand was recorded at Blackdog. Bird skeletons, fish vertebrate and fragments of starfish (Jamieson, 1882) have been found in the Aberdeen city deposits, while fragments of Arctic species of molluscs have been obtained from the more arenaceous horizons at Blackdog (Jamieson, 1906).

Bremner (1915) found that small masses of dark, shelly till occur within the Blackdog deposit and as the fossils are usually fragmentary, it is possible that the entire fauna in the argillaceous sequences was derived from till of eastern origin which contained material from the sea bed. The deposits may therefore be entirely glacio-lacustrine in origin, although the possibility that they may be wholly or partly glaciomarine (Peacock, 1975) cannot be excluded.

Higher-level coastal deposits (above 15 m above OD)

Temporary exposures in trenches in the valley between Torry and Tullos Hill showed (Simpson, 1948) that predominantly fine-grained sediments are present over a considerable area near Middleton Farm [NJ 955 042]. These deposits lie about 15 to 20 m above sea level and probably have a maximum thickness of c. 8 to 10 m in the centre of the valley but become attenuated as the sides of the valley are approached. Three sedimentary units were encountered in the trenches; an upper brown unit composed mainly of silts and clays, a middle red unit consisting largely of fine sand and silt, but including several clay horizons, and a lower brown unit composed almost entirely of laminated clay. The red unit sometimes has highly disturbed bedding, often shows marked variations in thickness and frequently has irregular upper and lower surfaces. The bedding in the overlying brown unit is generally draped over the irregularities at the top of the red unit. Draping also occurs at the base of the lower brown unit where the lamination in the clay is flexed into an anticline above an underlying sand lens. Rock clasts 20 to 50 mm or more in diameter are scattered throughout the red unit and the upper brown unit. Soliflucted material overlies the entire series, while grey till (till unit (b)) occurs beneath the basal clays. In the east the deposits are truncated by a lensoid body of coarse, current-bedded sand.

North of Aberdeen, horizons of red clay are occasionally interbedded with the predominant sands and gravels in a number of the mounds of meltwater sediments (see above) at elevations of 15 to 20 m above sea level. A more extensive argillaceous unit (c. 0.5 km2) occurs near Eigie Links c. [NJ 965 165], where a body of laminated red clay up to 5 m in thickness is exposed close to outcrops of red till and red sands and gravels (Figure 30). Red clay also occurs extensively near Wester Hatton at [NJ 961 157] and at Fife Hill [NJ 958 147], where Synge (1956) has stated that the clay is horizontally laminated and contains marine shells.

One of the largest of these higher level coastal deposits is well exposed in a clay pit at Tipperty [NJ 971 268] near the northern border of the map area at an elevation of c. 15 to 20 m above sea level (Figure 37). This body of clays lies in the valley of the Tarty Burn, and seems to be topographically controlled in the north and west where the argillaceous sediments wedge out on the hillside above the underlying till (Figure 37)b. To the south and east however, the clays show a more limited decrease in thickness and terminate in a steep slope above the valley of the Tarty Burn. The red clay (5 YR 4/4) of this deposit generally rests directly on red till, but locally overlies grey/brown till. In places it is separated from the till by lenses of grey or red sand and gravel up to 0.3 m thick (Figure 37)b. Red till can also be seen overlying the clays in a small hummock immediately east of the main clay pit [NJ 972 268].

In the main clay pit 4 m of red clay is exposed above red till, being massive in the upper 2 m, probably because of pedogenic and biogenic disturbance, and well laminated towards the base (Figure 37)b. The laminae are horizontal, are between 1.5 and 3 mm thick, and show no evidence of disruption. The laminated sequence consists largely of units which commence with a basal erosion surface and show an upward gradation from silt to clay. 147 of these varve-like couplets were identified in the lower part of the exposure.

The mineralogy of the entire clay sequence is closely similar to that of the clay and fine silt fractions of the red till (Glentworth and others, 1964) but the Tipperty sediments tend to be richer in mica. Jamieson (1882) recorded a fish skeleton from this deposit and stated that small pebbles and fragments of shells occur throughout the clay. However, examination of samples by the Institute of Geological Sciences in 1970, showed that foraminifera and shell fragments are present only in rare arenaceous bands and are absent in the clay and silt horizons, once again suggesting that the fauna is derived and that the deposit is glaciolacustrine rather than glaciomarine.

Inland deposits

Trench sections show that small bodies of argillaceous sediments occur at a number of inland localitieswhich are often interlayered with till and meltwater sands and gravels. At [NJ 934 213] near Middle Ardo the clays occur in association with grey/brown till some kilometres west of the margin of the area occupied by red till (Figure 32), but all the other examples occur within the area of red till. The laminated red clay exposed at Middle Ardo was traced for c. 100 m in the trench, and clearly forms a small hummock overlying grey/brown till (Figure 32). To the north-east, at Darrahill [NJ 938 220], an upper red till unit is separated from a lower grey/brown till unit by 1 m of well-laminated red clays and red and grey silts, while near Kincraig at [NJ 958 248] a composite unit, 0.4 m thick, consisting of thin, interstratified red till, sand and laminated clay horizons occurs within an upper unit of red till. Elsewhere in this general area laminated red clay sometimes occurs resting on bedrock beneath the lower unit of grey/brown till (Figure 32).

Relatively fine-grained sediments also occur in the topographic depressions at [NJ 845 065] near Westhill, where surface layers of peat and gravel are underlain by up to 3 m of silts and clays and at Kinellar [NJ 825 120]. It is likely that these are post-glacial lacustrine deposits, differing in origin from the argillaceous deposits that are intimately associated with the till and the meltwater sands and gravels.

Interpretation of the relations of the Quaternary deposits

The hypothesis that NE Scotland had been subjected to several separate glaciations during the Pleistocene was developed by earlier workers (e.g. Jamieson, 1906; Bremner, 1938) as knowledge was acquired of the distribution and diversity of the glacial and meltwater deposits in NE Scotland and of the possible directions of ice movement. This hypothesis was later sustained and developed by Synge (1956), who, with Charlesworth (1956), introduced the concept that a considerable area in Buchan had been ice-free during the final glaciation (or glaciations). However, the validity of some of the supporting evidence was questioned by Simpson (1948; 1955) and a subsequent reappraisal of all the available evidence by Clapperton and Sugden (1977), which incorporated much new data on till characteristics and meltwater phenomena, has led to the formulation of a much modified interpretation of the Quaternary record in NE Scotland.

Clapperton and Sugden suggest that the till stratigraphy of earlier investigators is largely based on dubious correlations from one area to another and point out that no section showing that the different deposits are clearly of different age has been observed <span data-type="footnote">Bremner (1943, p. 18) stated that a temporary exposure near Balmedie Village showed that a lower till unit derived from the north-west is separated from an overlying red till unit by a peat horizon. No other section showing such relationships is known, however, and as a detailed description was not provided by Bremner, some doubts must remain as to the precise nature of the uppermost unit (the 'red till').</span>

Other weaknesses in the earlier interpretations arise from the assumptions that certain erratics (notably of 'Younger Basic' rocks) can be ascribed to specific source areas with certainty, that meltwater channels are invariably ice-margin or proglacial features, and that mounds of meltwater sediments can be regarded as terminal moraines. Clapperton and Sugden (1977) conclude that nearly all the glacial and meltwater phenomena in the Aberdeen area can be ascribed to a single late-Devensian glacial episode.

The very dark till that locally underlies the typical grey/brown till in the map area, may be equivalent to Jamieson's (1906) 'indigo boulder clay' and be the product of an earlier glacial episode when ice moved westwards from the floor of the North Sea (Bremner, 1928). The till-filled channels in the Tullos-Nigg Bay, Denburn and Seaton Park areas of Aberdeen (Figure 35) and the cliff-base gravels with Scandinavian erratics in Nigg Bay may also be preDevensian in age, but no other evidence of possible preDevensian glacial and meltwater events has been recognised in the map area.

Devensian glacial and meltwater phenomena

It seems probable that the Aberdeen area was essentially free of ice by 13 000 BP (Gray and Lowe, 1977), though buried glacier ice may have survived as late as, or even beyond, 11 500 BP, the radio-carbon age of the plant bed at St. Fergus [NJ 875 150]. The final major glaciation to affect this area must therefore have occurred at least in the late-Devensian, and probably had a maximum c. 17 000 to 18 000 BP (Sissons, 1976). The following relationships suggest that virtually all the glacial and meltwater phenomena in the map area can be ascribed to this glacial episode:

(i) Glacial features. The relatively argillaceous, basal till (till unit (a)) that has been recognised locally in the centre of the map area has the characteristics of lodgement till (Boulton, 1970) formed at the base of an ice mass. The strong NW–SE orientation of the clasts in this deposit suggest that the ice movement was south-easterly across the map area, and the transport of andalusite schist erratics into the north-western corner of the area and the formation of NW–SE-trending striae and ice-moulded topographic features elsewhere (Figure 31) can be related to this glacial event. The restricted occurrence of this deposit may reflect the localised development of the appropriate thermal regime at the base of the ice mass, but, as there is evidence that widespread erosion has accompanied the movement of ice from the west at a later stage in the glaciation, it is possible that only fragmentary remnants of a formerly extensive layer of lodgement till have survived erosion.

The poor fabric and variable, often arenaceous, character of the grey/brown till (till unit (b)) suggest that it formed primarily as a melt-out till when the ice eventually retreated. It might be anticipated that the processes of wastage would also lead to the development of flow till (Boulton, 1972) and it is possible that the grey/brown till is in part a flow till, despite the very poor orientation of the clasts. The weak fabric in this deposit suggests that ice movements were W–E, and there movements probably also produced the W–E glacial erosion features in the map area (Figure 31).

Although it has been inferred that the direction of ice movement changed between the formation of the basal till and the grey/brown till there is no evidence to suggest that the ice retreated during this period. It is possible therefore that these two tills formed at different stages in the history of a single ice mass, and that they comprise a lodgement till/melt-out till couplet (Drake, 1971).

In the coastal area it is clear from the orientation of striae and elongated rock knolls and from the characteristic fabric, colour and erratic content of the red till (till unit (c)), that the direction of movement of the ice has been predominantly from south to north. The relations of the superimposed striae near Cove show that, at least locally, the northwards movement of the ice had supplanted an earlier eastwards movement ((Figure 31) and Jamieson, 1882), and this evidence, in conjunction with the common occurrence of red till as a layer overlying grey/brown till in the north-eastern part of the map, might be taken as indicating that there is a considerable difference in the ages of these two tills. However, the widespread occurrence of gradational and interdigitated boundaries between the two deposits suggests that the two tills formed virtually simultaneously, possibly when two ice masses, from different source areas and with different directions of movement, coalesced near the present coastline (Figure 38). The superposition of striae in the Cove area and of red till on grey/brown till further north probably indicate that the movement of the north-flowing ice mass was more powerful in the later stages of the glaciation than that of the ice derived from the west. The northward movement of ice in the coastal area may reflect the influence of a Scandinavian ice sheet lying offshore to the east (Jamieson, 1882) or may merely represent the divergence of the large mass of ice flowing eastwards from the Midland Valley of Scotland at this time (Clapperton, personal communication). Scandinavian erratics in till near Nigg Bay may have been derived from the underlying meltwater sands and gravels if the latter are pre-Devensian, and there is no other evidence to suggest that ice from Scandinavia reached the coast in the map area during the Devensian glaciation.

The red till (unit (c)) is relatively argillaceous and at first sight would seem to have more affinities with lodgement tills than with melt-out or flow tills. However, the interdigitation of thin layers of red till with bedded clays and sands (e.g. at Kincraig [NJ 958 248]) and the frequent occurrence of a surface layer of this till on bodies of meltwater sands, gravels and clays suggests that the red till was formed during the final retreat and wastage of the ice. Furthermore, since the red and grey/brown tills appear to have formed virtually simultaneously, both deposits probably have a similar mode of origin, with the red till, like the grey/brown till, being primarily a melt-out deposit. The processes of till formation appear to have resulted in the mingling of the detritus from the two ice sheets at the localities where the boundary between the two till units is gradational, but little admixture of material from the different ice masses has occurred in much of the area where the red till overlies the grey/brown till as a sharply defined layer (Figure 32).

(ii) Meltwater features. The orientation of the meltwater channels and of the elongated bodies of meltwater sediments differs in two broad regions in the map area: a western region, comprising much of this area, where many of the channels are orientated approximately W to E, and a narrow coastal strip, where the alignment of the channels and deposits is mainly S to N (Figure 33). As noted above, the predominant directions of movement of the ice in these two regions were also W to E and S to N (Figure 31) and many of the channels display 'ice-directed' characteristics (Sugden and John, 1976) that suggest sub-glacial flow, notably the examples with arched longitudinal profiles (Table 5), (Figure 34) that are discordant to the topography.

In inland areas, the exploitation of the main river valleys by meltwater seems to have occurred mainly where these valleys are aligned approximately W to E (Figure 34). This is particularly noticeable in the Don valley, where meltwater appears to have flowed eastwards in the valley in the western part of the map area, but, near Parkhill [NJ 892 139], where the trend of the valley changes from easterly to southerly, the distribution of sand and gravel deposits (Figure 36) suggests that meltwater abandoned the valley and flowed uphill towards the Corby Loch area [NJ 925 145]. A few kilometres downstream, evidence of the utilisation of the river valley by meltwater can be recognised again below Stoneywood [NJ 899 110], where the course of the river becomes easterly again and the meltwater channels from the TullochTyrebagger area join the river (Figure 34)a.

All these features suggest that much of the meltwater activity in the map area occurred at the base of the ice, with the direction of flow being controlled by a sub-glacial hydraulic pressure gradient, which, in turn, was dependent on the hydrostatic pressure gradient determining the regional direction of ice movement (Sugden and John, 1976; Clapperton and Sugden, 1977). The meltwater systems associated with the two ice sheets recognised in the map area appear to have coalesced near the coast, where the meltwater sediments are particularly abundant (Figure 30) and the morphology of some of the mounds of sand and gravel suggests that the meltwater activity was subject to both S to N and W to E control (Figure 36).

Although most of the meltwater channels probably formed sub-glacially, the channels leading from the topographic depressions at Kinellar [NJ 825 120] and Westhill [NJ 845 065] (Figure 33)-6, 11 may have been formed when temporary lakes were drained during the final stages of the retreat of the ice. The channels with a roughly W to E trend in the coastal area north of Aberdeen (Figure 33)-18 to 21 that cut across the prevailing S to N trend of the mounded sand and gravel deposits also display exceptional relationships. It is possible that these W to E channels were formed when meltwater flowing at the surface rather than sub-glacially, eroded earlier meltwater deposits, till and bedrock exposed during the final retreat of the ice.

The bodies of meltwater sediments in the map area probably formed over a considerable time interval, ranging from the period when ice still covered the entire area to the final stages of deglaciation, although most appear to be ice contact deposits. Thus, the eskers and elongated mounds of sand and gravel associated with meltwater channels or orientated parallel to the direction of ice movement seem likely to have formed sub-glacially. Other sand and gravel bodies, such as the middle unit in the St. Fergus sandpit, which contain highly disturbed sedimentary structures, were probably laid down en-glacially or supra-glacially, while the sand and gravel terraces in the Dee and Don valleys, were probably formed largely as ice-marginal deposits during the final retreat of the ice.

Clapperton and Sugden (1977) have pointed out that since 'ice-directed' meltwater features can be recognised throughout NE Scotland, the entire area must have been covered by ice during the last glacial episode, thereby making it more difficult to sustain the concept that Buchan was 'ice-free' during this episode (Synge, 1956; Charlesworth, 1956).

(iii) Features of the finer-grained deposits. The field relations of the bodies of clay and silt suggest that they were deposited in a variety of environments. Some of the relatively small inland bodies may have been laid down en-glacially or even sub-glacially; others, such as the body of clay near Middle Ardo at [NJ 934 213], probably formed supra-glacially. The abrupt eastern and southern terminations of the Tipperty clays may be due to stream erosion (Figure 37), but the presence of an overlying till unit suggests that the truncation of the sediments is an ice contact feature, and that they were deposited in a temporary lake at the margin of an ice mass, (Murdoch, 1977). At Tullos the draping and disturbance of the lower two sedimentary units suggests deposition supraglacially, and the abrupt eastern termination of this deposit may also be an ice contact feature (cf. Simpson, 1948). The pebbles in the clays lying near sea level were probably ice-rafted, and the occurrence of masses of shelly till in the Blackdog deposit (Bremner, 1915) may also indicate formation in the proximity of ice.

The concentration of the bodies of fine-grained sediments in the coastal area suggests that widespread meltwater activity continued here even when the wastage of the ice was far advanced. The red colour of many of the clay deposits suggests that much of this sediment was derived from the southern ice, in some cases even although the clay lies to the west of the area underlain by red till (e.g. at Middle Ardo). However the variation in colour of some of the clay deposits from grey-brown to red may be an indication that sediment from different sources reached particular localities at different times.

Late Devensian and post-Devensan events

The plant bed in the St. Fergus sand pit provides a probable minimum age for the general deglaciation of the map area (c. 11 500 years BP) and, indeed, it has been deduced that following the retreat of the late Devensian ice-sheet Scotland was largely free of ice by c. 12 500 years BP (Gray and Lowe, 1977). Jamieson (1865) inferred that the virtual absence of red till in the Aberdeen area was due to erosion accompanying a late readvance of the ice from the west down the Dee valley, and this interpretation was later favoured by Bremner (1938) and by Synge (1956) who designated this glacial episode the 'Aberdeen Readvance'. However, this idea was challenged by Simpson (1948, 1955), and is difficult to sustain in view of the lack of evidence of separate glacial events of different age. The main post-Devensian glacial event that is generally recognised in Scotland-the Loch Lomond Readvance (Gray and Lowe, 1977)-does not appear to have affected the Sheet 77 area, and it has been argued (Clapperton and Sugden, 1977) that the only effect of this glaciation in north-east Scotland was the production of small corrie glaciers in parts of the Cairngorms. It is possible that evidence of widespread periglacial activity in the map area, notably gravel-filled, ice wedge casts and involutions in meltwater deposits, may be associated with this readvance. However Fitzpatrick (1975) established that soils at a number of localities in north-east Scotland, including a locality at Craigiebuckler [NJ 902 053] in the map area, display two discontinuities which he ascribed to an early periglacial episode when freeze-thaw conditions penetrated to depths of 2 m, and a later episode when the freeze-thaw activity was less vigorous. It is possible (Clapperton and Sugden, 1977) that the earlier event was associated with the decay of the Devensian ice sheet and the later event with the climatic deterioration in the Loch Lomond Stadia.

Synge (1963) used evidence of widespread periglacial activity as a criterion for distinguishing between the area of 'moraineless Buchan' that escaped glaciation during the supposed 'Aberdeen Readvance' and the area that had been glaciated. However, evidence of periglacial activity is as widespread within his supposed ice limits (e.g. the evidence of ice wedging in sand pits at [NJ 902 137] near Dyce and at [NO 821 995] near Peterculter) as in the areas which were thought to be unaffected by this glacial event.

Studies elsewhere in eastern Scotland (Sissons, 1976) have shown that a record of extensive late-glacial and post-glacial variations in sea level is preserved at many localities where raised beach deposits and shorelines have been recognised, or where deposits of forest peat occur at, or below, the present low-water mark. Within the map area, raised beach deposits consisting of sand and gravel adjoin the present shoreline between the mouths of the Dee and of the Don (Figure 30), elevated shorelines and beach deposits have been recognised near the mouth of the Ythan at heights of between 16 m and 27 m (Ritchie and Walton, 1972), and peat has been recorded below present-day beach deposits between the Dee and the Ythan (Jamieson, 1858; Bremner, 1943). However, the evidence of variations in sea level in the Aberdeen area is much more limited than in the coastal areas to the south (e.g. Cullingford and Smith, 1980; Smith and others, 1980), probably because the Aberdeen area is more remote from the inferred centre of maximum post-glacial isostatic readjustment (e.g. Sissons, 1976, fig. 9b), and, as yet, has not been correlated with the detailed sequence of events recognised elsewhere.

Recent deposits include, not only the present day beach sediments, but also the bodies of alluvium that occur in the valleys of most of the streams and form extensive terraces in the valleys of the Dee and the Don (Figure 30). Wind-blown sand forms a series of dunes near the coast from the mouth of the Dee northwards, and covers a more extensive area to the north of the Ythan, where the sand has extended progressively inland with the passage of time and has engulfed the medieval landscape near Forvie [NK 020 265] (Figure 30). Deposits of post-glacial peat have also formed at many localities in the map area, particularly in the poorly-drained rock basins produced by glacial erosion (Figure 30), (Figure 31).

Chapter 14 Economic geology

Metalliferous minerals

The survey of the soils near Aberdeen (Glentworth and Muir, 1963) showed that the growth of crops was stunted in several small areas because of the high nickel content of the soils. This discovery, in conjunction with the knowledge that sulphides were associated with the 'Younger Basic' rocks of north-cast Scotland (Rice, 1975), has led to a widespread programme of mineral exploration being undertaken in the area. To the north of the Sheet 77 area, disseminated nickel and copper sulphides of an average grade of less than 0.5 per cent Ni or Cu have been found at Arthrath [NJ 965 365], but at the only locality within the map area where the soils have a high nickel content (Beauty Hill [NJ 908 205], on the ultramafic rocks of the Belhelvie mass), the nickel occurs mainly in olivine and magnetite, and only minor amounts of heazlewoodite (Ni3S2) have been found.

Molybdenite has been recorded as a minor constituent in quartz veins cutting the Souter Head breccia [NJ 962 018] (Porteous, 1973c). This occurrence may be related in origin to that recorded by Rice (1975) near Quilquox [NJ 902 382], to the north of the map area, where a quartz vein containing molybdenite and pyrites was traced for several miles.

Manganese-bearing metasediments were formerly worked on a small scale near Laverockbraes [NJ 917 111] (Figure 30), where Heddle (1901) recorded the occurrence of manganite and psilomelane. Analysed material from this locality contains 12.96 per cent MnO (B.G.S. records). Chew (1978) recorded the presence of small (< 0.5 m) masses of manganese ore in till near the Bridge of Don [NJ 9407 1087] over 2 km to the east. The ore at this locality consists largely of well crystallised manganite and pyrolusite with associated psilomelane and barytes, and often overlies red clay horizons in the till, appearing to have formed in situ. Chew suggested that the manganese was probably dissolved from the local bedrock (possibly including the Mn-rich horizon at Laverockbraes) by groundwater, being separated from iron in the anaerobic conditions prevailing in the local peat mosses, before being transported and deposited in the vicinity of the impervious clay horizons in the till.

Quarried rock

Aberdeen is traditionally associated with the quarrying of granite, but none of the quarries within the city boundary (Figure 23); (Plate 23) is now (1981) in operation. Elsewhere in the map area, extraction of granitic rock is confined to relatively small scale operations (e.g. near Cove at [NO 948 998]) and the only major quarry is in the 'Younger Basic' rocks of the Belhelvie mass at Balmedie [NJ 944 181]. The decline in quarrying has been to a large extent associated with the decreasing use of massive blocks of solid rock for constructional purposes, and although Aberdeen is still an important centre for the production of dressed and polished stone, this side of the industry has been largely dependent on imported rock for many years (Hamilton, 1963). At present most of the output of the quarries in the Grampian area, including the quarries in the map area, consists of crushed rock for use as aggregate in road construction or the production of concrete (Harris, 1977).

Apart from the large-scale quarrying of igneous rocks, smaller-scale operations have been carried on throughout the map area. These quarries are often situated in bodies of migmatitic granite in the Aberdeen Formation (e.g. Allathan [NJ 898 272], Cults [NJ 885 035], Craigingles Wood [NO 880 995]) (Figure 23), but have also been sited in felsite (e.g. Parkhead [NO 852 992]) and in the 'Younger Basic' rocks of the Belhelvie mass (e.g. Braeside of Balnakettle [NJ 904 213]). Quarries in metamorphic rocks include Shiels Quarry [NJ 937 195] in hornfelses, and the excavations in ampibolites of the Ellon Formation near Waterside Bridge at [NK 000 268], but, in general, the highly variable nature of the metamorphic rocks has led to these formations being exploited on a more limited scale. (Figure 23) shows that there are large areas of uniform granitic rock in the map area which would yield quarried material of high quality at localities where the depth of weathering is limited, but it seems likely that economic and environmental factors will prevent the exploitation of these reserves.

Sand and gravel

A review of the deposits of sand and gravel in the Aberdeen area that appear to be of a size and quality to justify commercial exploitation has been provided by Peacock and others (1977) (see also Merritt, 1981). Most of these deposits consist of meltwater sediments, but reference is also made to alluvial deposits in the Dee and Don valleys and to the dune sands that adjoin the coast north of the Don (Figure 30). Many of these deposits, particularly in the area of mounded meltwater sediments near the coast, have been worked extensively down to the watertable in the past, and are now virtually exhausted.

Clay

Deposits of argillaceous sediments have been commercially exploited in the past near Aberdeen at Torry [NJ 951 052], Ferryhill [NJ 941 055] and Seaton [NJ 946 087] and north of the city at Blackdog [NJ 962 139] and Tipperty [NJ 971 268] (Figure 30) but none of these clay pits is still (1981) in operation. All of these deposits are thought to have formed during the retreat of the Devensian ice sheet and have been briefly described in Chapter 13. More detailed descriptions are provided in Eyles and Anderson (1946).

During the remapping, a body of red clay covering an area of c. 0.5 km2 and locally ranging up to more than 5 m in thickness was identified at Eigie Links [NJ 965 165] (Figure 30), but this body has yet to be fully investigated. Eyles and Anderson (1946) record that clays occur near Balmedie House c. [NJ 965 180] and Middlemuir House c. [NJ 940 210], but like other argillaceous deposits described in Chapter 13, there are probably too small to be commercially viable.

Peat

No deposits of peat of a size sufficient to justify commercial exploitation are known to occur in the Aberdeen area, although a drill hole near Belhelvie [NJ 9155 2118] that encountered 7.5 m of peat below a surface layer of soil suggests that some of these deposits might repay more detailed investigation. The main occurrences are shown in (Figure 30) and in many cases (e.g. Harestone Moss [NJ 930 195]; Red Moss [NJ 920 155]) are situated in rock basins produced by glacial erosion (Figure 31).

References

ALBUQUERQUE, C. A. R. DE. 1973. Geochemistry of biotites from granitic rocks, northern Portugal. Geochim. Cosmochim. Acta, Vol. 37, 1779–1802.

ALLEN, J. R. L. 1971. A theoretical and experimental study of climbing-ripple cross-lamination, with a field application to the Uppsala esker. Geogr. Ann., A., Vol. 53, 157–187.

ANDERTON, R., BRIDGE, P.H., LEEDER, M. R. and SELLWOOD, B.W. 1979. A dynamic stratigraphy of the British Isles. 301pp. (London: Allen and Unwin.)

ASHCROFT, W. A. and BOYD, R. 1976. The Belhelvie mafic igneous intrusion, Aberdeenshire-a re-investigation. Scott. J. Geol., Vol. 12, 1–14.

ASHCROFT, W. A. and MUNRO, M. 1978. The structure of the eastern part of the Insch mafic intrusion, Aberdeenshire. Scott. J. Geol., Vol. 14, 55–79.

ASHWORTH, J. R. 1975. Staurolite at anomalously high grade. Contrib. Mineral. Petrol. , Vol. 53, 281–291.

ASHWORTH, J. R. and CHINNER, G. A. 1978. Co-existing garnet and cordierite in migmatites from the Scottish Caledonides. Contrib. Mineral. Petrol. , Vol. 65, 379–394.

ATHERTON, M. P. 1968. The variation in garnet, biotite and chlorite composition in medium grade pelitic rocks from the Dalradian, Scotland, with particular reference to the zonation in garnet. Contrib. Mineral. Petrol. , Vol. 18, 347–371.

ATHERTON, M. P. 1977. The metamorphism of the Dalradian rocks of Scotland. Scott. J. Geol., Vol. 13, 331–370.

ATHERTON, M. P. and BROTHERTON, M. S. 1972. The composition of some kyanite-bearing regionally metamorphosed rocks from the Dalradian. Scott. J. Geol., Vol. 8, 203–213.

ATHERTON, M. P. 1982. Major element composition of the pelites of the Scottish Dalradian. Geol. J., Vol. 17, 185–221.

BAILEY, E. B. 1925. Perthshire tectonics: Loch Tummell, Blair Atholl and Glen Shee. Trans. R. Soc. Edinburgh, Vol. 53, 671–698.

BAILEY, E. B. 1960. The geology of Ben Nevis and Glen Coe. (2nd Edition). Mem. Geol. Surv. G. B., Sheet 53. 307pp.

BAI.TATZIS, E. 1979. Distribution of Fe and Mg between garnet and biotite in Scottish Barrovian metamorphic zones. Mineral. Mag., Vol. 43, 155–157.

BARROW, G. 1912. Lower Deeside and the Highland Border. Proc. Geol. Assoc. , Vol. 23, 268–273.

BASHAM, I. R. 1968. Deeply weathered rock and associated soils in the Insch and Boganclogh masses. 237pp. Unpublished PhD thesis, University of Aberdeen.

BISSET, C. B. 1934a. A contribution to the study of some granites near Aberdeen. Trans. Edinburgh Geol. Soc. , Vol. 13, 72–88.

BISSET, C. B. 1934b. Some minor intrusions in south central Aberdeenshire. Trans. Edinburgh Geol. Soc. , Vol. 13, 133–147.

BOULTON, G. S. 1970. The deposition of subglacial and meltout tills at the margins of certain Svalbard glaciers. J. Glaciol., Vol. 9, 231–245.

BOULTON, G. S. 1972. Modern Arctic glaciers as depositional models for former ice sheets. J. Geol. Soc. , Vol. 128, 361–393.

BOWIE, S. H. U., DAWSON, J., GALLAGHER, M. J., LAMBERT, R. ST. J. and LAWSON, R. I. 1966. Potassium-rich sediments in the Cambrian of northwest Scotland. Trans. Inst. Min. Metal. , Vol. 75, Section B, 125–145.

BOYD, R. 1972. The geology of the Belhelvie mafic intrusion and its environs, Aberdeenshire. 142pp. Unpublished PhD thesis, University of Aberdeen.

BOYD, R. and MUNRO, M. 1978. Deformation of the Belhelvie mass, Aberdeenshire. Scott. J. Geol., Vol. 14, 29–44.

BREEMEN, O. VAN, and BOYD, R. 1972. A radiometric age for pegmatite cutting the Belhelvie mafic intrusion, Aberdeenshire. Scott. J. Geol., Vol. 8, 115–120.

BREMNER, A. 1915. Problems in the glacial geology of NE Scotland and some fresh facts bearing on them. Trans. Edinburgh Geol. Soc. , Vol. 10, 334–347.

BREMNER, A. 1928. Further problems in the glacial geology of northeastern Scotland. Trans. Edinburgh Geol. Soc. , Vol. 12, 147–164.

BREMNER, A. 1934. The glaciation of Moray and ice movements in the north of Scotland. Trans. Edinburgh Geol. Soc. , Vol. 13, 17–56.

BREMNER, A. 1938. The glacial epoch in north-east Scotland. The Deeside Field, No. 8, 64–68.

BREMNER, A. 1939. Notes on the glacial geology of east Aberdeenshire. Trans. Edinburgh Geol. Soc. , Vol. 13, 474–475.

BREMNER, A. 1943. The glacial epoch in the North-East. Pp.10–30 in The Book of Buchan (Jubilee volume). TOCHER, J. F. (editor). 330pp. (Aberdeen: University Press.)

BUCHAN, S. 1932. On some dykes in east Aberdeenshire. Trans. Edinburgh Geol. Soc. , Vol. 12, 323–328.

BUSREWIL, M. T., PANKHURST, R. J. and WADSWORTH, W. J. 1975. The origin of the Kennethmont granite-diorite series, Insch, Aberdeenshire. Mineral. Mag., Vol. 40, 363–376.

CAMERON, J. 1945. Structural features of the grey granites of Aberdeenshire. Geol. Mag., Vol. 82, 189–204.

CHARLESWORTH, J. K. 1956. The late-glacial history of the Highlands and Islands of Scotland. Trans. R. Soc. Edinburgh, Vol. 62, 769–928.

CHATTERJEE, N. D. and JOHANNES, W. 1974. Thermal stability and standard thermodynamic properties of synthetic 2M1 muscovite, K Al2 (Al Si3O10(OH)2). Contrib. Mineral. Petrol., Vol. 48, 89–114.

CHEW, K. J. 1978. Crystalline manganese oxides in till, from Bridge of Don, Aberdeen. Scott. J. Geol., Vol. 14, 329–334.

CHINNER, C. A. 1960. Pelitic gneisses with varying ferrous-ferric ratios from Glen Clova, Angus, Scotland. J. Petrol., Vol. 1, 178–217.

CHINNER, C. A. 1961. The origin of sillimanite in Glen Clova, Angus. J. Petrol., Vol. 2, 312–323.

CHINNER, C. A. 1962. Almandine in thermal aureoles. J. Petrol., Vol. 3, 316–340.

CHINNER, C. A. 1965. The kyanite isograd in Glen Clova, Angus, Scotland. Mineral. Mag., Vol. 34, 132–143.

CHINNER, C. A. 1966. The distribution of pressure and temperature during Dalradian metamorphism. Q. J. Geol. Soc. London, Vol. 122, 159–186.

CHINNER, C. A. and HESEI.TINE, F. H. 1979. The Grampide andalusite/ kyanite isograd. Scott. J. Geol., Vol. 15, 117–127.

CLABAUGH, S. E. 1952. Corundum deposits of Montana. Bull. U. S. Geol. Sure. No. 983. 100pp.

CLAPPERTON, C. M. and SUGDEN, D. E. 1977. The late Devensian glaciation of north-east Scotland. Pp.1–13 in Studies in the Scottish lateglacial environment. GRAY, J. M. and LOWE, J. J. (editors). 197pp. (Oxford: Pergamon Press.)

COOPE, G. R. 1975. Climatic fluctuations in north-west Europe since the last interglacial, indicated by fossil assemblages of Coleoptera. Pp.153–168 in Ice Ages, Ancient and Modern. WRIGHT, A. E.and MOSELEY, F. (editors). Geol. J. Spec. Issue. No. 6. 320pp.

CULLINGFORD, R. A. and SMITH, D. E. 1980. Late Devensian raised shorelines in Angus and Kincardineshire, Scotland. Boreas, Vol. 9, 21–38.

DRAKE, L. D. 1971. Evidence for ablation and basal till in east central New Hampshire. Pp.73–91 in Till- a symposium. GOLDTHWAITE, R. P. (editor). 402pp. (Ohio: State Univ. Press.)

DUNCAN, I. G. 1974. Studies in metamorphic and basic igneous rocks in eastern Aberdeenshire. 119pp. Unpublished PhD thesis, University of Aberdeen.

DUNNING, F. W. 1972. Dating events in the metamorphic Caledonides: impressions of the symposium held in Edinburgh, September 1971. Scott. J. Geol., Vol. 8, 179–191.

EVANS, B. W. and GUIDOTTI, C. V. 1966. The sillimanite-potash feldspar isograd in western Maine, U.S.A. Contrib. Mineral. Petrol., Vol. 12, 25–62.

EYLES, V. A., ANDERSON, J. G. C. , BONNER, D. G. R. and BUTTERWORTH, B. 1946. Brick clays of north-east Scotland. Wartime pamphlet Geol. Sure. G.B. No. 47, 86pp.

FITCH, F. J., MILLER, J. A. and WILLIAMS, S. C. 1970. Isotopic ages of British Carboniferous rocks. C. R. 6me. Congr. Stratigr. Geol. Carbonif, Sheffield 1967, Vol. 2, 771–789.

FITZPATRICK, E. A. 1963. Deeply weathered rock in Scotland; its occurrence, age and contribution to the soils. J. Soil Sci., Vol. 14, 33–43.

FITZPATRICK, E. A. 1975. Particle size distribution and stone orientation patterns in some soils of north-east Scotland. Pp.49–60 in Quaternary studies in north-east Scotland. GEMMEL, A. M. D. (editor). 87pp. Annual field meeting, Quaternary Research Association, Aberdeen. (Aberdeen: Dept. of Geography, Univ. of Aberdeen.)

FLINT, R. F. 1971. Glacial and Quaternary geology. 892pp. (New York: Wiley.)

FRANCIS, E. H. 1967. Review of Carboniferous-Permian volcanicity in Scotland. Geol. Rundsch. , Vol. 57, 219–246.

FRANCIS, G. H. 1956. Facies boundaries in pelites at the middle grades of regional metamorphism. Geol. Mag., Vol. 93, 353–368.

GEMMELL, A. M. D. 1975. Quaternary studies in north-east Scotland:-an introduction. Pp.1–13 in Quaternary studies in northeast Scotland. GEMMELL, A. M. D. (editor). 87pp. Annual field meeting, Quaternary Research Association, Aberdeen. (Aberdeen: Dept. of Geography, Univ. of Aberdeen.)

GLENTWORTH, R. and MUIR, J. W. 1963. The soils of the country round Aberdeen, Inverurie and Fraserburgh. Mem. Soil. Sure, G.B. 371pp.

MITCHELL, W. A. and MITCHELL, B. D. 1964. The red glacial drift deposits of north-east Scotland. Clay Mineral. Bull., Vol. 5, 373–381.

GODWIN, N. 1977. Quaternary history of the British flora. Pp.107–118 in British Quaternary studies: recent advances. SHOTTON, F. W. (editor). 298pp. (Oxford: Clarendon Press.)

GOODE, A. D. T. and MOORE, A. C. 1975. High pressure crystallisation of the Ewarara, Kalka and Gosse Pile intrusions, Giles Complex, central Australia. Contrib. Mineral. Petrol., Vol. 51, 77–97.

GRAHAM, C. M. 1976. Petrochemistry and tectonic significance of Dalradian metabasaltic rocks of the SW Scottish Highlands. J. Geol. Soc. , Vol. 132, 61–84.

GRAY, J. M. and LOWE, J. J. 1977. The Scottish lateglacial environment: a synthesis. Pp.163–181 in Studies in the Scottish lateglacial environment. GRAY, J. M. and LOWE, J. J. (editors). 197pp. (Oxford: Pergamon Press.)

GRIBBLE, C. D. 1968. The cordierite-bearing rocks of the Haddo House and Arnage districts, Aberdeenshire. Contrib. Mineral. Petrol., Vol. 17, 315–330.

GRIBBLE, C. D. 1970. The role of partial fusion in the genesis of certain cordierite-bearing rocks. Scott. J. Geol., Vol. 6, 75–82.

GRIBBLE, C. D. and O'HARA, M. J. 1967. Interaction of basic magma and pelitic materials. Nature, London, Vol. 214, 1198–1201.

GROOME, D. R. and HALL, A. 1974. The geochemistry of the Devonian lavas of the northern Lorne plateau, Scotland. Mineral. Mag., Vol. 39, 621–640.

GUPPY, E. M. and SABINE, P. A. 1956. Chemical analyses of igneous rocks, metamorphic rocks and minerals 1931–1954. Mem. Geol. Sure. G.B. 78pp.

HALL, A. 1972. New data on the composition of Caledonian granites. Mineral. Mag, Vol. 38, 847–862.

HALLIDAY, A. N., AFTALION, M., BREEMEN, O. VAN, and JOCELYN, J. 1979. Petrogenetic significance of Rb-Sr and U-Pb isotopic systems in the 400 Ma old British Isles granitoids and their hosts. Pp.653–661 in The Caledonides of the British Isles-reviewed. HARRIS, A. L., HOLLAND, C. H. and LEAKE, B. E. (editors). Geol. Soc. London Spec. Issue, No. 8, 768pp.

HAMILTON, H. 1963. Industries and commerce. Pp.140–192 in The north-east of Scotland. 256pp. Br. Assoc. Handbook (Aberdeen: Central Press.)

HANCOCK, J. M. 1975. The petrology of the Chalk. Proc. Geol. Assoc. , Vol. 86, 499–535.

HARKNESS, D. D. and WILSON, H. W. 1979. Scottish Universities Research and Reactor Centre Radiocarbon Measurements III. Radiocarbon, Vol. 21, 203–256.

HARRIS, A. L., BALDWIN, C. T., BRADBURY, H. J., JOHNSON, H. D. and SMITH, R. A. 1978. Ensialic basin sedimentation: the Dalradian Supergroup. Pp.115–138 in Crustal evolution in northwestern Britain and the adjacent regions. Bowls, D. R. and LEAKE, B. E. (editors). Geol. J. Spec. Issue, No. 10. 492pp.

HARRIS, A. L., BRADBURY, H. J. and McGONIGAL, M. H. 1976. The evolution and transport of the Tay Nappe. Scott. J. Geol., Vol. 12, 103–113.

HARRIS, A. L. and PITCHER, W. S. 1975. The Dalradian Supergroup. Pp.52- 75 in A correlation of the Precambrian rocks of the British Isles. HARRIS, A.L., SHACKLETON, R.M., WATSON, J., DOWNIE, C. , HARLAND, W. B. and MOORBATH, S. (editors). Spec. Rep. Geol. Soc. London, No. 6. 136pp.

HARRIS, P. M. 1977. Igneous and metamorphic rock. Miner. Dossier, Miner. Resour. Consult. Comm., No. 19, 62pp.

HART, R. 1941. Soil studies in relation to geology in an area of north-east Scotland. J. Agric. Sci., Vol. 31, 438–447.

HARTS, B. 1975. Determination of a petite petrogenetic grid for the eastern Scottish Dalradian. Carnegie. Inst. Washington Yearbook, No. 74, 438–446.

HARTE, B. 1979. The Tarfside succession and the structure and stratigraphy of the eastern Scottish Dalradian rocks. pp.221–228 in The Caledonides of the British Isles-reviewed. HARRIS, A. L., HOLLAND, C. H. and LEAKE, B. E. (editors). Geol. Soc. London. Spec. Issue No. 8, 768pp.

HARTE, B. and HUDSON, N. F. C. 1979. Pelite facies series and the temperatures and pressures of Dalradian metamorphism in E Scotland. Pp.323–337 in The Caledonides of the British Isles-reviewed. HARRIS, A. L., HOLLAND, C. H. and LEAKE, B. E. (editors). Geol. Soc. London Spec. Issue, No. 8, 768pp.

HOLLAND, C. H. and JOHNSON, M. R. W. 1969. Metamorphic history of Dalradian rocks in Glens Clova, Esk and Lethnot, Angus, Scotland. Scott. J. Geol., Vol. 5, 54–80.

HEDDLE, M. F. 1901. The mineralogy of Scotland. Vol. 1. 148pp. (Edinburgh: David Douglas.)

HINXMAN, L. W. and WILSON, J. S. G. 1890. Explanation of Sheet 76, Central Aberdeenshire. 43pp. Mem. Geol. Sure. G.B.

HOLDAWAY, M. J. and LEE, S. M. 1977. Fe-Mg cordierite stability in high grade pelitic rocks based on experimental, theoretical and natural observations. Contrib. Mineral. Petrol., Vol. 63, 175–198.

HOLLAND, J. G. and BROWN, G. M. 1972. Hebridean tholeiitic magmas: a geochemical study of the Ardnamurchan cone sheets. Contrib. Mineral. Petrol., Vol. 37, 139–160.

INSTITUTE OF GEOLOGICAL SCIENCES. 1968. Aeromagnetic map of parts of Great Britain and Northern Ireland. Sheets 13 and 14. (London: Institute of Geological Sciences.)

INSTITUTE OF GEOLOGICAL SCIENCES. 1977a. Map of solid geology, 1:250 000 series, Moray Buchan, Sheet 57°N-04°W. (London: Institute of Geological Sciences.)

INSTITUTE OF GEOLOGICAL SCIENCES. 1977b. Bouguer gravity anomaly map, Moray-Buchan, Sheet 57°N-04°W. (London: Institute of Geological Sciences.)

JACKSON, E. D. and THAYER, T. P. 1972. Some criteria for distinguishing between stratiform, concentric and alpine peridotite-gabbro complexes. Proc. 24th. Int. Geol. Congr. Montreal. Sect.2, 289–296.

JAMIESON, T. F. 1858. On the Pleistocene deposits of Aberdeenshire. Q. J. Geol. Soc. London, Vol. 14, 509–532.

JAMIESON, T. F. 1860. On the drift and rolled gravels of the North of Scotland. Q. J.Geol. Soc. London, Vol. 16, 347–371.

JAMIESON, T. F. 1865. On the history of the last geological changes in Scotland. Q. J.Geol. Soc. London, Vol. 21, 161–203.

JAMIESON, T. F. 1874. On the last stage of the glacial period in north Britain. Q. J.Geol. Soc. London, Vol. 30, 317–338.

JAMIESON, T. F. 1882. On the red clay of the Aberdeenshire coast and the direction of the ice-movement in that quarter. Q. J. Geol. Soc. London, Vol. 38, 160–177.

JAMIESON, T. F. 1906. The glacial period in Aberdeenshire and the southern border of the Moray Firth. Q. J. Geol. Soc. London, Vol. 62, 13–39.

JANSEN, J. B. H. and SCHUHANG, R. D. 1976. Metamorphism on Naxos: petrology and geothermal gradients. Am. J. Sci., Vol. 276, 1225–1253.

JOHNSON, M. R. W. 1962. Relations of movement and metamorphism in the Dalradians of Banffshire. Trans. Edinburgh Geol. Soc. , Vol. 19, 29–64.

JOHNSTONE, C. S. 1966. British regional geology: the Grampian Highlands (3rd edition). 103pp. (Edinburgh: HMSO for Institute of Geological Sciences.)

KAMP, P. C. VAN DE. 1970. The Green Beds of the Scottish Dalradian Series: geochemistry, origin and metamorphism of mafic sediments. J. Geol., Vol. 78, 218–303.

KERRICK, D. M. 1972. Experimental determination of muscovite + quartz stability with PH2O<P total. Am. J. Sci., Vol. 272, 946–958.

LAW, G. R. 1962. The sub-surface geology of the Bay of NiggTullos area. 88pp. Unpublished MSc thesis, University of Aberdeen.

LEAKE, B. E. 1964. The chemical distinction between orthoand para-amphibolites. J. Petrol., Vol. 5, 238–254.

LEAKE, B. E. 1965. The relationship between composition of calciferous amphibole and grade of metamorphism. Pp.299–318 in Controls of metamorphism. PITCHER, W. S. and FLINN, G. W. (editors). Geol. J. Spec. Issue No.l. 368pp.

LEAKE, B. E. 1972. Garnetiferous striped amphibolites from Connemara, Western Ireland. Mineral. Mag., Vol. 38, 649–665.

LEAKE, B. E. 1978. Nomenclature of amphiboles. Mineral. Mag., Vol. 42, 533–563.

LINTON, D. L. 1951. Problems of Scottish scenery. Scott. Geogr. Mag., Vol. 67, 65–85.

MACDONALD, C. A. and KATSURA, T. 1964. Chemical composition of Hawaiian lavas. J. Petrol. Vol. 5, 82–133.

MACDONALD, R., GOTTFRIED, D., FARRINGTON, M. J., BROWN, F. W. and SKINNER, N. G. 1981. Geochemistry of a continental tholeiite suite: late Palaeozoic quartz dolerite dykes of Scotland. Trans. R. Soc. Edinburgh, Vol. 72, 57–74.

MACKIE, W. 1926. The heavier accessory minerals in the granites of Scotland. Trans. Edinburgh Geol. Soc. , Vol. 12. 22–40.

MCLEAN, F. 1977. The glacial sediments of a part of east Aberdeenshire. 152pp. Unpublished PhD thesis, University of Aberdeen,

MATHER, J. D. 1970. The biotite isograd and the lower greenschist facies in the Dalradian rocks of Scotland. J. Petrol., Vol. 11, 253–275.

MERRITT, J. W. 1981. The sand and gravel resources of the country around Ellon, Grampian Region. Description of 1:25 000 resource sheets NJ 93 with parts of NJ 82, 83, and 92 and NK 03 and parts of NK 02 and 13. Miner. Assess. Rep. Inst. Geol. Sci., No. 76.

MILNE, J. 1902. Old Red Sandstone in Aberdeen and the neighbourhood. Trans. Edinburgh Geol. Soc. , Vol. 8, 221–225.

MURDOCH, W. M. 1977. The glaciation and deglaciation of south-east Aberdeenshire. 238pp. Unpublished PhD thesis, University of Aberdeen.

NICOL, J. 1860. On the geological structure of the vicinity of Aberdeen and the north-east of Scotland. Rep. Br. Assoc. Adv. Sci., 29th Meeting, Misc. Commun., 116–119.

NOCKOLDS, S. R. 1941. The Garabal Hill-Glen Fyne igneous complex. Q. J. Geol. Soc. London, Vol. 96, 451–511.

NOCKOLDS, S. R. and MITCHELL, R. L. 1949. The geochemistry of some Caledonian plutonic rocks. Trans. R. Soc. Edinburgh, Vol. 61, 533–575.

PANKHURST, R. J. 1982. Geochronological tables for British igneous rocks. Appendix C in Igneous rocks of the British Isles. SUTHERLAND, D. S. (editor). 560pp. (Chichester: Wiley.)

PANTIN, H. M. 1956. The petrology of the Ben Vrackie epidiorites and their contact rocks. Trans. Geol. Soc. Glasgow, Vol. 22, 48–79.

PATTERSON, S. H. 1967. Bauxite reserves and potential aluminium resources of the world. Bull. U. S. Geol. Surv., No. 1228, 176pp.

PEACOCK, J. D. 1975. Scottish late and post-glacial marine sediments. Pp.45–48 in Quaternary studies in north-east Scotland. GEMMELL, A. M. D. (editor). 87pp. Annual Field Meeting, Quaternary Research Association, Aberdeen. (Aberdeen: Dept. of Geography, Univ. of Aberdeen.)

PEACOCK, J. D. CLARK, G. C., MAY, F., MENDUM, J. R., ROSS, D. L. and RUCKLEY, A.E. 1977. Sand and gravel resources of the Grampian region. Rep. Inst. Geol. Sci., No. 77/2, 24pp.

PETTIJOHN, F. J. 1963. Chemical composition of sandstones-excluding carbonate and volcanic sands. Prof. Paper U.S. Geol. Sum., No. 440-S. 21pp.

PHEMISTER, T. C. , FRASER, W. E. and WILLIAMSON, D. H. 1960. Dalradian metamorphism and structure-Stonehaven to Aberdeen. Rep. 21st Int. Geol. Congr., Norden, Part 13, 352–361.

PHEMISTER, T. C. and SIMPSON, S. 1949. Pleistocene deep weathering in NE Scotland. Nature, London. Vol. 164, 318–319.

PORTEOUS, W. G. 1973a. Metamorphic index minerals in the eastern Dalradian. Scott. J. Geol., Vol. 9, 29–43.

PORTEOUS, W. G. 1973b. Studies in metamorphic and migmatitic rocks from the area south of Aberdeen. 96pp. Unpublished PhD thesis, University of Aberdeen.

PORTEOUS, W. G. 1973c. A breccia pipe in the Dalradian series, East Kincardineshire. Scott. J. Geol., Vol. 9, 233–237.

PRICE, R. J. 1960. Glacial meltwater channels in the upper Tweed drainage basin. Geogr. J., Vol. 126, 483–489.

PRICE, R. J. 1963. A glacial meltwater systernTn Peebleshire, Scotland. Scott. Geogr. Mag., Vol. 79, 133–141.

RAMSAY, D. M. and STURT, B. A. 1979. The status of the Banff nappe. Pp.145–151 in The Caledonides of the British Isles-reviewed. HARRIS, A. L., HOLLANDS, C. H. and LEAKE, B. E. (editors). Geol. Soc. London. Spec. Issue No. 8, 768pp.

READ, H. H. 1919. The two magmas of Strathbogie and Lower Banffshire. Geol. Mag., Vol. 56, 364–371.

READ, H. H. 1923a. The petrology of the Arnage district in Aberdeenshire; a study in assimilation. Q. J. Geol. Soc. London., Vol. 79, 446–486.

READ, H. H. 1923b. The geology of the country round Banff, Huntly and Turriff. Mem. Geol. Sum G. B. 240pp.

READ, H. H. 1928. The Highland schists of middle Deeside and east Glen Muick. Trans. R. Soc. Edinburgh, Vol. 55, 755–772.

READ, H. H. 1931. On corundum-spinel xenoliths in the gabbro of Haddo House, Aberdeenshire. Geol. Mag., Vol. 68, 446–453.

READ, H. H. 1952. Metamorphism and migmatisation in the Ythan Valley, Aberdeenshire. Trans. Edinburgh Geol. Soc. , Vol. 15, 265–279.

READ, H. H. 1955. The Banff nappe: an interpretation of the structure of the Dalradian rocks of north-east Scotland. Proc. Geol. Assoc. , Vol. 66, 1–29.

READ, H. H. and FARQUHAR, O. C. 1956. The Buchan anticline of the Banff nappe of Dalradian rocks in north-east Scotland. Q. J. Geol. Soc. London, Vol. 112, 131–156.

READ, H. H. and HAQ, B. T. 1963. The distribution of trace elements in the Dunite-Syenite Differentiated Series of the Insch Complex, Aberdeenshire. Proc. Geol. Assoc. , Vol. 74, 203–212.

READ, H. H., SADASHIVAIAH, M. S. and HAQ, B. T. 1961. Differentiation in the olivine gabbro of the Insch mass, Aberdeenshire. Proc. Geol. Assoc. , Vol. 72, 391–413.

READ, H. H. and SADASHIVAIAH, M. S.-1965. The hypersthene gabbro of the Insch Complex, Aberdeenshire. Proc. Geol. Assoc., Vol. 76, 1–11.

RICE, R. 1975. Geochemical exploration in an area of glacial overburden at Arthrath, Aberdeenshire. Pp.82–86 in Prospecting in areas of glaciated terrain 1975. JONES, M. J. (editor). 155pp. (London: Institution of Mining and Metallurgy).

RITCHIE, W. and WALTON, K. W. 1972. The evolution of the sands of Forvie and the Ythan estuary. Pp.12–14 in North-east Scotland: geographical essays. CLAPPERTON, C. M. (editor). 47pp. (Aberdeen: Dept. of Geography, Univ. of Aberdeen.)

ROCK, N. M. S. 1977. The nature and origin of lamprophyres: some definitions, distinctions and derivations. Earth Sci. Rev., Vol. 13, 123–169.

ROTHSTEIN, A. T. V. 1962. Magmatic facies in ultrabasic igneous rocks of the tholeiitic series: a naturally occurring example in the peridoties of Dawros, Connemara, Eire, and Belhelvie, Aberdeenshire, Scotland. 48pp. (Acad. Sci. U.S.S.R.) [Russian with English abstract.]

RUTHERFORD, M. J. 1973. The phase relations of aluminous iron biotites in the system KAl Si3O8-KAlSiO4-Al2O3-Fe-O-H. J. Petrol., Vol. 14, 159–180.

SALLOMY, J. T. and PIPER, J. D. A. 1973. Palaeomagnetic studies in the British Caledonides-II. The Younger Gabbros of Aberdeenshire, Scotland. Geophys. J. R. Astr. Soc. , Vol. 34, pp.13–26.

SCHREYER, W. 1976. Experimental metamorphic petrology at low pressures and high temperatures. Pp.261–331 in The evolution of the crystalline rocks. BAILEY, D. K. and MACDONALD, R. (editors). 484pp. (London: Academic Press).

SERDYUCHENKO, D. P. and POLUNOVSKIY, R. M. 1971. Sillimanite-corundum gneiss of the central Azov region and its mode of origin. Dokl. Akad. Nauk. S.S.S.R., Vol. 197, 156–159.

SHAW, D.M. 1956. Geochemistry of pelitic rocks. Part III. Major elements and general geochemistry. Bull. Geol. Soc. Am., Vol. 67, 919–934.

SHIDO, F. and MIYASHIRO, A. 1959. Hornblendes of basic metamorphic rocks. J. Fac. Sci. Univ. Tokyo, Vol. 12, Sect.2, 85–102.

SIMPSON, S. 1948. The glacial deposits of Tullos and the Bay of Nigg, Aberdeen. Trans. R. Soc. Edinburgh, Vol. 61, 687–698.

SIMPSON, S. 1955. A re-interpretation of the drifts of north-east Scotland. Trans. Edinburgh Geol. Soc. , Vol. 16, 189–199.

SISSONS, J. B. 1958. Supposed ice-dammed lakes in Britain, with particular reference to the Eddleston valley, southern Scotland. Geogr. Ann., Vol. 40, 159–187.

SISSONS, J. B. 1961. A sub-glacial drainage system by the Tinto Hills, Lanarkshire. Trans. Edinburgh Geol. Soc. , Vol. 18, 175–193.

SISSONS, J. B. 1976. The geomorphology of the British Isles: Scotland. 150pp. (London: Methuen.)

SMITH, D. E., MORRISON, J., JONES, R. L. and CULLINGFORD, R. A. 1980. Dating the main postglacial shoreline in the Montrose area, Scotland. Pp. 225–245 in Timescales in geomorphology. CULLINGFORD, R. A., DAVIDSON, D. A. and LEWIN, J. (editors). (Chichester: Wiley). 360 pp.

SMITH, D. G. W. 1965. The chemistry and mineralogy of some emery-like rocks from Sithean Sluaigh, Strachur, Argyllshire. Am. Mineral., Vol. 50, 1982–2022.

STEWART, F. H. 1942. Chemical data on a silica-poor argillaceous hornfels and its constituent minerals. Mineral. Mag., Vol. 26, 260–266.

STEWART, F. H. 1947. The gabbroic complex of Belhelvie in Aberdeenshire. Q. J.Geol. Soc. London, Vol. 102, 465–498.

STEWART, F. H. 1970. The 'younger' basic igneous complexes of north-east Scotland and their metamorphic envelope. Scott. J. Geol., Vol. 6, 3–6.

STRECKEISEN, A. 1967. Classification and nomenclature of igneous rocks. Neues Jahrb. Mineral. Abh., Vol. 107, 144–240.

STURT, B. A., RAMSAY, D. M., PRINGLE, I.R. and TEGGIN, D. E. 1977. Precambrian gneisses in the Dalradian sequence of north-east Scotland. J. Geol. Soc. , Vol. 134, 41–44.

SUGDEN, D. E. and JOHN, B. S. 1976. Glaciers and landscape. 376pp. (London: Arnold.)

SUZUKI, M. and KOJIMA, G. 1970. On the association of potassium feldspar and corundum found in the Hida metamorphic belt. J. Jap. Assoc. Mineral. Petrol. Econ. Geol., Vol. 63, 266–274.

SYNGE, F. M. 1956. The glaciation of north-east Scotland. Scott. Geogr. Mag., Vol. 72, 129–143.

SYNGE, F. M. 1963. The Quaternary succession round Aberdeen, northeast Scotland. Rep. 6th. Int. Quaternary Congr. Geomorphol., Sect.3, 353–361.

WADSWORTH, W. J., STEWART, F. H. and ROTHSTEIN, A. T. V. 1966. Cryptic layering in the Belhelvie intrusion, Aberdeenshire. Scott. J. Geol., Vol. 2, 54–66.

WAGER, L. R. and BROWN, G. M. 1968. Layered igneous rocks. 588pp. (Edinburgh: Oliver and Boyd.)

WAGER, L. R. and WADSWORTH, W. J. 1960. Types of igneous cumulates. J. Petrol., Vol. 1, 73–85.

WALKER, F. 1935. The late Palaeozoic quartz-dolerites and tholeiites of Scotland. Mineral. Mag., Vol. 24, 131–159.

WALKER, F. 1952. Differentiation in a quartz dolerite sill at Northfield Quarry, Stirlingshire. Trans. Edinburgh Geol. Soc., Vol. 15, 393–405.

WALKER, F. 1965. The part played by tholeiitic magma in the CarboPermian vulcanicity of central Scotland. Mineral. Mag., Vol. 34, 498–516.

WALLIS, R. H., HARLAND, W. B., GEE, D. G. and GAYER, R. A. 1968. A scheme of petrographic nomenclature for some metamorphic rocks in Spitsbergen. Nor. Polarinst., Arb. 1966, 25–37.

WADSWORTH-BELL, E. B. 1974. Some studies of various Aberdeenshire granitic rocks. 224pp. Unpublished PhD thesis, University of Aberdeen.

WAI.TON, K. W. 1963. Geomorphology. Pp.16–31 in The north-east of Scotland. 256pp. Br. Assoc. Handbook. (Aberdeen: Central Press.)

WEEDON, D. S. 1970. The ultrabasic/basic igneous rocks of the Huntly region. Scott. J. Geol., Vol. 6, 26–40.

WELLS, P. R. A. and RICHARDSON, S. W. 1979. Thermal evolution of metamorphic rocks in the Central Highlands of Scotland. Pp.339–344 in The Caledonides of the British Isles-reviewed. 768pp. HARRIS, A. L., HOLLAND, C. H. and LEAKE, B. E. (editors). Geol. Soc. London. Spec. Issue No. 8.

WEST, R. G. 1977. Pleistocene geology and biology. (2nd edition revised). 44Opp. (London: Longman.)

WILSON, J. S. G. 1882. Explanation of Sheet 97, Northern Aberdeenshire, Eastern Banffshire. Mem. Geol. Sure. G.B. 15pp.

WILSON, J. S. G. 1886. Explanation of Sheet 87, North-east Aberdeenshire. Mem. Geol. Sure. G.B. 32pp.

WISEMAN, J. D. H. 1934. The central and south-west Highland epidiorites: a study in progressive metamorphism. Q. J. Geol. Soc. London, Vol. 90, 354–417.

WRIGHT, A. E. and BOWES, D. R. 1979. Geochemistry of the appinite suite. Pp.699–704 in The Caledonides of the British Isles-reviewed. 768pp. HARRIS, A. L., HOLLAND, C. H. and LEAKE, B. E. (editors). Geol. Soc. London Spec. Issue. No. 8.

Figures, plates and tables

Figures

(Figure 1) Sketch map of the Aberdeen area showing the main topographic features. Boundaries of 100km National Grid squares NJ, NK and NO are shown. Contours in metres.

(Figure 2) Outline geological map of the Aberdeen area. The Aberdeen Formation consists largely of psammitic and semi-pelitic metasediments (QS) but sub-units, consisting largely of pelitic and semi-pelitic rocks in one instance (PS) and of psammitic rocks in the other (Q), have been recognised in the Northern unit of this formation. To the west of the Don the boundary between the Northern and Southern units is drawn at the southern margin of the psammitic sub-unit (dashed line), but this boundary cannot be traced to the east of the river. The occurrences of important index minerals in pelitic and semi-pelitic metasediments and of garnet in amphibolites in this formation are shown. h-hornfelsed regionally metamorphosed rocks; O.R.S.-Old Red Sandstone sedimentary rocks. The inset map shows the interpretation of the relationships in the Newburgh-Collieston area in earlier maps (e.g. Read and Farquhar, 1956, fig. 2)

(Figure 3) Diagrams depicting the compositional variation of 36 analysed pelitic and semi-pelitic metasediments from the Aberdeen area. Data mainly from Boyd, 1972; Porteous, 1973b; Duncan, 1974. Four speciments of hornfcls (148, 150, 247, St) discussed in the text (Chapter 7) are identified in (a) and (h). The greywacke-shale boundary in (c) is taken from Pettijohn, 1963, figs.2,3. In (d), A = Al2O3–(CaO + Na2O + K2O), K = K2O, F = FeO + MgO + MnO.

(Figure 4a) Magnetic map of the area near the boundary between the Aberdeen and Ellon formations. Anomalies in the total field intensity are expressed relative to an arbitrary zero level (see Ashcroft and Boyd, 1976, p. 4).

(Figure 4b) Magnetic profiles along traverses A-A etc. shown in a. The position of the 94 Grid line is shown in each profile.

(Figure 4c) Geological map of the area near the boundary between the Aberdeen and Ellon formations.

(Figure 5) Equal area projections of poles to foliation planes in metamorphic rocks in the map area. Poles to foliation planes are plotted in the lower hemisphere in equal area projection. The number of observations incorporated in each diagram is specified. These observations were made in areas which vary in size: in the inland regions the diagrams are positioned approximately in the centre of the area in which the data were acquired: on the coast the diagrams are positioned adjacent to the section of coastline where the observations were made except in the case of V where the source area is shown by an arrow.

(Figure 6) Compositional relationships of garnets in regionally and thermally metamorphosed pelitic and semi-pelitic rocks from the eastern Dalradian. The composition range in zoned crystals is shown by tie lines. Data for the Aberdeen and Kincardine coast areas from Stewart, 1942; Boyd, 1972; Porteous, 1973b and Baltatzis, 1979: for Glen Clova from Chinner, 1960; 1965: for Perthshire from Atherton, 1968: for Lochnagar from Chinner, 1962; Ashworth and Chinner, 1978: for the Huntly-Portsoy area from Ashworth, 1975; Ashworth and Chinner, 1978.

(Figure 7) Compositional relationships of corundum-bearing metasediments (2330, 2338) in the Northern unit of the Aberdeen Formation. Analyses of corundum-bearing regionally metamorphosed gneisses from Montana (Clabaugh, 1952) and Russia (Serdyuchenko and Polunovskiy, 1971) are also plotted. F-analysed Fucoid Bed (Bowie and others, 1966), K-potash feldspar. In (a) the composition fields of clays, shales and slates (pel; Shaw, 1956), kaolinites (k) and illites (i) are shown. In (b) the composition field of politic and semipelitic metasediments in the Aberdeen area (metased.) is shown. M-muscovite, C-cordierite, B-biotite, G-garnet.

(Figure 8) Compositional range of amphiboles (a and b), and garnets (c), in amphibolites from the Aberdeen area and from Dalradian areas to the south-west. Niggli mg values are plotted in (a) and (b). Based mainly on data from Duncan, 1974 but also incorporates analyses from Wiseman, 1934; Pantin, 1956; Shido and Miyashiro, 1959.

(Figure 9) Compositional relationships of amphibolites from the Aberdeen area. a and b are based on diagrams by Leake (1964; 1972), mg, c, alk, at are Niggli values, W is 2Fe2O3/2Fe2O3 + FeO. Mn0 is espressed as weight per cent. Specimens 27, 29 and 59 from the Northern unit of the Aberdeen Formation, which display distinctive mineralogical and chemical features, are identified. Data mainly from Duncan, 1974

(Figure 10) Cr, Ni, TiO2, mg relations in amphibolites from the Aberdeen area. Relationships between content of Cr and Ni and weight per cent Ti02 and Niggli mg. The fields of metasedimentary amphibolites are taken from Leake, 1964. Distinctive specimens 27, 29 and 59 as in (Figure 9). Data from Duncan, 1974.

(Figure 11) Diagrams showing tholeiitic affinities of the amphibolites from the Aberdeen area. In the alkali basalt-tholeiite field boundary is taken from MacDonald and Katsura, 1974, fig. 1. In b normative Di, 01 and Q are plotted and the tholeiite field is based on Holland and Brown, 1972, fig. 6. Distinctive speciments 27, 29 59 as in (Figure 9). Data mainly from Duncan, 1974.

(Figure 12) Compositional characteristics of metasediments and migmatitic rocks from coastal exposures of the Southern unit of the Aberdeen Formation. Data from Porteous, 1973b.

(Figure 13) Simplified map of coastal exposure of the Collieston Formation.

(Figure 14) P-T fields and facies series of critical mineral assemblages in pelites in the eastern Scottish Dalradian. Based on fig. 3 in Harte and Hudson, 1979. B-B1 Barrovian sequence in Glen Clova, S-S1 Stonehavian sequence on the Kincardine coast, C-E Buchan sequence from Collieston to Ellon. A-Al2SiO5, B-Biotite, C-Cordierite, Chl-Chlorite, G-Garnet, S-Staurolite. Curve for breakdown of muscovite (Ms) to potash feldspar (Or) and corundum (Cor) from Chatterjee and Johannes, 1974.

(Figure 15) Magnetic map of the Belhelvie 'Younger Basic' mass. Anomalies in the total field intensity are expressed relative to an arbitrary zero level (see Ashcroft and Boyd, 1976, p. 4). AB and AC are discontinuities in the magnetic map which are discussed in the text. Magnetic profiles and summaries of the geology on traverses X-X′-X″, Y-Y′ and Z-Z′-Z″ are shown below.

(Figure 16) Geological map of the Belhelvie 'Younger Basic' mass. Based on outcrop, drill hole and magnetic data (Boyd, 1972). The aeromagnetic map of the Aberdeen area (Institute of Geological Sciences, 1968) shows that an extensive positive anomaly (shaded area in the inset map) lies immediately offshore from the igneous mass, and suggests that the intrusion has a considerable submarine extension to the east.

(Figure 17) Geological models to explain magnetic anomalies in the Belhelvie 'Younger Basic' mass. Profiles of the anomaly in total magnetic field (solid lines) along traverses x-x′(a) and z-z′ (h) in (Figure 15) compared to anomalies calculated from the two-dimensional model geological structures (.) shown below each profile. The direction of magnetisation is the vector sum of an induced component and the NRM of Sallomy and Piper (1973) and is approximately vertical. The magnitude of magnetisation (in units of amp/m x 10–2) is shown for each lithological unit in the models. G-Gabbroic rocks; M-Metamorphic country rocks; P-Ultramafic rocks; T-Troctolitic rocks

(Figure 18) Variations in the composition of the cumulus phases in the Belhelvie 'Younger Basic' mass. Internal lithological boundaries are shown by dotted lines (see (Figure 16)). Data from Rothstein, 1962 and Wadsworth and others, 1966 are underlined. Otherwise from Boyd, 1972.

(Figure 19) Diagrams showing compositional variations in the rocks of the Belhelvie 'Younger Basic' mass. Cpx = Mg51 Fe8Ca41; 01 = FO85; Opx = En78; Plag = An81 Incorporates analyses from Stewart, 1947 and is largely based on figs. 25, 27, 29 and 31 in Boyd, 1972.

(Figure 20) Diagrams showing compositional relationships of rocks from the Belhevie and Insch 'Younger Basic' masses. Belhevie data from Stewart, 1947 and Boyd, 1972. Insch data from Read and others, 1961; 1965.

(Figure 21) Simplified map of 'Younger Basic' rocks and associated hornfelses in the area north of Aberdeen. The dotted line in the Insch mass c. [NJ 820 270] is the boundary between noritic and peridotitic cumulates (see text).

(Figure 22) Simplified map showing shearing affecting the ‘Younger Basic’ intrusions. The broad structural relations of the ‘Younger Basic’ masses and the hornfelses are also shown.

(Figure 23) Outline map of the major granitic intrusions in the Aberdeen area. Relatively small (< 10m thick) bodies of migmatitic granite are also abundant throughout the Aberdeen Formation. The breccia at Souter Head [NJ 962 018] and the minor lamprophyric and felsitic intrusions which probably were formed at a late stage in the sequence of Caledonian igneous events are also shown.

(Figure 26)). Data from Porteous, 1973b and Walsworth-Bell, 1974." data-name="images/P999791.jpg">(Figure 24) Mineralogical and compositional relationships in the granitic rocks from the Aberdeen area. (See (Figure 26)). Data from Porteous, 1973b and Walsworth-Bell, 1974.

(Figure 26)). Fe2O3 is expressed as FeO and included in 'FeO total'. Field of biotites associated with muscovite, topaz etc. from Albuquerque, 1973. Data from Walsworth-Bell, 1974. " data-name="images/P999792.jpg">(Figure 25) Compositional relationships of biotites in granitic rocks from the Aberdeen area and from the area to the west ((Figure 26)). Fe2O3 is expressed as FeO and included in 'FeO total'. Field of biotites associated with muscovite, topaz etc. from Albuquerque, 1973. Data from Walsworth-Bell, 1974.

(Figure 26) Simplified map of major granitic intrusions in the Aberdeen area and in the area to the west. The Aberdeen, Clinterty, Kemnay and Hill of Fare masses each consist of a single lithological unit, but the Tillyfourie, Crathes and Torphins masses are more heterogeneous and possibly consist of several separate intrusions. Based on Walsworth-Bell, 1974, fig. 3.1.

(Figure 27) Simplified map of the occurrences of Old Red Sandstone rocks near Aberdeen. Drill holes and temporary exposures) show that sedimentary rocks resembling the rocks exposed in the Brig O' Balgownie area occur widely beneath Aberdeen city. 1 Much of the following information has been obtained since Milne (1902) published his account. The records of drill holes and trenches are mainly held by the City Engineer.

(Figure 28) Simplified map showing occurrences of quartz-dolerite dykes and faults in the Aberdeen area. The magnetic surveys ((Figure 15); Institute of Geological Sciences, 1968) suggest that there are no major dykes in the area between Grid line 20 and the Dee. Faults have been identified mainly from circumstantial evidence (magnetic discontinuities, outcrop distribution, topographic features).

(Figure 29) Diagrams showing the tholeiitic affinities of the late-Carboniferous quartz dolerite dykes. Specimens 1396, 2130 and 2441 are from the Aberdeen area (see text), I is from near Insch (Read, 1923, p. 164). Other analyses are from Walker, 1935; 1952; Guppy and Sabine, 1956. The dotted line in (a) is the alkali basalt-tholeiite field boundary from MacDonald and Katsura, 1964.

(Figure 30) Simplified map of the superficial deposits in the Aberdeen area. Areas of till unornamented.

(Figure 31) Simplified map showing evidence of glacial erosion and the direction of ice movements in the Aberdeen area.

(Figure 32) Sections through the superficial deposits temporarily exposed in trenches near Ardo and Dyce. Based on Murdoch, 1977.

(Figure 33) Main meltwater channels and bodies of meltwater sediments in the Aberdeen area. Based on Murdoch, 1977. The channels are numbered as in Table 5.

(Figure 34) Simplified maps of systems of meltwater channels near Dyce (a) and Straloch (b). Both of these systems include channels with arched longitudinal profiles (e.g. the channels near [NJ 850 114] in (a) and near [NJ 846 1991 in (b). Contour heights in metres. Based on Murdoch, 1977, fig. 4.2 and fig. 4.3.

(Figure 35) Contoured rockhead surface beneath the city of Aberdeen. HJ-Holburn Junction. MC-Marischal College, St-Railway Station, Th-Theatre, KC-Kings College. Based largely on drill hole data lodged with the City Engineer or in the Geology Department, Aberdeen University.

(Figure 36) Meltwater deposits in the Corby Loch area, c. 5 km north of Aberdeen. Note that the deposits extend over the ridge of high ground between Parkhill and Campla Hill. Contours in metres. Based on Murdoch 1977, fig. 6.4.

(Figure 37) Simplified map (a) and sections (b) of clay deposit near Tipperty, c. 20 km north of Aberdeen on the A92. The surface deposit in most of the unornamented area of the map is red till. Small bodies of sand and gravel (e.g. near A) and of red till overlying red clay (at C) also occur at the surface but are not shown on the map. Based on Murdoch, 1977, fig. 7.2 and fig. 7.3.

(Figure 38) Postulated directions of ice flow during the late Devensian. Based on Clapperton and Sugden, 1977, fig. 6. The boundaries of the Sheet 77 area are shown.

Plates

(Plate 1) General view of the Aberdeen area looking north from near the southern boundary of Sheet 77 at Hillside [NJ 927 008]

(Plate 2) Trench section showing fresh psammitic metasediments of the Northern unit of the Aberdeen Formation occurring over an extensive area beneath virtually no overburden. Near Whitlam [NJ 888 220]. (Aberdeen University Geology Department photograph)

(Plate 3) Tightly appressed, rootless folds (F1) with axial planar foliation in varied metasediments of the Southern unit of the Aberdeen Formation. Looking south, east of the sea wall at Cove Harbour [NJ 956 006] (D3122)

(Plate 4) Asymmetric fold (F2) in predominantly psammitic metasediments of the Southern unit of the Aberdeen Formation. Looking west, east of the sea wall at Cove Harbour [NJ 956 006] (D3134)

(Plate 5) Microfolds and crenulations (F2) in a micaceous horizon in the Southern unit of the Aberdeen Formation. Looking north, east of the sea wall at Cove Harbour [NJ 956 006] (D3121)

(Plate 13)) Looking west, The Priest, south of Cove Harbour [NJ 954 004] (D3135)" data-name="images/P999811.jpg">(Plate 6) Asymmetric fold (F2) in the metasediments of the Southern unit of the Aberdeen Formation. The inclined granitic sheet in the left centre of the photograph cuts through this structure (see (Plate 13)) Looking west, The Priest, south of Cove Harbour [NJ 954 004] (D3135)

(Plate 7) Open folding (F3) in predominantly psammitic metasediments of the Southern unit of the Aberdeen Formation Fold axes approximately parallel to the line of sight. The rocks have also been affixted by F2 folds with axes approximately at right angles to the line of sight. An altered lamprophyric intrusion is visible in the left centre of the photograph as a massive, slightly discordant lithological unit. Looking north, Cove Shure, south of Cove Harbour [NJ 954 0051] (D3123)

(Plate 8) Amphibolite in the Southern unit of the Aberdeen Formation. The upper and lower contacts of this body have been exploited by intrusions of migmatitic granite which now contain disorientated metasedimentary and amphibolite xenoliths. Headland 300m south of Barest& Point [NO 947 9901] (D3129)

(Plate 9) Photomicrographs of metamorphic rocks of the Aberdeen Formation. a. Slice No. 2306*. Magnification x 20. Plane polarised light. Pelitic rock consisting largely of biotite and oligoclase, but with minor quartz and garnet. Most of the biotite crystals are aligned parallel to the lithological banding in the rock and define a foliation that trends obliquely (top left to bottom right) across the photograph, but a considerable number of these crystals are aligned at large angles to this foliation and define a second planer structure. Trench at Balgownie Home Farm [NJ 9275 1065]. b. Slice No. 2303. Magnification x 25. Plane polarised light. Highly micaceous pelite consisting mainly of small (< 0.5 mm) crystals of muscovite and biotite. The main foliation in the rock is defined by the alignment of these small mica crystals but is overgrown by larger (c. mm) idioblasitic muscovite crystals. Lochhills Sandpit [NJ 914 147]. c. Slice No. 2418. Magnification x 16. Plane polarised light. Highly micaceous pelite consisting largely of undeformed crystals of muscovite and biotite which overgrow an F2 inicrofOld. Southeast of the sea wall, Cove Harbour [NJ 956 006]. d. Slice No. 2420. Magnification x 20. Plane polarised light. Pelite with relatively large (2–3 mm) porphyroblastic garnets in a matrix which is mainly composed of plagioclase and biotite but also contains fibrolite and numerous small (0.1 0.2 mm) crystals of apatite. South-east of the sea wall, Cove Harbour [NJ 956 006]. e. Slice No. 2412. Magnification x 20. Plane polarised light. Porphyroblastic (1–2 mm) staurolite crystals with sericitised margins, overgrowing a foliation defined by the imperfect preferred orientation of biotite crystals. Pelitic metasediment from the north bank of the Don, 100 in east of the Brig CY Balgownie [NJ 943 096]. f. Slice No. 605. Magnification x 19. Plane polarised light. Irregular porphyroblasts of andalusite and garnet in a matrix consisting mainly of biotite, which shows an imperfect preferred orientation, and oligoclase. Pelite from Cran Hill [NJ 911 007]. g. Slice No. 609. Magnification x 45. Crossed polarisers. Fibrous sillimanite developing within porphyroblastic andalusite in pelite from Brodie Wood, Cran Hill [NJ 914 005]. h. Slice No. 665. Magnification x 40. Plane polarised light. Porphyroblasts of muscovite are intergrown with biotite and contain small (mainly < 0.1 mm) crystals of sillimanite which in many instances are equant and have round terminations. Migmatitic pelite, 400 m south-west of Cloghill Farm [NJ 855 074]

(Plate 10) Photomicrographs of metamorphic rocks of the Aberdeen and Elton Formations. a. Slice No. 2338. Magnification x 60. Plane polarised light. Pelitic metasediment of the Aberdeen Formation consisting mainly of muscovite, biotite and potash feldspar. Relict grains (c. 0.1 mm) of corundum which occur within the muscovite crystals in clusters are generally in optical continuity. Outcrop adjoining a forestry track, 300m south-west of Mount joy, southeast slopes of Tyrebagger Hill [NJ 852 116]. b. Slice No. 2330*. Magnification x 20. Plane polarised light. Pelitic metasediment of the Aberdeen Formation containing irregular corundum porphyroblasts (c. 2mm) within a matrix of biotite, plagioclase and potash feldspar. Part of a veinlet consisting entirely of potash feldspar is also visible. Temporary exposure in trench, 200 m south-east of Tillyfar [NJ 914 269]. c. Slice No. 97. Magnification x 26. Plane polarised light. Corundum pseudomorph (c. 1 mm) in a pelitic metasediment of the Aberdeen Formation. The pseudomorph consists largely of margarita, but granules of colourless epidote occur in the core and muscovite crystals form a clear peripheral area. The matrix to the pseudomorphs is composed mainly of biotite, plagioclase and potash feldspar. Roadside quarry, 300m north-east of Broomiehillock [NJ 829 227]. d. Slice No. 29* Magnification x 26. Plane polarised light. Schistose amphibolite of the Aberdeen Formation containing crystals of mica which display a good preferred orientation and are intergrown with plagioclase and small (0.1 to 0.2 mm) granules of pale green amphibole. Temporary exposure in trench, north-east margin of Burreldale Moss [NJ 8322 2413]. e. Slice No. 2433. Magnification x 18. Plane polarised light. Amphibolite with irregular garnet porphyroblasts (c. 2 mm) which are locally surround by quartz. Amphibole, showing an imperfect preferred orientation, and plagioclase are the other main constituents but grains (c. 0.5 mm) of opaque oxide also occur, mostly in linear trails. Aberdeen Formation, Oldman Hill, Marvcutter [NO 8330 9905]. f. Slice No. 2430. Magnification x 20. Plane polarised light. Amphibolite of the Elton Formation showing the overgrowth of microfolds (F2?) by undeformed crystals of amphibole, plagioclase and opaque constituents. Ythan Lodge, Newburgh [NJ 995 266]. g. Slice No. 2363. Magnification x 25. Plane polarised light. Pelitic metasediment of the Elton Formation containing cordierite (altered crystals left centre), andalusite (small (c. 0.5 mm) crystals bottom left, top right), biotite, quartz and plagioclase. Knoll in field, c. 500 m west-north-west of Kinknockie [NJ 943 258]. h. Slice No. 1338. Magnification x 18. Plane polarised light. Amphibolite of the Elton Formation. Consists essentially of an intergrowth of small (0.1 to 0.2 mm) crystals of plagioclase, amphibole and opaque constituents, but the outlines of former plagioclase phenocrysts can still be recognised even although the original large (c. 2 mm) crystals have been replaced by aggregates of small feldspar grains. Road-cutting at Waterside [NK 003 280] vapour phase in the vicinity of the lavers of calcareous metasediment that are often (Figure 2) intimately associated with the corundum-bearing rocks (Kerrick, 1972).

(Plate 11) Highly migrnatised metasediments of the Southern unit of the Aberdeen Formation. Foreshore at Girdle Ness [NJ 971 051] (D3133)

(Plate 12) Body of migmatitic granite in the Southern unit of the Aberdeen Formation containing highly modified metasedimentary xenoliths. Foreshore at Girdle Ness [NJ 971 051] (D3131)

(Plate 13) Sheet of migmatitic granite displaying sharp, transgressive contracts against folded metasediments of the Southern unit of the Aberdeen Formation. This intrusion cuts a large F2 structure (see (Plate 13)) Looking west, The Priest, south of Cove Harbour [NJ 954 004] (D3135)" data-name="images/P999811.jpg">(Plate 6)). The Priest, south of Cove Harbour [NJ 954 004] (D3124)

(Plate 14) Granitic intrusion in the metasediments of the Southern unit of the Aberdeen Formation showing local attenuation. Note discordant relations at the upper surface of the granite and the presence of quartz veins near the thinnest part of the intrusion. 100 m south of the Cave of Red Rocks [NJ 965 028] (D3132)

(Plate 15) Exposure of metasediments of the Ellon Formation. The imperfect fissility in the rocks dips gently to the lower right corner of the photograph but the absence of a strong foliation, and of well-defined lithological banding are features typical of the rocks of this formation. Looking north, Foveran Links, 400m southeast of Mains of Foveran [NJ 997 233] (D3358)

(Plate 16) Flat-lying amphibolite (foreground) and metasediments (background and headland on the left) of the Collieston Formation. The Smithy, c. 3 ktn south-west of Collieston INK 027 2661 (D3355)

(Plate 17) Pelitic metasediment of the Collieston Formation with 'knotted' appearance due to the presence of abundant porphyroblasts of andalusite and cordierite. South Broad Haven [NK 031 274] (D3352)

(Plate 18) Coarse psammitic metasediment of the Collieston Formation containing thin impersisten micaceous laminae which are often slightly discordan to the lithological boundaries (e.g. overhanging surface). The formation of these laminae is ascribed to pressure solution. The Smithy [NK 027 266] (D3354)

(Plate 19) Photomicrographs of metamorphic rocks of the Collieston. Formation and of igneous rocks of the Belhelvie 'Younger Basic' mass. a. Slice No. 2428. Magnification x 20. Plane polarised light. 'Knotted' petite of the Collieston Formation. Irregular porphyroblasts (c. 2 mm) of andalusite, which contain many opaque inclusions, and of altered.cordierite (e.g. right centre) are set in a matrix consisting largely of smaller (c. 0.5mm) crystals of biotite, feldspar, quartz and muscovite. The alignment of the small biotite and muscovite crystals defines a foliation, but larger (c. 1 mm), sub-idioblastic muscovite crystals are markedly oblique to this structure. Shore of South Broad Haven [NK 031 274]. b. Slice No. 2427. Magnification x 20. Crossed polarisers. 'Knotted' petite of the Collieston Formation containing a cordierite porphyroblast (c. 2.5 mm) which shows sector twinning and is enclosed in a matrix of biotite, muscovite, quartz and feldspar. North shore of Hackley Bay [NK 028 271]. c. Slice No. 2424. Magnification x 20. Plane polarised light. Metagreywacke of the Collieston Formation. Quartz occurs as relatively large (c. 1 mm) elongate crystals and crystal aggregates which show preferred orientation and are set in an abundant, very fine-grained (c. 0.1 mm) matrix of biotite, quartz, highly altered feldspar and cordierite. Sea stack, 150m east of Sanyne [NK 025 265]. d. Slice No. 1324. Magnification x 19. Plane polarised light. Amphibolite of the Collieston Formation showing relict ophitic texture. The original ferromagnesian crystals are entirely replaced by aggregates of amphibole and opaque constituents. Original plagioclase laths (c. 2 mm) survive in some places, but, in others, have been replaced by aggregates of small (c. 0.1 mm) feldspar grains. Foreshore near the Smithy INK 026 265]. e. Slice No. 1695*. Magnification x 19. Crossed polarisers. Noritic, orthopyroxene-augite cumulate from the western marginal facies of the Belhelvie mass. Idiomorphic to subidiomorphic crystals of the two pyroxenes, which sometimes are partially uralitised, are poikilitically enclosed in large (7–10 mm) crystals of bytownitic plagioclase. Shallow drill hole, 400 m north of Kingseat Hospital [NJ 9053 1970]. f. Slice No. 321. Magnification x 20. Plane polarised light. Olivine cumulate from the major ultramafic unit in the eastern part of the Belhelvie mass. Round to sub-idiomorphic crystals (1 to 1.5 mm) of highly serpentinised olivine are partially mantled by orthopyroxene and, with small (0.1–0.2 mm) idiomorphic crystals of spinet, are enclosed in a completely altered leucocratic matrix which was probably plagioclase originally. 200 m northwest of Backhill of Overhill [NJ 933 190]. g. Slice No. 153. Magnification x 27. Crossed polarisers. Gabbroic rock from the western part of the Belhelvie mass. A primary igneous lamination is defined by the preferred orientation of plagioclase laths which are intergrown with equant augite crystals. 150m north-east of Little Craigie [NJ 920 197]. h. Slice No. 151. Magnification x 18. Plane polarised light. Gabbroic, plagioclase-augite-olivine cumulate from the western part of the Belhelvie mass. Orthopyroxene mantles are widely developed at the boundaries of spongy olivine crystals (2–4 mm) and are sufficiently extensive in places to enclose crystals of plagioclase poikilitically. Coronas of vermicular amphibole are also conspicuous and have developed at original olivine-plagioclase boundaries. The plagioclase laths are subidiomorphic, and although the augite crystals show little tendency to be idiomorphic they do not appear to be interstitial to the other phases. 400m north-east of Little Craigie [NJ 922 198]

(Plate 20) Photomicrographs of 'Younger Basic' and hornfelsic rocks. a. Slice No. 1875*. Magnification x 20. Plane polarised light. Quartz-biotite norite from an isolated small body of basic igneous rock east of Udny. Consists essentially of intergrown crystals of orthopyroxene, biotite, bytownite and quartz. Some of the orthopyroxene crystals show an approach to a lath-like form but many are irregular and adjoining grains of this phase are often in optical continuity. Shallow drill hole, 100 m south-east of Hillhead of Mosstown [NJ 9291 2767]. b. Slice No. 81. Magnification x 25. Plane polarised light. Hornfelsed pelite from the zone of structural and lithological complexity between the Ellon and Aberdeen formations, 4 km north-east of the Belhelvie mass. Randomly orientated sillimanite laths (up to 4–5 mm) occur in a matrix of biotite, quartz, opaque constituents and the very fine-grained alteration products of feldspar and cordierite. Farmyard at Fiddesbeg [NJ 943 244]. c. Slice No. 2407*. Magnification x 70. Plane polarised light. Prismatic sillimanite crystals overgrowing biotite and an associated plexus of fibrolite needles in a hornfelsed pelite, 2 km north-east of the Belhelvie mass. From a shallow trench at Darrahill [NJ 9345 2205]. d. Slice No. 325. Magnification x 57. Plane polarised light. Pelitic hornfels from the Belhelvie aureole. Shows prismatic sillimanite laths overgrowing irregular (c. 1 mm) crystals of andalusite. 400 m north-east of Bruntlandpark [NJ 932 190]. e. Slice No. 247. Magnification x 20. Plane polarised light. Silica-poor, pelitic hornfels from the aureole of the Belhelvie mass. Xenoblastic garnet porphyroblasts (up to 2 mm) are set in a matrix of biotite, plagioclase, cordierite, opaque constituents and spine]. From the eastern contact of the mass at Sparcraigs [NJ 934 197]. f. Slice No. 193. Magnification x 18. Plane polarised light. Pelitic hornfels from the Belhelvie aureole. Idioblastic sillimanite porphyroblasts (c. 0.5 mm) are surrounded by aggregates of potash feldspar crystals. Cordierite porphyroblasts (up to 3 mm) with inclusions of biotite are also conspicuous. The matrix of the rock consists largely of finer-grained (0.1–0.2 mm) sillimanite, cordierite and opaque oxide. From Bruntland Wood [NJ 930 182] in the septum of country rocks in the Belhelvie mass. g. Slice No. 247. Magnification x 18. Plane polarised light. Banded, silica-poor hornfels in the aureole of the Belhelvie mass. Bands rich in orthopyroxene, in plagioclase and in plagioclase and spinel are conspicuous. Eastern contact of the intrusion at Sparcraigs [NJ 934 197]. h. Slice No. 85. Magnification x 28. Plane polarised light. Gneissose matrix of rock with hornfelsic xenoliths from the zone of structural and lithological complexity between the Ellon and Aberdeen formations, 5km north of the Belhelvie mass. A large (c. 2 mm) irregular porphyroblast of andalusite is intergrown with biotite, quartz and plagioclase which form much of the remainder of the rock. From outcrop 200 m south of Auchindarg [NJ 937 254] near Hillhead of Mosstown is associated with the occurrence of amphibolites (1873*) and deformed amphibolitic (possibly dioritic) rocks (1876*). There are no prominent magnetic features associated with either of the occurrences of 'Younger Basic' rock (1817*; 1875*) in this area (Figures 4a,4c), and there is thus no magnetic evidence to support the view that large unexposed bodies of 'Younger Basic' rock are responsible for the widespread development of hornfelses in this area (see Chapter 7).

(Plate 21) Hornfelsed metasediments of the Aberdeen Formation in the thermal aureole of the Belhelvie 'Younger Basic' mass Porphyroblastic minerals (probably mainly cordierite) which have developed during thermal metamorphism weather out to give the rocks a pock-marked appearance. These porphyroblasts overgrow folds formed during the earlier episode of regional metamorphism. In the septum of metasedimentary rocks on Harestone Moss [NJ 930 193] (D3120).

(Plate 22) Photomicrographs of deformed 'Younger Basic' rocks and hornfels, rocks of major granitic intrusions and an associated hornfcls, felsite and quartz dolerite. a. Slice No. 227. Magnification x 20. Crossed polarisers. Moderately deformed noritic rock of the Belhelvie mass. Plagioclase crystals have been bent and marginally granulated, pyroxene crystals are largely unmodified. Balmedie Quarry [NJ 944 181]. b. Slice No. 343. Magnification x 20. Plane polarised light. Strongly deformed mafic igneous rock of the Belhelvie mass. Original plagioclase crystals have survived as lensoid relict grains (1–2 mm) but the original ferromagnesian minerals have been completely replaced by a fine-grained (< 0.5 trim) aggregate of amphibole, biotite and opaque constituents. Foliation is defined by the preferred orientation of the plagioclase relicts and of the amphibole and biotite crystals. Balmedie Quarry [NJ 944 181]. c. Slice No. 187. Magnification x 19. Plane polarised light. Mylonitised pelitic hornfels from the septum of country rocks in the Belhelvie mass. Round relict grains of plagioclase and garnet in a strongly foliated, fine-grained (c. 0.1 mm) matrix containing leucocratic quartzose and melanocratic biotitic bands. 200 m north-east of Bruntland Park [NJ 929 189]. d. Slice No. 2444*. Magnification x 18. Plane polarised light. Hornfelsed migmatitic metasediment of the Aberdeen Formation from the eastern contact of the Crathes granitic mass. An aggregate of sillimanite needles and of grains of cordierite and opaque constituents is conspicuous in the lower part of the photograph while round porphyroblasts (c. 1 mm) of altered cordierite are conspicuous in the leucosome in the upper part. From a temporary exposure in a trench near Anguston [NJ 8135 0184]. e. Slice No. 715. Magnification x 18. Crossed polarisers. Megacryst of potash feldspar in a granitic rock of the Aberdeen mass. The megacryst has irregular outlines and contains subidiomorphic biotite and plagioclase inclusions (< 0.5 mm) and a zone of small (c. 0.05mm), round quartz inclusions. Sclattie Quarry [NJ 893 099]. f. Slice No. 756. Magnification x 20. Crossed polarisers. Granite of the Aberdeen mass. Textural relations in this rock are dominated by the presence of aggregates of strained quartz crystals. Plagioclase, altered potash feldspar and biotite are also recognisable. Rubislaw Den [NJ 917 059]. g. Slice No. 644. Magnification x 20. Crossed polarisers. Quartz porphyry intrusion ('felsite'). Idiomorphic and subidiomorphic phenocrysts of quartz, plagioclase, potash feldspar (upper left) and muscovite (lower left centre) in a very fine-grained (c. 0.05mm) quartzo-feldspathic groundmass. Intruded into the Souter Head breccia [NJ 961 017]. h. Slice No. 1396*. Magnification x 27. Plane polarised light. Quartz dolerite dyke containing plagioclase in sub-ophitic intergrowth with clinopyroxenes (pigeonite and augite) and opaque oxide. Occasional small (c. 0.1–0.3 mm) interstitial areas appear turbid in the photograph but examination at higher power shows that they contain micrographic intergrowths of quartz and alkali feldspar. From a shallow drill hole, 400 m south-east of Monkshill [NJ 9189 2509]

(Plate 23) Rubislaw Quarry excavated to a depth of c. 150 m in the granite of the Aberdeen mass. The tree-lined valley of the Denburn, an incised glacial meltwater channel, is visible in the upper part of the photograph. Looking northeast. [NJ 912 055]. (Aberdeen University Geology Department photograph)

(Plate 24) Sub-horizontal felsite intrusion in highly migmatised metasediments of the Southern unit of the Aberdeen Formation. The boundaries of the intrusion are slightly undulose and are locally discordant to structures in the surrounding rocks. Looking west, Cave of Red Rocks [NJ 965 028] (D3357)

(Plate 25) Breccia consisting largely of fragments of migmatised metasediments but also including blocks of amphibolite and lamprophyre (top centre). Souter Head [NJ 961 017] (D3125)

(Plate 26) Quartz dolerite dyke intruded into the migmatised metasediments of the Southern unit of the Aberdeen Formation. The dyke is c. 15m wide and is flanked on either side by zones of shattering and alteration in the country rocks which have been preferentially eroded. Looking south at the Bridge of One Hair [NJ 970 039] (D3127)

(Plate 27) Meltwater channel, incised into flat-lying landscape to depths of 15–20 m Now followed by the Leuchar Burn 2 km north-west of Peterculter [NJ 826 030] (D3356)

(Plate 28) Meltwater sediments, mainly sands and gravels, overlain by till (top right). Lochills sandpit [NJ 912 148] (D3126)

(Front cover)

(Rear cover)

Tables

(Table 1) Analyses of pelitic metasediments from the Aberdeen area including corundum-bearing rocks.

(Table 2) Analyses of amphiboles in amphibolites from the Aberdeen area.

(Table 3) Analyses of amphibolites from the Aberdeen area.

(Table 4) Cumulate sequences in the Belhelvie, Insch and Huntly 'Younger Basic' masses.

(Table 5) Details of meltwater channels in the Aberdeen area.

Tables

(Table 1) Analyses of two corundum-bearing metasediments of the Aberdeen Formation and of typical pelitic metasediments and hornfelses from the Aberdeen area

Fucoid Beds (Bowie and Others, 1966)

Regionally metamorphosed pelitic rocks from the Kincardinecoasts

Pelitic hornfelses from the Belhelvie aureole
2330 2338 554 604 605 684 1816 Stewart1942, 148 150 193 238 247
Biot-plag-corundum mig- matised Biot-K-feld-corundum And-gar And-gar-sill Gar Gar-spinel-cord-plag-biot Gar-cord-biot-K-feld Sill-cord-biot-plag-qtz Sill-spinel-biot-cord-K-feld And-biot-cord-gar-K-feld Biot-cord-gar-plag-spinelopx
SiO2 53.71 47.86 61.48 54.9 45.1 47.0 52.3 48.6 40.77 48.20 56.70 47.65 50.14 37.20
TiO2 0.64 0.67 — 1.24 1.23 1.21 1.24 0.78 2.13 1.34 0.86 1.62 1.25 2.51
Al2O3 19.77 28.53 17.6 21.7 27.4 26.1 22.7 23.5 25.90 27.59 20.37 28.53 24.21 26.55
Fe2O3 1.38 — 3.65 1.54 1.51 1.00 1.42 — 1.02 0.80 0.57 1.20 2.02 3.01
FeO 4.95 4.52 — 6.84 7.37 9.59 6.13 7.92 17.52 8.64 5.24 8.50 7.91 15.45
MnO 0.08 0.02 — 0.10 0.13 0.24 0.10 0.17 0.24 0.15 0.18 0.06 0.25 0.26
MgO 3.37 0.87 1.40 3.36 2.70 3.00 2.78 3.40 4.95 4.96 3.12 3.90 5.76 5.29
CaO 2.52 0.41 0.47 1.45 0.61 2.50 1.24 5.74 2.07 0.79 2.49 0.03 1.48 3.30
Na2O 1.70 1.56 0.22 2.16 0.59 1.92 1.69 3.38 1.09 0.75 1.11 0.48 1.01 0.83
K2O 8.72 10.59 12.0 4.49 7.24 3.99 6.60 3.13 2.73 3.73 5.62 6.17 3.50 2.55
H2O+ 0.52 1.61 2.07 2.55 4.85 2.31 2.34 1.49 0.97 1.30 1.49 1.44 2.53 1.19
H2O− 0.58 — — — — — 0.26 — — — — —
P2O5 0.13 0.04 0.24 0.13 0.42 0.15 0.23 0.17 0.02 0.05 0.10 0.08 0.17 0.08
CO2 0.98 0.37 0.07 — — — — — — 0.24 0.15 0.27 0.18 0.01
Total 98.47 98.21 99.78 100.46 99.15 99.01 98.77 98.28 99.67 98.54 98.00 99.93 100.41 98.23
  • All contain quartz-plagioclase-muscovite-biotite
  • Mineral assemblages given for each specimen. Corundum-bearing rocks analysed by 'wet' chemistry, other specimens by XRF. Data for rocks from Kincardine coast from Porteous (1973b); homfels data from Boyd (1972).

(Table 2) Microprobe analyses of amphiboles in amphibolites from the Aberdeen area, mainly from Duncan, 1974. The naming of the minerals follows Leake (1978)

Aberdeen Formation

Elton Formation

 Collieston Formation
509 631 1305 1351 1837 2433 66 90 1324
SiO2 43.78 44.50 43.12 44.14 43.53 42.59 45.33 47.51 45,83
TiO2 1.16 0.74 0.85 0.79 0.65 0.93 1.44 0.87 0.38
Al2O3 10.90 12.19 11.53 11.19 11.01 12.77 7.11 9.01 9.40
tot. FeO 20.38 17.97 17.11 14.46 18.72 21.40 18.73 16.26 13.05
MnO 0.22 0.28 0.17 0.08 0.06 0.22 0.10 0.10 0.15
MgO 8.63 8.78 11.20 13.97 10.06 6.72 10.32 11.99 14.22
CaO 12.20 11.45 11.17 10.70 12.28 10.79 11.51 10.38 11.04
Na2O 1.11 0.80 1.18 1.55 1.29 1.01 1.26 1.37 1.71
K2O 0.86 0.60 0.49 0.24 0.39 0.52 0.81 0.47 0.07
Total 99.24 97.31 96.82 97.12 98.00 96.95 96.61 97.96 95.85

Cation proportions on the basis of 23 oxygen ions
Si 6.56 6.66 6.50 6.53 6.55 6.53 6.84 6.97 6.81
Ali' 1.44 1.34 1.50 1.47 1.45 1.47 1.16 1.03 1.19
Al" 0.49 0.77 0.55 0.48 0.50 0.83 0.29 0.53 0.46
Ti 0.13 0.08 0.09 0.09 0.08 0.11 0.17 0.10 0.04
tot.Fe2+ 2.56 2.25 2.16 1.79 2.36 2.74 2.36 2.00 1.62
Mn 0.03 0.04 0.02 0.01 0.01 0.03 0.01 0.01 0.02
Mg 1.93 1.95 2.52 3.08 2.26 1.54 2.31 2.63 3.15
Ca 1.96 1.84 1.81 1.69 1.98 1.77 1.86 1.63 1.76
Na 0.33 0.23 0.34 0.44 0.38 0.30 0.37 0.39 0.50
K 0.16 0.11 0.09 0.05 0.08 0.10 0.16 0.09 0.02
Mg/Mg + Fe2+ 0.48 0.53 0.71 0.84 0.56 0.45 0.53 0.68 0.82
Ferro-Hornblende —

Magnesio-hornblendes

 — Ferro- hornblende Edenite Magnesio-hornblende Edenite

 -

 

(Table 3) Chemical analyses and norms of a representative group of amphibolites from the Aberdeen area. Specimens 631 and 2433 contain garnet and were analysed by 'wet' chemical methods. Other analyses are by XRF and are taken from Duncan, 1974

Aberdeen Formation

Elton Formation

Collieston Fm.
Spec. No. 27 29 59 98 509 631 1305 1351 1837 2433 66 90 1338 1324 1330
Wt%
SiO2 54.60 59.03 47.50 49.18 51.72 50.11 49.72 49.35 54.33 48.87 51.26 57.50 47.87 49.70 55.00
TiO2 2.11 0.65 1.07 1.66 2.10 2.49 1.32 0.69 1.04 3.85 1.94 1.07 0.83 0.64 1.39
Al2O3 12.85 14.76 8.70 14.83 13.37 14.23 14.40 17.44 14.20 13.07 13.52 14.48 14.14 15.04 13.35
Fe2O3 1.29 1.74 1.09 1.62 1.53 1.76 4.36 1.88 2.62 3.38 2.27 4.21 3.32 1.78 1.70
FeO 8.92 4.42 8.61 8.95 10.46 12.73 8.08 6.72 7.95 15.55 10.52 6.20 8.44 7.87 7.12
MnO 0.13 0.20 0.14 0.17 0.22 0.33 0.19 0.13 0.28 0.29 0.18 0.14 0.19 0.14 0.14
MgO 3.51 7.32 22.00 7.22 5.15 5.85 6.32 7.90 5.07 4.49 5.72 4.34 11.25 9.21 5.58
CaO 14.10 5.30 8.70 10.80 10.45 8.79 10.05 12.06 8.45 8.03 10.62 7.90 9.71 11.18 8.66
Na2O 1.89 4.35 0.22 2.12 2.03 0.70 2.04 1.95 2.47 0.95 1.60 1.79 1.22 1.89 2.04
K2O 0.19 1.52 0.02 0.66 0.46 1.80 0.76 0.14 0.34 0.50 0.57 0.65 0.83 0.16 1.10
P2O, 0.38 0.17 0.58 0.28 0.24 0.16 0.16 0.18 0.17 0.30 0.27 0.16 0.14 0.15 0.25
CO, 0.03 0.08 0.05 0.11 0.14 0.14 0.11 0.06 0.06 0.07 0.09 0.10 0.23
H2O+ 0.81 1.44 2.45 1.91 1.35 0.71 2.02 1.57 1.23 0.52 1.59 1.10 2.83 1.81 1.93
Total 100.81 100.98 101.13 99.51 99.22 99.66 99.56 100.12 98.21 99.80 100.12 99.16 100.86 99.73 98.49
ppm
Cr 120 75 2993 401 63 300 334 216 83 197 1329 319 11
Ni 45 46 762 142 106 120 224 94 166 92 308 130 138
Rb 2 43 19 11 18 2 8 9 12 22 6 31
Sr 345 268 58 176 280 245 132 109 244 93 126 240
Q 12.07 5.49 0.49 7.41 6.57 4.83 0.34 11.65 11.26 7.71 20.71 0.15 12.22
OR 1.12 8.98 0.12 3.90 2.72 10.64 4.49 0.83 2.01 2.96 3.37 3.84 4.91 0.95 6.50
AB 15.99 36.81 1.86 17.94 17.18 5.92 17.26 16.50 20.90 8.04 13.54 15.15 10.32 15.99 17.26
AN 26.02 16.27 22.70 29.01 26.02 30.38 27.90 38.43 26.66 29.93 28.03 29.56 30.66 32.09 24.03
DI WO 17.31 3.72 6.96 9.50 10.13 5.09 8.73 8.44 5.91 3.32 9.56 3.58 6.93 9.35 7.22
EN 7.37 2.61 5.09 5.39 4.71 2.28 5.25 5.32 3.10 1.24 4.69 2.16 4.57 5.80 4.13
FS 9.97 0.80 1.22 3.70 5.32 2.79 3.02 2.61 2.63 9.95 4.69 1.23 1.86 3.00 2.78
HY EN 1.37 15.62 32.64 12.59 8.12 12.29 10.49 14.36 9.52 17.26 9.55 8.65 20.42 17.28 6.59
FS 1.85 4.80 7.83 8.65 9.16 15.03 6.04 7.04 8.08 11.26 9.55 4.91 8.30 8.93 12.22
OL FO 11.95 - - 2.12
FA 3.16 - - 0.95
MT 1.87 2.52 1.58 2.35 2.22 2.55 6.32 2.73 3.80 4.90 3.29 6.10 4.81 2.58 2.47
IL 4.01 1.24 2.03 3.15 3.99 4.73 2.51 1.31 1.98 7.31 3.69 2.03 1.58 1.22 2.64
NP 0.90 0.40 1.37 0.66 0.57 0.38 0.38 0.43 0.40 0.71 0.64 0.38 0.33 0.36 0.59

(Table 4) Sequences of cumulate assemblages and composition of cumulus phases in Mg-rich rocks in 'Younger Basic' intrusions in north-east Scotland

Belhevie

Insch

Huntly–Knock–Portsoy
Main culmulus phases

Mineral compositions

 Main culmulus phases

Mineral compositions

 Main culmulus phases

Mineral compositions
Fo An En Fo An En Fo An
Ol-plag-opx- 76 74 76 Ol-plag-opx-cpx 76 75 80 Ol-plag-opx
cpx 85 78 77 Ol-plag-opx 82 81 82 Ol-plag-opx 75 73
Ol-plag 87 83 — Ol-plag 84 84 — Ol-plag 81 75
Ol 86 — — Ol 83 — — 82 79

En-enstatite content of cumulus orthopyroxene, no clinopyroxene compositions are given. Belhelvie data from (Figure 18); Insch data from Ashcroft and Munro, 1978; Huntly-Knock-Portsoy data from unpublished work by Munro.

(Table 5) Details of meltwater channels shown in (Figure 33)

General characteristics Length  (km) Max. depth (m)
1  Concordant, in Dee Valley 1.5 20
2  Concordant, in Dee Valley 2 30
3   Concordant, in Dee Valley. Associated eskers 2 30
4  Concordant, in Dee Valley 1 4
5  Partly discordant. Series of channels 4 15
6  Lies in col. Cut in drift. May have drained a lake 1.5 4
7  Originates in col. Discordant in part 8 15
8  Originates in col. Discordant in part 4 3
9  Complex system of over 30 channels.
Main branches have arched profiles and cross topographic divides 4 8
10 Originates in col. Several tributaries.
Deviates into Don valley 1.5 4
11 Appears to have been outlet from a lake in the Kinellar area 2.6 10
12 Series of channels that turn to flow into the Don Valley 1 2
13 Discordant, cuts through a ridge 1.5 4
14 Arched profile, traverses a col where an anastomosing channel pattern develops 1.5 3
15 Originates in a col. 0.6 3
16 Concordant. Several tributaries 1 10
17 One channel in this system is discordant with an arched profile 1 4
18 Concordant in the main, although discordant to mounds of fluvioglacial deposits. Locally contains an esker system 4 8
19 Generally concordant, but cuts the trend of the fluvioglacial mounds 0.8 8
20 Generally concordant, but cuts the trend of the fluvioglacial mounds 2.2 20
21 Generally concordant, but cuts the trend of the fluvioglacial mounds 1 6

All channels are cut at least partly in bedrock, apart from channel 6. Based on Murdoch, 1977, table A2