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Geology of the Hamilton district. Memoir for 1:50 000 geological sheet 23W (Scotland)

By I B Cameron

Bibliographical reference: Paterson, I B, Mcadam, A D, and Macpherson, K A T. 1998. Geology of the Hamilton district. Memoir of the British Geological Survey, Sheet 23W (Scotland).

British Geological Survey

Memoir For 1:50 000 Geological Sheet 23W (Scotland)

I B Cameron, Palaeontology P J Brand M T Dean, Petrology E R Phillips, Geophysics M J Arthur K E Rollin, Hydrogeology N S Robins

Authors: I B Paterson, BSc formerly British Geological Survey, Edinburgh A D McAdam, BSc, CGeol K A T MacPherson, BSc, MSc, CGeol British Geological Survey, Edinburgh

Contributors: M J Arthur, BSc, MSc, DIC P J Brand, Bsc formerly British Geological Survey, Edinburgh London: The Stationery Office 1998 I B Cameron, BSc M T Dean, BSc, MPhil E R Phillips, BSc, PhD British Geological Survey, Edinburgh K E Rollin, Bsc British Geological Survey, Keyworth N S Robins, BSc, MSc, CGeol British Geological Survey, Wallingford

NERC copyright 1998 First published 1998. ISBN 0 11 884533 0

The grid used on the figures is the National Grid taken from the Ordnance Survey map. (Figure 1) is based on material from Ordnance Survey 1:50 000 scale maps number 64, 65, 71 and 72.  © Crown copyright reserved. Ordnance Survey licence no. GD272191/1998. Printed in the UK for the Stationery Office J37396 C6 4/98.

Other publications of the Survey dealing with this district and adjoining districts

Books

• British Regional Geology
• The Midland Valley of Scotland, 3rd edition, 1985
• Memoirs
• Geology of the Airdrie district, 1996
• Economic geology of the Central Coalfield of Scotland, Area VIII, East Kilbride and Quarter, 1917.
• Economic geology of the Central Coalfield of Scotland, Area VII, Rutherglen, Hamilton and Wishaw, 1920 Economic geology of the Central Coalfield of Scotland, Area IX, Carluke, Strathaven and Larkhall, 1921
• BGS Report
• Lithostratigraphy of the late Devonian and early Carboniferous rocks in the Midland Valley of Scotland. Vol. 18, No. 3, 1986.

Maps

• 1:625 000
• United Kingdom (North Sheet) Solid geology, 1979
• Quaternary geology, 1977
• Aeromagnetic anomaly, 1972
• Bouguer gravity anomaly, 1981
• 1:250 000
• Clyde Sheet (55N06W)
• Solid geology, 1986
• Aeromagnetic anomaly, 1980
• Bouguer gravity anomaly Map, 1985
• Borders Sheet (55N04W)
• Solid geology, 1986
• Aeromagnetic anomaly, 1980
• Bouguer gravity anomaly, 1981
• 1:63 000
• Sheet 22 (Kilmarnock) Solid, 1928
• Sheet 23 (Hamilton) Solid, 1929; Drift, 1929
• 1:50 000
• Sheet 14E (Cumnock) Solid, 1986; Drift, 1980
• Sheet 15W (New Cumnock) Solid, 1986; Drift, 1982
• Sheet 15E (Leadhills) Solid, 1987; Drift, 1983
• Sheet 23W (Hamilton) Solid, 1995; Drift, 1993
• Sheet 30E (Glasgow) Solid, 1994; Drift, in press
• Sheet 31W (Airdrie) Solid, 1992; Drift, 1992

Preface

Geology underpins a wide range of activities vital to the creation of wealth, particularly in relation to the exploration for and exploitation of resources. It is also vital that we have the best possible understanding of the geology of the United Kingdom if we are to maintain the quality of life whether through the identification of potential hazards prior to development or helping to ameliorate the problems created by future developments. The British Geological Survey is funded by central Government to improve our understanding of the three-dimensional geology of the UK through a national programme of geoscience surveying, data collection, interpretation, publication and archiving. One aim of this programme is to provide coverage of the UK land area by modern 1:50 000 scale geological maps, together with explanatory memoirs, by the year 2005. This memoir on the Hamilton district of the Midland Valley of Scotland, the first ever general account of the geology, is part of the output from that programme.

The past development and early prosperity of the district, which extends south from Hamilton towards the Southern Uplands, resulted from its favourable geology. Carboniferous coal, ironstone, seat-clay and limestone fuelled extractive industries during the 19th and early 20th centuries, and opencast exploitation of the coal continues to the present day. However, this past development is also the cause of many present-day problems, particularly in the north-west and south-east parts of the district and detailed knowledge of the abandoned underground mine-workings is essential when planning for development. Therefore, just as geology was vital to the past development of the Hamilton district, so it is essential to its future prosperity and well-being.

However, the geology of the district is also renowned for reasons other than its practical aspects. It has long been famous for the Silurian inliers of Lesmahagow and the Hagshaw Hills with their rich faunas of unusual fish and arthropods. The inliers provide information on the development of northern Britain in the waning phases of the Caledonian orogenesis during the late Silurian and early Devonian, and subsequently.

Major extrusion of lavas occurred only during the early Carboniferous. Large Lower Devonian intrusive sheet-like masses are the main source of hard rock aggregate. Many of the landforms result from the last ice sheet and its deglaciation, and meltwaters during deglaciation laid down extensive economic deposits of sand and gravel. A major glacial lake developed in the valleys of the River Clyde and the Avon Water, impounded by retreating ice, making it an important area for understanding the deglaciation history of the whole Midland Valley.

David A Falvey, PhD Director. British Geological Survey Kingsley Dunham Centre Keyworth Nottingham NG12 5GG

Previous surveys

The Hamilton district is covered by Sheet 23W of the Geological Map of Scotland at the 1:50 000 scale. The primary geological survey of the area covered by the sheet was carried out by J Geikie and B N Peach and published as Sheet 23 of the Geological Map at the 1:63 360 scale in 1872. An explanatory account was published in the following year (Geikie et al., 1873). The area covered by Sheet 23W was resurveyed by C T Clough, L W Hinxman, J S Grant Wilson, E M Anderson, R G Carruthers, M Macgregor, B Lightfoot, C H Dinham, J E Richey, G V Wilson, G Ross, A G MacGregor, J B Simpson and J Phemister and the results published in Drift and Solid versions of 1:63 360 Sheet 23 in 1929. No explanatory memoir was produced.

The rocks in parts of Sheet 23W were covered in the course of a mining revision of the Douglas Coalfield by G I Lumdsen in 1950 and of the Muirkirk Coalfield in 1960 by A Davies. The volcanic rocks north-west of East Kilbride were remappcd by P M Craig in 1967 and a reappraisal of the drift geology of part of the sheet was carried out by A D McAdam and P Stone in the course of an assessment of sand and gravel resources (Nickless et al., 1978). The second resurvey proper of the sheet commenced in 1976, however, with a re-examination of the rocks in the Lesmahagow Inlier by J D Floyd, M J Gallagher and A D McAdam, accompanied by extensive palaeontological collecting mainly by P J Brand and D E White. The resurvey was completed in the period from 1978 by M Armstrong, J M Dean, I H Forsyth, D N Halley, A D McAdam, A A McMillan, K A T MacPherson, Dr S K Monro and I B Paterson, much of the fieldwork being done in connection with a series of projects commissioned by the Department of the Environment (Davies et al., 1982; Paterson and Hall, 1982; Paterson et al., 1989; 1990b).

Drift and Solid editions of Sheet 23W have been published.

Notes

• The word 'district' is used in this memoir to denote the area included in the 1:50 000 Geological Sheet 23W (Hamilton).
• National Grid references are given in square brackets throughout the memoir. Unless otherwise stated, all lie within the 100 km square NS.
• Numbers preceded by the letter S refer to the Sliced Rock Collection of the British Geological Survey.

Acknowledgments

This memoir was compiled by I B Paterson and edited by D J Fettes and A D McAdam.

Chapters in this memoir have been written by the following authors: Introduction, Devonian and Structure by I B Paterson; Silurian by A D McAdam (Lesmahagow Inlier), I B Cameron (Hagshaw Hills Inlier); P J Brand (Biostratigraphy) and I B Paterson (Conditions of sedimentation); Carboniferous by K A T MacPherson and I B Paterson; Carboniferous biostratigraphy by P J Brand; Intrusive igneous rocks by I B Paterson and E R Phillips; Quaternary by A D McAdam and I B Paterson; Economic geology by K A T MacPherson (Mineral deposits), N S Robins (Groundwater) and M J Arthur (Geothermal energy); Geophysics by M J Arthur and K E Rollin.

The cores and samples from the Ross House Borehole, drilled by the BGS in 1986 to investigate the stratigraphy of the Quaternary sediments were examined by M A E Browne and A A McMillan. The cores from the Fore Hareshaw Borehole, drilled in 1993 to determine the stratigraphical position of the Carboniferous succession, were examined by I B Paterson and K A T MacPherson. The faunas were collected and identified by M T Dean. The cores from boreholes drilled by civil engineering contractors to investigate foundation conditions for roads, buildings and other works in the district were examined by D N Halley, P M Halpin and K I G Lawrie.

The Silurian macrofauna was identified by D E White, S P Tunnicliff and P J Brand and the Carboniferous macrofauna was revised by P J Brand. Thin sections of the igneous rocks of the district were examined by E R Phillips who contributed the petrography section in Chapter 6. The photographs were mainly taken by T Bain and F I MacTaggart.

Geology of the Hamilton district—summary

The district described in this memoir extends south from Hamilton to the valley of the Greenock Water. It is covered by Sheet 23W of the 1:50 000 geological map of Scotland.

The introductory chapter briefly describes the physical features of the district and outlines its geological development. The oldest rocks in the district, consisting of marine and continental strata of Silurian age, occur in inliers in the south. They are described with particular regard to their place in the structural development of southern Scotland and their biostratigraphy. A brief description of the Lower Devonian rocks is followed by an account of the Carboniferous sequence, which includes a thick pile of lavas, and its biostratigraphy. The structural development of the district as a whole is discussed, emphasising the manner in which sedimentation was influenced and at various times interrupted by local or regional uplift or by movements on faults. An account of the intrusive rocks of various ages is followed by a chapter on the Quaternary features and deposits in the district. There is an outline of the economic geology of the district and the final chapter reviews the available geophysical data.

Chapter 1 Introduction

The Hamilton district described in this memoir is contained in Sheet 23W of the 1:50 000 geological map of Scotland. The district is still largely rural, urban development being essentially confined to low ground in the north. The southern part of the district, consisting of undulating upland country reaching heights of almost 500 m, is mainly given over to agriculture and forestry. In the north the chief centres of population are Hamilton, Larkhall, parts of Motherwell and Wishaw, and the rapidly growing 'New Town' of East Kilbride. The important centres in the south are the old market town of Strathaven, Stonehouse and the villages of Glassford, Chapelton and Kirkmuirhill (Figure 1). Apart from small areas around Loudoun Hill and in the south, the district lies entirely within the catchment of the River Clyde, which flows north-westwards passing between Larkhall and Wishaw. The Avon Water and the River Nethan are tributaries which flow north-eastwards to join the Clyde. The Greenock Water flows south and west towards the River Ayr. All are deeply incised in places and provide good exposures.

The oldest rocks in the district are of Silurian age, occurring in the upland area in the south in the inliers of Lesmahagow and the Hagshaw Hills (Figure 2). They are of particular interest because of the information they provide regarding the development of sedimentary basins in northern Britain during the period when it was being assembled from fragments of continental crust brought together by subduction processes. The oldest Silurian strata were laid down in a basin which initially was marine but with somewhat restricted access to the open sea. As the basin filled, mainly with greywacke and siltstone delivered by turbiditic flow, water depths decreased and, following a period when nearshore deposits were laid down, terrestrial conditions were established under which formations of conglomerate, sandstone and lacustrine mudstones and siltstones were deposited. Terrestrial conditions also prevailed while strata considered to be of Lower Devonian age were laid down, consisting of a basal conglomerate formation overlain by a mainly arenaceous sequence. The conglomerate differs from earlier comparable deposits in that it consists almost exclusively of well-rounded greywacke clasts. Elsewhere, there is a marked unconformity at an equivalent stratigraphical position but no angular break can be detected within the district. That a non-sequence exists at this position in the Hamilton district also is suggested by the marked attenuation of the underlying Plewland Sandstone in the Hagshaw Hills as compared with the Lesmahagow Inlier.

Throughout this early period, the district lay within a sinistral transpressive stress regime. Major NE-trending crustal fractures were formed, movements on which had long-lasting effects on the sedimentation and structural development of the district. Some of these fractures were doubtless reactivated during the episode of intense tectonic activity which affected northern Britain during the Middle Devonian. As a consequence of the associated uplift, sedimentation in the district was suspended until late in the Upper Devonian or during the early Carboniferous when subsidence affected the Midland Valley as a whole. As a result the fluviatile cornstone-bearing sandstones and silty mudstones of the Kinnesswood Formation in the lower part of the Inverclyde Group were deposited, probably throughout the entire district. These were succeeded by the grey mudstones and dolomitic limestones of the Ballagan Formation, laid down in a coastal environment as continuing subsidence of the basin allowed access of the sea to most parts of the Midland Valley. Towards the end of the Tournaisian, the Midland Valley and adjacent upland areas were subjected to earth movements. The initial consequence of these was to cause a resumption of fluvial sedimentation and the Clyde Sandstone Formation was deposited over much of the Midland Valley basin. Subsequently, as a result of continuing uplift accompanied by movements of fault-bounded blocks, the lnverclyde Group strata were exposed to erosion and were entirely eliminated from large areas within the Midland Valley basin, including most of the Hamilton district.

The erosive episode was followed by widespread volcanicity in the Midland Valley and northern parts of the district were covered by the mainly basaltic lavas of the Clyde Plateau Volcanic Formation and the volcanic detrital rocks of the Kirkwood Formation, which are derived from them. During the later Carboniferous, sandstones, mudstones, seatrocks and coals were laid down in cyclic sequences in a fluviodeltaic environment. From time to time Shelly limestones and mudstones were laid down as a result of marine incursions and at other times sedimentation was interrupted during periods of renewed uplift. In some cases, it can be demonstrated that re-activation of faults influenced sedimentation. The presence of highly productive coal seams in the north and in the Muirkirk and Douglas basins to the south has had a profound effect on the demographic development of the district.

Eruption of the Clyde Plateau Volcanic Formation was accompanied by the emplacement of a number of intrusive igneous bodies. However, the most numerous and extensive intrusions in the district belong to the suite of mainly acid rocks which cut the rocks of the Silurian and Devonian. These include the dioritic rocks of the Distinkhorn complex, the only major Caledonian pluton in the Midland Valley. In the later Carboniferous and in the Tertiary, dykes and sills, mainly of doleritic composition, were intruded.

During the Quaternary, the district was covered on several occasions by ice sheets which moulded the topography and laid down widespread deposits of till and fluvioglacial sand and gravel. During deglaciation of the last major ice sheet, the district lay across the line of separation between ice withdrawing to the north into the Highlands and south towards the Southern Uplands and a large lake developed in the intervening ground. When the ice impounding the lake finally melted, the sea entered the district for a time and marine deposits were laid down in lowlying ground in the north.

Chapter 2 Silurian

Sedimentary rocks of Silurian age occur in a series of inliers along the southern margin of the Midland Valley. Parts of two of these — almost the whole of the large Lesmahagow Inlier and the northern part of the Hagshaw Hills Inlier — lie within the Hamilton district. The inliers afford some insight into the development of early Phanerozoic basins in northern Britain but the geological setting of the inliers, in terms of the relative positions of lithospheric plates in Silurian times, remains conjectural.

Overall, the sequences in the Lesmahagow and Hagshaw Hills inliers are similar in that both record a transition from marine to continental sedimentation. However, the transition is abrupt in the Hagshaw Hills as compared with the Lesmahagow Inlier and it is apparent that several formations recognised in the Lesmahagow Inlier are not represented in the Hagshaw Hills sequence where there is a marked non-sequence (Figure 3). The inliers lie on opposite sides of the NE-trending Kerse Loch Fault which clearly was active in late-Silurian times, with an effective downthrow towards the north. There is no evidence for determining whether the fault was active over a prolonged period while the transition to terrestrial conditions was taking place or whether the area to the south was uplifted and eroded after the transition was complete. It is possible that the fault movements involved lateral displacement but the similarities between the successions in the two inliers are such that any such displacement is likely to have been small.

The sequences in both inliers differ markedly, however, from that in the adjacent Southern Uplands. Furthermore, it does not appear possible that any existing terrane in the area south of the Midland Valley can have given rise to the full range of clasts, notably those consisting of quartzite and acid volcanic rocks, which occur in southerly derived conglomerates in the inliers (Bluck, 1983; Heinz and Loeschkc, 1988). It has been suggested that in Lower Palaeozoic times the Midland Valley and Southern Uplands depositional basins lay some distance apart and were juxtaposed by sinistral strike-slip fault movements associated with the oblique collision in late-Silurian times of the East Avalonian and Laurentian plates (Soper and Woodcock, 1990; Soper et al., 1992).

The Silurian rocks of both inliers are disposed in asymmetrical anticlines, faulted along their NE-trending axes. The fold structure in the Hagshaw Hills is tighter than in the case of the Lesmahagow Inlier, the strata being overturned in places along the length of the south-east limb.

Lesmahagow Inlier

A lithostratigraphy of the Lesmahagow Inlier was first set out by Peach and Home (1899). It provided a basis for the resurvey of Sheet 23, published in 1929, on which, however, only certain of the divisions were depicted. The stratigraphy was greatly elaborated by Jennings (1961) who named many new formations which he placed in three groups, in ascending order: Priesthill, Waterhead and Dungavel groups. With only minor changes, this stratigraphy has been adopted. It should be noted that the thickness estimated for many of the units has been revised as a result of the resurvey.

The lower part of the succession is of marine origin and contains moderately abundant faunas that are useful for subdividing the sequence, for correlation on a local and regional basis, and for providing information on the environment of deposition.

Priesthill Group

The stratigraphical classification of the group used here differs in part from that proposed by Jennings (1961). The lower part of the sequence of greywackes and siltstones formerly assigned to the Patrick Burn Formation is referred instead to the newly recognised Ponesk Burn Formation, on the grounds that derived faunas found in greywacke beds in the latter include brachiopods, trilobites and crinoids, indicative of an open sea marine environment, whereas in the former the derived faunas in the greywacke beds are mainly molluscan and of low diversity — typical of a restricted marine basin. The distribution of the fossil localities strongly suggests that the boundary between the two formations is a NE-trending fault with a displacement equivalent to a downthrow of several hundred metres. As the oldest outcropping strata of the Ponesk Burn Formation are juxtaposed by faulting against much younger strata, the base of the group is nowhere seen. The boundaries of the Castle, Kip Burn and Blaeberry formations have also been redefined to take account of the new fossil evidence and a reassessment of the faulting that affects the rocks around the Logan Reservoir. At the top of the group, the Passage Formation has been eliminated and the predominantly argillaceous strata in its lower part assigned to the Dunside Formation. The top of this formation, which also marks the top of the Priesthill Group, is arbitrarily taken at the incoming of red beds into the sequence.

Ponesk Burn Formation

This formation is confined to an elongate fault-bounded tract along the axis of the anticlinal structure which affects the rocks of the inlier and in consequence neither the top nor base of the formation is seen. The preserved thickness of the formation exceeds 200 m.

The rocks are well seen in the Ponesk Burn [NS 717 304] to [NS 733 320], upstream from Priesthill, and in the Greenock Water downstream from Greenock Bridge [NS 698 296]. They consist predominantly of beds, up to 1.5 m thick but mainly from 0.2 to 0.3 m thick, of grey quartzose greywacke which pass up into grey siltstone. The greywacke beds are commonly graded and ripple-and slump-bedding may be present. The erosive bases of many beds show load, groove, flute and prod casts, and the structures termed longitudinal ripples by Jennings (1961). Intercalated between sequences of greywacke beds, which are considered to have been laid down by turbiditic flows, are beds up to 15 m thick consisting of grey siltstone and silty mudstone with laminae of darker grey carbonaceous siltstone.

Some greywacke beds contain a derived shelly fauna including brachiopods, trilobites and crinoids indicative of a fully marine, shelf environment. The argillaceous units have yielded a fauna characterised by Ceratiocaris. The fauna differs significantly from the mainly molluscan, low-diversity fauna found in the overlying Patrick Burn Formation.

Patrick Burn Formation

The strata in this formation are generally similar to those in the Ponesk Burn Formation, consisting of series of turbiditic flow units with intercalations, up to 20 m thick, of grey siltstone and silty mudstone with laminae of carbonaceous siltstone. As in the Ponesk Burn Formation, the greywacke beds are up to 1.5 m thick, with a general thickness of 0.15 to 0.3 m and show a similar range of sedimentary structures. In some cases, the fine-grained upper part of flow units is absent. A derived, mainly molluscan fauna occurring in the pebbly base of some of the turbiditic flow units is markedly less diverse than that in the Ponesk Burn Formation.

In the siltstones, the dark grey carbonaceous laminae are commonly fossiliferous and in some cases exude hydrocarbon from freshly broken surfaces. Siltstone makes up approximately 25–30 per cent of the sequence but is more abundant in the upper part, being the predominant lithology in beds up to 20 m thick, commonly containing concretions. Four such units with a combined thickness of 55 m around Priesthill [NS 720 311] are generally correlatable with siltstone beds in Leaze Burn [NS 732 330] and on Nutberry Hill [NS 743 338], and with the Jamoytius Fish Bed at Birk Knowe [NS 738 346]. As in the case of the Ponesk Burn Formation, the siltstones contain a fauna, apparently indigenous to the basin, which consists of fish and Ceraliocaris. Fossils are especially common in ovate calcareous septarian concretions up to 0.20 m across lying on the bedding planes. These beds probably lie within the lower part of the 'Ceratiocaris Beds' of Peach and Horne (1899).

Siltstone beds with the characteristic fauna including Ceratiocaris occur throughout the Patrick Burn Formation. The derived, predominantly molluscan fauna occurs less commonly but is equally widely distributed through the succession and on the available evidence the two assemblages must be regarded as broadly contemporaneous.

The formation has an estimated thickness of about 850 m in the area around Nutberry Hill but this must be regarded as a minimum as the outcrop is affected by a number of strike faults. As a result of the large fault which forms the southern margin of its outcrop, little more than 100 m of strata in the upper part of the formation are present at Priesthill.

Castle Formation

According to Jennings (1961), this formation consists of about 60 m of massive fine-grained micaceous siltstones with shale partings which can be seen underlying the Teratiocaris Beds' at 'Shank's Castle' [NS 745 362]. However, at this locality, the 'massive siltstones' are seen to consist of quartzose, siltstone grade, turbiditic flow deposits, in some cases more than a metre thick, with bases showing load and flute casts. The distinctive massive siltstone beds, which are interbedded with bedded grey siltstones, occur through a thickness of only about 25 to 30 m, but the formation is widespread and maintains its stratigraphical position in relation to the succeeding Kip Burn Formation.

In addition to exposures in the type locality at Shank's Castle, there are good, though incomplete, sections in the Kip and Patrick burns. A complete, but imperfectly exposed, section of the formation occurs high on the eastern valley side [NS 710 307] of an unnamed tributary of the Greenock Water and there are also good exposures [NS 712 308] in a tributary, a little to the east. Siltstones in the Eaglin Burn [NS 760 349], placed in the Castle Formation by Jennings (1961), are now considered part of the Blaeberry Formation on faunal evidence obtained during the resurvey. No fossils have been recovered from the Castle Formation.

Kip Burn Formation

The formation consists of grey and olive-grey mudstones and silty mudstones with laminae of dark grey carbonaceous siltstone or silty mudstone. Calcareous nodules are common. The strata are richly fossiliferous and have yielded a fauna characterised by Ceratiocaris, fish and eurypterids. The base of the formation is seen at Shank's Castle where carbonaceous siltstones pass up by rapid transition from the massive siltstones and mudstones of the Castle Formation. The uppermost beds in Kip Burn [NS 732 347] are transitional, the top of the formation being defined at the point where greenish grey siltstone laminae predominate in the sequence. This lithological change is accompanied by a change in faunal content, the fisheurypterid-Ceratiocaris fauna being replaced by an assemblage of Lingula, ostracods and molluscs. The boundary must lie at a somewhat lower stratigraphical level than that proposed by Jennings (1961) as his fossil lists for the upper part of the Kip Burn Formation, as defined by him, includes forms, such as Lingula minima and Platyschisma which are characteristic of his Blaeberry Formation.

The Ceratiocaris-bearing strata at Shank's Castle were singled out for special mention by Peach and Horne (1899) but it is clear that they formed only one of a number of fossiliferous beds within their Teratiocaris Beds'. It follows that, according to the definition of its boundaries proposed by Jennings (1961) and adopted here, the Kip Burn Formation includes only the uppermost part of the Teratiocaris Beds' of Peach and Home, the remainder lying within the upper part of the Patrick Burn Formation. The carbonaceous laminated siltstones of the Kip Burn section constitute their Pterygotus Beds'.

As defined here, the formation is approximately 50 m thick and shows little lateral variation in thickness within its outcrop. The strata in its upper part are well seen in the Blaeberry Burn section [NS 736 356], which is terminated to the east by a fault which throws down strata of the Blaeberry Formation. The formation is also exposed in the Leaze Burn [NS 725 328], where the strata are repeated by faulting, in the headwaters [NS 720 316] of the Patrick Burn and in Lamon Burn [NS 689 301].

Blaeberry Formation

The type locality of the formation is in Blaeberry Burn [NS 735 356] to [NS 732 357] where about 130 m of poorly bedded green-grey mudstones, silty mudstones and siltstones with calcareous ribs are exposed. Although carbonaceous laminae occur, they are nowhere as conspicuous as in the underlying beds. The strata are richly fossiliferous, containing a fauna that is characterised by modiolid bivalves, gastropods, Lingula and ostracods. The outcrop of the formation, displaced by numerous faults, can be traced eastwards to the Logan Water, and south-westwards by Kip Burn, Leaze Burn, Patrick Burn and the streams above Waterhead to those on the lower slopes of Middlefield Law, including Lamon Burn.

Dunside Formation

This formation is a sequence of mostly green-grey micaceous sandstone, with subordinate beds of siltstone and mudstone which are especially common in the lower part. The sandstones are flat and cross-bedded in beds from a few centimetres to 1.5 m thick, with rare layers of mudstone flakes. The strata in the lower part of the succession are bioturbated and fossiliferous, the fauna resembling that in the underlying Blaeberry Formation. The formation takes its name from a section [NS 748 373] at Dunside Reservoir where it is about 35 m thick. The passage from the Blaeberry Formation is transitional, the base being taken at the base of the lowest prominent greyish green sandstone bed. The upper boundary of the Dunside Formation is also transitional being taken at the incoming of red beds into the sequence.

There are good exposures of the formation in the Blaeberry Burn [NS 732 357], the Leaze Burn [NS 721 328], the Greenock Water [NS 699 308] and the Lamon Burn [NS 693 301].

Waterhead Group

The strata of the group are of continental origin and consist mainly of red and purple siltstones and sandstones. Six formations have been recognised, two containing strata most probably of lacustrine origin, the others being fluvial.

Leaze Formation

This is the first dominantly 'red' formation in the succession, consisting of purple to grey-purple, fine- to medium-grained sandstone with thin beds of dark purple siltstone and mudstone. The sandstone beds are mostly from 10 to 50 cm thick but in some cases exceed 1 m. They are mainly planar bedded, in some cases showing parting lineation, or ripple-bedded with the thickest beds being cross-bedded. Mudstone flakes are commonly present. Mudcracks are common in the mudstones, and trace fossils, possibly arthropod tracks, occur at a few localities. At the type locality in Leaze Burn [NS 721 328] to [NS 710 328], the formation is approximately 400 m thick.

The formation is best exposed in the Leaze Burn. Farther east, the rocks are seen in the Kip and Blaeberry burns and other tributaries of the Logan Water, and in the head waters of the Birkenhead Burn. In the west, cliff sections occur along the Greenock Water, notably opposite its confluence with the Dippal Burn and also below Waterhead. The formation also crops out in streams on the eastern and southern slopes of Middlefield Law.

Birkenhead Sandstone

This distinctive formation persists throughout the inlier. As the name Yellow Sandstone given by Peach and Horne (1899) implies, the formation is composed of yellow, orange or brown-weathering, soft, fine- to medium-grained sandstone. Large-scale, low-angle cross-bedding and ripple-bedding are common. Pebbly horizons occur with subangular pebbles up to several centimetres across, mainly of vein-quartz and rarely of jasper, chert and acid igneous rocks. This assemblage is comparable to that in the Parishholm Conglomerate of the Hagshaw Hills Inlier. Thin bands of pale yellow clay and layers with green or red mudstone flakes occur in a few places between sandstones.

At its maximum, the formation is somewhat more than 100 m thick thinning to about 50 m in the south-west of the inlier. There are good exposures of the upper part at the type locality in the much-faulted section in the Birkenhead Burn [NS 761 359]. The sandstones are also exposed in Hareshaw Burn in the north-east, in the Kype Water and in the Back Burn. Exposures in the Dippal Burn and its tributary the Leaze Burn are discontinuous but the transitional top and base of the formation are both seen. Some of the beds in this section show ripple-lamination. In the banks of Greenock Water, pebbly beds occur, and there are exposures of sandstone on the eastern and southern slopes of Middlefield Law.

Dippal Burn Formation

The sequence consists of green and grey beds of sandstone, siltstone and mudstone. The Dippal Burn Fish Bed near the base is a dark grey fissile mudstone, 1 to 2 m, thick with a rich fauna. In the overlying sequence, beds of grey ripple-bedded siltstone or very fine-grained sandstone, 2 to 30 cm thick, are interbedded with grey or greenish grey silty mudstone and mudstone. The uppermost beds are dominantly greenish grey, silty, micaceous, planar and ripple-bedded sandstone, commonly with sharp erosional bases. A few of the thicker beds are cross-bedded and load structures also occur (Plate 1). The total thickness of the formation is about 70 m at its type section along the Dippal Burn, strata in the lower part being well seen in a spectacular gorge [NS 694 318]. However, the contact of the formation with the underlying Birkenhead Sandstone is nowhere exposed.

The most westerly exposures of the Dippal Burn Formation are on the easterly slopes of Middlefield Law. Eastwards, the formation can be traced with certainty only as far as the head of Kip Burn and Auchingilloch Glen. However, grey sandstone in Birkenhead Burn [NS 762 359] may lie within the upper part of the formation. The Fish Bed contains a diverse fauna mainly of fish and eurypterids.

Monument Formation

The sequence consists mainly of brick red and red-purple, fine- to medium-grained, micaceous sandstone with subordinate beds of purple silty mudstone and siltstone. The sandstones are commonly planar bedded, but ripple- or cross-bedding also occurs. Mudstone clasts are common, scattered through the sandstone beds or in layers.

The formation is named after the Covenanters' Monument near the type locality in Auchingilloch Glen [NS 713 356] where, however, only a small part of the formation is present. A more complete section of about 140 m occurs in a small unnamed tributary to the Dippal Burn [NS 696 324]. The base of the formation is taken where the beds change from grey or green to purple or red.

The Monument Formation is also exposed in ditches on the eastern slopes of Middlefield Law and, together with the Dippal Burn Formation, is involved in a complicated fault structure in the headwaters of the Kip Burn. It also occurs in the Birkenhead Burn where about 40 m of beds are exposed. Throughout the inlier, the sequence is barren of fossils.

Slot Burn Formation

These beds are similar in lithology and fauna to the Dippal Burn Formation and prior to the work of Jennings (1961) the two sequences were regarded as the same. Most of the sequence is thin alternating beds of greenish grey mudstone, siltstone and silty sandstone. The sandstones show planar bedding, or are ripple- or cross-bedded. The bases of beds are commonly erosional and in some cases show load structures.

Near the base of the sequence are grey and red variegated mudstones which are transitional from the red Monument Formation beneath. Above these lies the Slot Burn Fish Bed, which consists of 5 m of grey mudstones and laminated siltstones with laminae containing well- preserved fish and arthropods. This lithology recurs a number of times in the lower part of the formation at the type locality [NS 679 319] in Slot Burn and its tributary Spindle Burn. The thickness of the formation in this area is approximately 85 m.

The formation can be traced north-eastwards by way of exposures in the northern tributaries of the Dippal Burn and in Auchingilloch Glen. The rocks were formerly quarried farther to the north but most of these sections are now overgrown. In the east, outcrops occur in the Logan Water, south of Auchrobert, and the Birkenhead Burn, where there are good sections of the fish beds.

Logan Formation

This formation shows a return of the conditions which existed while the Monument Formation was laid down. Purple or red-purple micaceous sandstones, from a few centimetres to a few metres thick, are interbedded with siltstones and silty mudstones. Planar bedding and ripple-bedding are characteristic and layers of mudstone flakes are common. The type locality of the formation is at good exposures in the Logan Water [NS 753 372] to [NS 764 378]. The base of the formation is marked by a change of colour to purple or red from the grey or green beds of the Slot Burn Formation, as in Slot Burn [NS 676 318].

The outcrop of the formation in the inlier as a whole is greatly affected by faulting and it is difficult to assess its thickness. At least 150 m of beds must be present, however, in the Regal Burn section [NS 689 334] to [NS 681 330] and the actual thickness may exceed 250 m. Jennings (1961) considered the formation to be only 8 m thick where exposed in the Birkenhead Burn [NS 767 360] but its contact with the Middlefield Conglomerate to the east is faulted.

In addition to the sections in the Logan Water, Regal Burn and the headwaters of Slot Burn and Spindle Burn, the formation crops out in the northern part of the inlier but here the ground is poorly exposed. No stratigraphically useful macrofossils have been found in these rocks, although spore assemblages have been recovered.

Dungavel Group

This group contains only two thick formations, which reflect the establishment of settled terrestrial conditions.

Middle Field Conglomerate

In this formation, conglomerates containing well-rounded boulders, 5 to 50 cm in diameter with haematite-stained pellicles, alternate with beds of soft yellow, grey or purple, coarse-grained pebbly sandstone (Plate 2). In the conglomerate beds, the boulders are predominantly of grey quartzite, less commonly of vein quartz, granite and jasper. The matrix, which forms only a small proportion of the rock, is a grey-purple coarse-grained feldspathic sandstone. In the sandstones, the pebbles are sub-angular, lack a red-stained skin and consist largely of vein quartz with some of chert, jasper, basic igneous rock and schistose grit.

The type locality of the formation is on the north-west slopes of Middlefield Law where yellow pebbly sandstone forms a spectacular gorge and waterfalls in the headwaters of the Spindle Burn [NS 676 313]. The base of the Middlefield Conglomerate is taken at the base of the lowest bed of pebbly sandstone or conglomerate. The formation has an extensive, much-faulted outcrop commonly forming high ground, such as Bibblon Hill and Millstone Rig in the western part of the inlier, and Auchrobert Hill in the east. It is poorly exposed and its thickness is not easily determined but may be as much as 250 m. However, at Craigs Hill [NS 605 304] in the extreme west, quartz-pebble conglomerate is estimated to be no more than 50 m thick.

In addition to the Spindle Burn, there are good exposures in the Glengavel Water, the Powbrone Burn and its headwaters — Dead Grain and Self Grain, and along Halls Burn and Dyke Burn in the north-west, in the Kype Water to the north, and in the Logan Water and the River Nethan to the east. The formation occurs in two fault-bounded outcrops in Birkenhead Burn. In the west, it forms scarp features on Craigs Hill and is exposed in the Wood, Pennel and Cove burns.

Plewland Sandstone

The sandstones that constitute the bulk of this formation are purple to grey-purple, fine to medium grained, micaceous and commonly cross-bedded, with layers of mudstone flakes and rare dark purple mudstone partings. The type locality is a cliff section in Glengavel Water [NS 657 355]. The base of the sequence is taken at the change from conglomerate to sandstone, although this is nowhere seen.

Large areas in the west of the inlier are underlain by the sandstone as well as smaller areas in the extreme west, north and east. There are numerous small exposures, but few good continuous sections, the best being in the Glengavel Water between High Plewland and Laigh Plewland, in Little Grain and Middle Grain, and in the headwaters of the Powbrone Burn. The thickness of the formation is hard to estimate across a wide poorly exposed outcrop, which may be affected by unrecognised faulting. However, in the south-west of the inlier between Pennel Burn and Meanlour Hill, the thickness may exceed 1400 m. In the eastern part of the inlier, the rocks are so severely affected by faulting that no realistic estimate of their thickness is possible.

The Plewland Sandstone is overlain with apparent conformity by the Greywacke Conglomerate, which is arbitrarily taken as the base of the Lower Devonian.

Hagshaw Hills Inlier

A stratigraphy for the Silurian rocks was first proposed by Peach and Horne (1899) and elaborated by Rolfe (1961). As in the Lesmahagow Inlier, the oldest formations are of marine origin but it is probable that none of the marine strata are younger than those in the oldest formation of the Lesmahagow succession. There is a considerable gap in the Hagshaw Hills sequence and the marine strata are succeeded abruptly by the Parishholm Conglomerate (the Igneous Conglomerate of Peach and Horne (1899), and the probable equivalent of the Birkenhead Sandstone of the Lesmahagow Inlier). The succeeding strata are predominantly red-coloured, fluvial or lacustrine sediments and include well-known fish-bearing beds.

The rocks are disposed in the form of an asymmetric anticline, aligned north-eastwards and with the southern limb locally inverted. The axial trace of the anticline is marked by a north-dipping reverse fault, the Parish Holm Fault, which throws down to the south-east. The whole area of the Hagshaw Hills inlier is described including the part to the south in the New Cumnock district (Sheet 15W).

Hagshaw Group

The group contains two fossiliferous marine formations.

Smithy Burn Siltstone

This formation includes the oldest strata seen in the inlier and is named from its occurrence in the Smithy Burn near Low Broomerside [NS 796 291]. The rocks consist of rather massive grey siltstones and muddy siltstones. The siltstone beds are about 0.4 m thick and in places there are thin discontinuous laminae of fine-grained sandstone or siltstone and dark films of mudstone. A faint parallel bedding is present locally.

The formation has a discontinuous outcrop on the north-west side of the Parish Holm Fault. The full thickness is not known as the outcrop is partially or in some places totally truncated by the fault. The greatest thickness is seen in the Ree Burn, where about 125 m of strata crop out between the fault and the base of the Ree Burn Formation. Thicknesses seen in the Podowrin Burn and the Smithy Burn are about 90 m and 85 m respectively. The siltstones can also be seen in the Monks Water.

The Smithy Burn Siltstone is poorly fossiliferous. It has yielded orthocone fragments and a collection of graptolites, subsequently lost, which was said to indicate the crenulata Biozone of the upper Llandovery.

Ree Burn Formation

The formation takes its name from the Ree Burn. In the section south of Parish Holm, it is divisible into three members but these have not been mapped in the Hamilton district. The lowest of the three, approximately 160 m thick, consists of dark grey mudstones and shales with thin fine- and medium-grained greywacke beds. Angular to subangular lithic clasts in the greywackes include rhyolite, spilite, keratophyre, andesite and various metamorphic rocks (Rolfe, 1961, p.247). The greywacke beds, which are considered to represent the lowest component of turbiditic flow units, in some cases display bottom structures and commonly pass rapidly by gradation into the overlying siltstone or mudstone. On average, the ratio of siltstone and mudstone to greywacke is approximately three to one.

In the overlying member, approximately 105 m thick, laminae of pale grey-green siltstone and silty mudstone alternate with thinner laminae of dark grey mudstone. In some instances there is gradation from siltstone to mudstone but in many cases the distinction is clear-cut. Some siltstone beds show signs of current action and there are also instances of intrastratal convolution. Concretions are present and have been found to enclose fossils including Ceratiocaris and orthocone fragments. Beds of siltstone and fine-grained sandstone up to 0.1 m thick also occur but are rare.

At the top of the formation there is a group of relatively thick greywacke beds up to 0.6 m thick. These are pale grey and generally medium grained but in some cases are coarse grained in the basal part. The greywacke beds are separated by thin greenish grey mudstone partings. The bases of the beds commonly show load-cast structures. About 11 m of strata of the subdivision are exposed in the Ree Burn south of Parish Holm. The greywacke member has also been recognised also in the Monks Water and in the Smithy Burn.

The best exposures of the formation are at the type section in Ree Burn [NS 763 281] to [NS 761 276], south of Parish Holm, where a transitional boundary with the underlying Smithy Burn Siltstone is seen. The formation crops out also in the Monks Water, the Podowrin Burn and the Smithy Burn, and there are sporadic exposures [NS 752 267] in a tributary of the Douglas Water, west of Urit Hill. According to Rolfe (1961, p.247, table 1) , directional structures such as cross-bedding, ripples and flute and load casts indicate a consistent southerly derivation. The formation is estimated to be about 290 m thick.

A fragmentary fauna, chiefly of trilobites, brachiopods and orthocones (below and see Rolfe, 1961), occurs as washed-in detritus in the basal parts of greywacke beds. The intervening siltstone beds have yielded a sparse fauna, indigenous to the sedimentary basin, which, in addition to Ceratiocaris from the concretions, is now known to include graptolites.

Glenbuck Group

As defined by Rolfe (1961), this group consisted of four formations — in upwards sequence, the Douglas Water Arenite, the Dovestone Redbeds, the Fish Bed Formation and the Gully Redbeds — all of terrestrial origin. They consist mainly of red-brown and red-purple siltstones and sandstones, with an intercalation, probably lacustrine, of grey-green siltstones and mudstones. It is proposed here that the definition of the group be altered so as to include the Parishholm Conglomerate at its base. The Glenbuck Group then becomes the approximate equivalent of the Waterhead Group of the Lesmahagow Inlier.

Parishholm Conglomerate

The Parishholm Conglomerate succeeds the Ree Burn Formation without apparent angular unconformity, although there is a sharp lithological contrast between the two formations and comparison with the succession in the Lesmahagow Inlier suggests that the boundary marks a major non-sequence. The type locality is in the valley of the Douglas Water close to Parish Holm [NS 761 281]. There are exposures at several other localities in the valley of the Douglas Water and in the Shiel Burn [NS 777 291], a tributary of the Monks Water. The formation is estimated to be about 40 m thick.

The formation is composed of reddish grey unsorted, apparently matrix-supported, pebble conglomerate. It is roughly bedded with beds up to 0.5 m thick. Clasts, up to 0.12 m in diameter but usually less than 0.03 m across, are mainly angular to subrounded, only a few being well rounded. They consist of fine-grained reddish brown acid igneous rocks, fine-grained intermediate and basic igneous rocks, quartzite, green, grey and black chert, jasper and greywacke. The commonest are the fine-grained acid igneous rocks and chert is also conspicuous. The matrix is a coarse grit. McGiven (1968) suggested that the conglomerate was deposited as a terrestrial alluvial fan, mainly by sheet floods and possibly also by stream floods flowing generally towards the north.

A few limestone pebbles found in the conglomerate are the only indication of age so far found (Rolfe and Fritz, 1966). Stromatoporoids and bryozoans in some pebbles indicate that the conglomerate is not older than Wenlock.

Douglas Water Arenite

The Douglas Water Arenite succeeds the Parishholm Conglomerate with apparent conformity. However, the contact between the two formations is nowhere seen. The arenite is well exposed [NS 757 278] in the Douglas Water and there are also exposures at other points along the outcrop to the north-east. The strata consist of reddish brown or reddish grey medium-grained sandstones, as beds which are either massive or show large-scale cross-bedding, particularly in their upper part. Locally the sandstones are yellowish grey or buff coloured and near the top of the formation fine-grained slightly micaceous planar bedded red sandstone occurs. The thickness is estimated to be about 80 m.

Dovestone Red Beds

The Dovestone Red Beds follow the Douglas Water Arenite without a noticeable break. The rocks consist of poorly bedded dark reddish brown muddy siltstone, silty mudstone and fine silty sandstone. They are finely micaceous and in some exposures show small-scale cross-bedding. The bedding is up to 0.14 m thick and there are also a few calcareous beds up to 50 mm thick. They are exposed near the dam at the east end of Glenbuck Loch [NS 762 283]. There are exposures on the south side of the loch beside the A70 and in the Douglas Water north-west of Urit Hill. There are also small exposures in the Shiel Burn tributary of the Monks Water and at the north end of the Smithy Burn. The formation is about 72 m thick.

Fish Bed Formation

The formation consists of greenish grey silty mudstones and siltstones interbedded with a few fine-grained sandstone beds up to 1 m thick, especially in the upper part. The siltstones are laminated and commonly show small scale cross-bedding and in a few places convolute bedding. They are considered to be lacustrine deposits. The basal bed of the formation in the Shiel Burn is a brownish grey medium-grained sandstone about 0.6 m thick, which rests with apparent conformity on dark reddish brown silty mudstones with pale green and cream beds of fine-grained sandstone. The formation is exposed at the east end of Glenbuck Loch [NS 762 284] and in the Shiel Burn tributary of the Monks Water and there are small exposures of the formation in the Douglas Water northwest of Urit Hill. It is estimated to be about 58 m thick.

Rolfe (1961) gives details of the position of two fossiliferous beds of finely laminated siltstone at Glenbuck Loch and one in the Shiel Burn. The fauna, mainly of fish and eurypterids, is very similar to the fauna of Ringerike, Norway, considered to be of Ludlow age. However, an assemblage of miospores recently collected from exposures at Glenbuck Loch is said to indicate an earliest Wenlock age for the strata (Molyneux, 1993). The fish bed probably correlates with the Dippal Burn Fish Bed of Lesmahagow.

Gully Red Beds

The Gully Red Beds is the topmost subdivision of the Glenbuck Group and indicates a return to conditions similar to those which existed during deposition of the Dovestone Red Beds. They are exposed on the eastern shore [NS 762 286] of Glenbuck Loch, and are also seen in small exposures in an unnamed burn on the south side of Hareshaw Hill and in an unnamed triburary of the Douglas Water [NS 768 284]. The strata on the eastern shore of Glenbuck Loch consist of reddish brown silty mudstone with some fine laminations. It weathers to a small blocky structure with thin irregular hard beds. Some silty beds show convolutions and others are slightly micaceous with small-scale cross-bedding. There are also sandstone beds up to 0.1 m and siltstone beds up to 0.04 m.

The exposures in the Monks Water consist of fine-grained red sandstone and dark red-brown silty mudstone which is finely micaceous, its colour being altered by reduction to pale grey-green in places. The bedding is thin and poorly developed. The formation is estimated to be about 95 m thick and is unfossiliferous.

Monks Water Group

In order to achieve a consistent approach to the stratigraphy of the Silurian inliers, it is proposed that the youngest formations, the Hareshaw Conglomerate and Quarry Arenite (Rolfe, 1961), be assigned to a new group, namely the Monks Water Group. Both formations are of terrestrial origin.

Hareshaw Conglomerate

The formation consists of beds of conglomerate, pebbly sandstone and coarse sandstone with a few scattered pebbles. In the Douglas Water [NS 752 272], an exposure shows about 12 m of massive conglomerate with a vague stratification. The rock is poorly sorted to unsorted and there is no obvious imbrication. It contains cobbles and boulders up to 0.4 m across. Elsewhere the conglomerate occurs in thinner beds with smaller clasts and is interbedded with pebbly sandstones. There are erosion surfaces within the formation with remnant patches of pebble conglomerate resting on the underlying beds. The Hareshaw Conglomerate is considered to be generally equivalent to the Middlefield Conglomerate of the Lesmahagow Inlier. The contact with the underlying Gully Red Beds is nowhere exposed but, as only a short distance separates exposures of the two formations, an abrupt lithological change is indicated.

The clast content of the conglomerate consists of rounded to well-rounded haematite-coated quartzite pebbles, cobbles and rarely boulders and, as clasts of smaller sizes, vein quartz, fine-grained acid igneous rocks, chert and greywacke. The matrix is a reddish grey or grey coarse quartzose sand with lithic grains. The conglomerate beds are usually clast supported.

On the northern limb of the Hagshaw Hills Anticline, where the formation is estimated to be 65 m to 70 m thick, there are exposures in the Douglas Water, northwest of Dovestone Rig [NS 753 273], at the east end of Glen-buck Loch [NS 762 286], in an unnamed right-hand tributary [NS 771 291] of the Monks Water, and in the Podowrin and Smithy burns. On the southern limb of the anticline, the conglomerate crops out alongside the Parish Holm Fault. It is nowhere present in its full thickness at crop and in places has been entirely eliminated by faulting.

Quarry Arenite

The Quarry Arenite overlies the Hareshaw Conglomerate. It is the youngest formation assigned to the Silurian in this area, and is considered to be generally equivalent to the Plewland Sandstone of the Lesmahagow Inlier. The rock consists chiefly of reddish brown, greyish brown and brownish grey, medium-grained slightly micaceous sandstones. They occur in beds 0.5 to 1.2 m thick, commonly massive in the lower part and with platy parallel bedding in the upper part. Large-scale cross-bedding also occurs. The lithology at the top of the formation is slightly different and consists of beds of relatively soft, friable red sandstone with grey-green or cream reduction spots; interbedded with dark reddish brown muddy siltstone.

The Quarry Arenite forms broad outcrops in the north-western part of the inlier and in a NE-trending strip along the south-east side of the Parish Holm Fault. The formation is exposed in the Monks Water and some of its tributaries, in the Podowrin Burn and in the Smithy Burn, south of Parish Holm. It is estimated to be about 450 m thick.

Biostratigraphy

Fossil assemblages recovered from the two Silurian inliers in the Hamilton district differ somewhat and they are therefore described separately. The most obvious differences are the presence of beds containing graptolites in the earlier rocks in the Hagshaw Hills Inlier and the variation in composition of the shelly faunas, in particular the trilobites. It would appear that the faunas in the lower part of the succession in the Hagshaw Hills Inlier are more closely allied to those of the Girvan and the Pentland Hills areas than are those of the Ponesk and Patrick Burn Formations of the Lesmahagow Inlier.

The bulk of the BGS collection from the area is housed in the Edinburgh office and most of the specimens have been named by S P Tunnicliff and D E White. Their conclusions have been used extensively in this account.

Lesmahagow Inlier

Ponesk Formation

Two differing faunal assemblages can be recognised in the rocks of this formation. Greywackes low in the sequence contain, in the coarser fraction at the base, a derived assemblage including Howellella sp., Pentlandella pentlandica, Protatopa ?, Skenidioides sp., Strophochonetes sp., Loxonema sp., Eoschizodus?, Cornulites sp., Tentaculites sp., Encrinurus hagshawensis, Podowrinella straitonensis and Beyrichia sp. This is similar to the assemblage described from the Knockgardner Formation of the Girvan area (Sheet 14W, Ayr) by Cocks and Toghill (1973, pp.238–239), who suggested an early Wenlock (Sheinwoodian) age for the formation and considered the fauna to be characteristic of deposition under fairly shallow marine conditions. Thomas and Lane (1984, pp.58 and 62) held that Podowrinella may well have lived in a marginal marine environment where high energy and/or varying salinity conditions prevailed. They also held that the Encrinuridae may have lived at least partially buried with a shallow burrowing or ploughing mode of life.

Recent work on the graptolite faunas of the Blair Shale Formation which underlies the Knockgardner Formation at Girvan suggests that this too may be of earliest Wen-lock rather than late Llandovery age (crenulata Biozone) (Zalasiewicz, 1992). However, Howells (1982), who revised the Scottish Silurian trilobite faunas, identified both E. hagshawensis and P. straitonensis in the upper part of the Deerhope Formation of the Pentland Hills (Sheet 32W, Livingston). Graptolites from this formation are of late Telychian age (upper part of the crenulata Biozone, the European spiralis Biozone) (Bull, 1987, p.117). Furthermore, acritarchs recovered from the Wetherlaw Linn Formation above the Deerhope Formation suggest a mid to late Telychian age, probably in the range crispus to crenulata graptolite biozones (Molyneux, 1992). It thus remains unclear whether the oldest known fossiliferous rocks in the Lesmahagow Inlier are of late Telychian or early Sheinwoodian age, no graptolites having, as yet, been found.

Beds thought to be higher in the Ponesk Formation contain a less varied fauna in thin-bedded greywacke units. The fauna includes Howellella anglica, Pentlandella pentlandica, Molinicola? and Orthoceras cf. araneosum. Associated with orthocone fragments are poorly preserved cricoconarids. The fauna is distinguished from that previously described by a greater presence of benthonic elements, although the brachiopods still suggest a source in which fairly shallow-water marine conditions prevailed as compared to the constricted marine environment indicated by the mainly molluscan fauna found in the greywacke beds of the succeeding Patrick Burn Formation.

The type material of Howellella anglica is of late Telychian age and, together with Pentlandella pentlandica, occurs in upper Telychian strata in the Llandovery area (Cocks et al., 1984, pp.158–159). However, these species continue into the lower Wenlock of the Girvan area, where H. anglica is quickly replaced by H. elegans, and are therefore of no great value in determining the precise age of the containing strata.

Patrick Burn Formation

Two distinct faunal facies are present in this formation, one a derived fauna occurring in beds of fine sandstone with mudstone clasts, the other in finely laminated sediments with rare calcareous nodules. Forms present in the first of these include Loxonema sp., Pleuromphalus? simulans, Colpomya?, cf. Ctenodonta sp., cf. Deceptrix sp., Eoschizodus?, cf. "Fuchsella" amygdalina, Goniophora?, Modiomorpha sp., Molinicola cf. gotlandica, Mytilarca cf. acutirostra, Praenucula sp., Vlasta?, Hyolithus sp., Dictyocaris slimoni and Ceratiocaris sp. The fauna also includes nuculoid genera such as Praenucula which Kriz (1984, pp.185–l88) held were infaunal deposit feeders, whilst many of the other genera were regarded by him as either epibyssate or endobyssate semi-infaunal suspension feeders living on unstable sediments. The fauna contains elements in common with the fauna described by Liljedahl (1984) from rocks of Homerian age in Gotland.

The second faunal association has been collected from a number of localities at different levels within the formation including the Jamoytius Fish Bed of Ritchie (1960) which lies near the top. The fauna consists mainly of Ceratiocaris papilio, Cyamocephalus loganensis„Slimonia acuminata, Erettopterus bilobus, Hughmilleria sp., Jamoytius kerwoodi and Logania scotica. Pleuromphalus?, Pteronitella sp. and orthocone fragments have also been found but are uncommon. The general aspect of the sediments indicates deposition in quiet-water conditions, the fauna being mainly benthonic. Rolfe and Beckett (1984, p.31) have suggested that deposition took place in a restricted marine environment at no great distance offshore, with foul bottom waters which were nutrient rich. The dominant species, Ceratiocaris papilio, specimens of which are to be found with the gut preserved, was regarded by Rolfe and Beckett (1984, p.34) as a detritus feeder. Turner (1973, pp.571–575) considered that the thelodont fish from the upper part of the formation were of lower Wenlock age.

Kipburn Formation

Beds in the basal part of the formation contain an assemblage of chelicerates, including Carcinosoma scorpioides, Erettopterus bilobus, Hughmilleria lanceolata, Paracarcinosoma obesa, Slimonia acuminata and Stylonurella sinipes. The fauna also includes Neolimulus falcatus, numerous specimens of Ceratiocaris papilio, Beyrichia (Beyrichia) sp. nov. and fish remains, including Birkenia elegans and Logania scotica. The fauna is distinguished from that in the earlier Patrick Burn Formation principally by the presence of Beyrichia (Beyrichia) sp. nov.

Blaeberry and Dunside Formations

The faunas denote a distinct change of environment from that of the preceding formation and consist mainly of Lingula minima, Pleuromphalus? simulans, small tapered tubes, which are possibly scaphopods of the Dentalium group, Modiomorpha sp. A and Spathella sp., together with swarms of Beyrichia (Beyrichia) sp. nov. Ceratiocaris sp. and fragments of eurypterids are still present, however, particularly in the lower part. Towards the upper part of the Blaeberry Formation the rocks become more sandy and pass upwards into the Dunside Formation. In this upper part the tapered tubes no longer occur and species of another beyrichacean genus, Gen. and sp. nov. A and sp. nov. B become more important in the ostracod fauna, although Beyrichia (Beyrichia) sp. nov. persists. The macro-fauna of the Dunside Formation is more restricted than that of the Blaeberry Formation and only Lingula sp. and Pleuromphalus? simulans are at all common. This illustrates the end of the change from a benthonic fauna in the Kip Burn Formation with only few bottom-living forms, to possibly shallower water with an abundant bottom fauna and clean waters in which ostracods could flourish.

Leaze Formation

In the basal beds of the formation, intercalations of greenish mudstone amongst reddened beds contain Lingula sp., indeterminate gastropods and the beyrichacean Gen. nov. noted earlier. The red beds are apparently unfossiliferous.

Dippal Burn Formation

Finely laminated siltstones and mudstones associated with greenish mudstones in the Dippal Burn and its minor tributaries [NS 6925 3170] to [NS 7109 3359] have yielded an assemblage consisting of Pachytheca sp., Parka sp., Mixopterus sp., Ateleaspis tesselata, Birkenia elegans, Lanarkia horrida, L. spinulosa, Lasanius problematicus and Logania scotica.

Slot Burn Formation

Fish-bearing finely laminated siltstones and mudstones, similar to those in the Dippal Burn Formation, are associated with greenish mudstones containing indeterminate plant debris, possibly algal. Exposures in the finely laminated beds occur in the Slot Burn [NS 6803 3205], east of South Hill [NS 727 407] and in the Birkenhead Burn [NS 7632 3593]. These beds and associated strata contain an assemblage consisting of Glauconome sp., Pachytheca sp., Taitia catena, Brachyopterella ritchiei, Lanarkopterus dolichoschelus, Birkenia elegans, Lanarkia horrida, L. spinulosa, Lasanius altos, L. annulus, L. problemalieus, Logania scotica and L. taiti. At both South Hill and in the Slot Burn an arthropod of uncertain affinities, Pseudarthron, also occurs. Selden and White (1983) suggest that the environment of deposition was fresh or brackish water, possibly quiet and lagoonal.

Richardson and Ford, quoted by Selden and White (1983, p.44), suggest that miospores from the overlying Logan Formation are no older than late Ludlow in age. However, Wellman and Richardson (1993), working on a larger series of samples, consider that the plant assemblages from the Dippal Burn, Slot Burn and Logan formations indicate an early Wenlock age, in the range earliest Sheinwoodian (lower part of the centrifuges Biozone) to Homerian (upper part of the tundgreni Biozone). They obtained similar floras in material from the Hagshaw Hills Fish Bed Formation and the Lynslie Burn Fish Bed in the Pentland Hills and assigned them the same general age. They concluded that the Dippal Burn, Slot Burn and Logan formations were laid down in a nonmarine environment and that the fish beds were laid down in freshwater lacustrine conditions. Turner (1973, pp.571–575) suggested that the thelodont fishes from the Dippal Burn and Slot Burn formations were of late Wenlock to early Ludlow age.

No biostratigraphically useful material has been recovered from Dungavel Group strata.

Hagshaw Hills Inlier

The succession and fauna were originally described by Rolfe (1961). This account includes references to parts of the inliers in the adjacent New Cumnock district to the south and Lanark district to the east.

Smithy Burn Siltstone

The formation is poorly fossiliferous with only orthocone and bivalve fragments to be found in existing collections. Rolfe and Fritz (1966) refer to a lost collection from a location in the Ree Burn near the faulted junction of the Smithy Burn Formation with the Hareshaw Conglomerate [NS 7614 2750] which was said to have contained Monoclimacis crenulata, M. griestoniensis, Monograptus marri, M. priodon and M. spiralis. Attempts to rediscover this assemblage have so far failed. If correctly determined M spiratis would suggest a late Telychian age.

Ree Burn Formation

The fauna includes Atrypa reticularis, Glassia sp., Howellella sp., Leptaena cf. rhomboidalis, Pentlandella pentlandica, Strophochonetes aff. edmundsi, ?Holopella, Cornulites sp., Ctenodonta sp., Nuculites sp., Acernaspis sp., Encrinurus hagshawensis, Hemiarges rolfei, Podowrinella straitonensis and orthocone fragments. This list includes forms identified by Rolfe (1961) and Howells (1982) as well as those in BGS collections. Except for the orthocone fragments, which are found in thin intercalations of finely laminated siltstone, the fauna occurs in greywacke beds. As in the case of the Ponesk Formation, the shelly faunas are presumed to have been derived from an area in which fairly shallow water marine deposition was taking place. The trilobite fauna bears a closer resemblance to that from the Pentland Hills and from the Knockgardner Formation of Girvan than does the fauna of the Ponesk Formation. In particular, species of Acernaspis and Hemiarges are present which have so far not been collected from Lesmahagow. Localities in the Monks Water [NS 7792 2911], Podowrin Burn [NS 7854 2949]; [NS 7863 2945], Smithy Burn [NS 7914 2977] and Windrow Burn [NS 7991 3008] all contain some or many of these species.

In the Ree Burn [NS 7613 2762] a silty mudstone horizon has yielded a number of examples of the graptolite Retiolites geinitzianus geinitzianus associated with orthocone fragments. The species occurs in rocks ranging in age from the crenulata Biozone to the centrifuges Biozone, thus spanning the Llandovery to early Wenlock interval, similar to that suggested by the non-graptolitic faunas.

The middle part of the formation is characterised by beds of thinly laminated siltstone with calcareous nodules containing Ceratiocaris papilio, similar to the beds in the upper part of the Patrick Burn Formation.

Parishholm Conglomerate

In the Douglas Water [NS 7576 2775], pebbles from this formation have yielded forms which Rolfe and Fritz (1966, p.161) regarded as indicating a Wenlock age.

Glenbuck Fish Bed Formation

Exposures at Glenbuck Loch [NS 7614 2842], Shiel Burn [NS 7770 2904] and Smithy Burn [NS 7885 3007]; [NS 7870 2998] have all yielded fish. The combined fauna consists of Glauconome sp., Pachytheca sp., Taitia catena, Lanarkopterus dolichoschelus, Ateleaspis tesselata, Birkenia elegans, Lanarkia horrida, L. spinulosa, Lasanius problematicus, Logania scotica and L. taiti. All the locations are thought to lie at the same horizon. Ritchie (1960) correlated the horizon on stratigraphical grounds with the Dippal Burn Fish Bed rather than with the Slot Burn Fish Bed. However, the general aspect of the flora and fauna more closely resembles that of the Slot Burn horizon.

A diverse assemblage of miospores and acritarch material including Ambitisporites sp., Nodospora?, Moyeria cabottii and Synsphaeridium? has been obtained from rocks at Glenbuck Loch [NS 7613 2840] (Molyneux, 1992). The presence of Ambitisporites and the absence of strongly ornamented trilete miospores suggests that a Telychian or Sheinwoodian age is likely for the host rocks. Wellman and Richardson (1993) also examined material from this and nearby localities and concluded that the plant assemblages obtained were of earliest Sheinwoodian to latest Sheinwoodian or Homerian age, similar to those from the Dippal Burn and Slot Burn formations of the Lesmahagow Inlier. None of the strata above the Fish Bed Formation have yielded any useful biostratigraphical ages and, as in the case of equivalent strata in the Lesmahagow Inlier, their precise age remains uncertain.

Conditions of sedimentation

The successions in all the Midland Valley Silurian inliers reflect a passage from a wholly marine to a terrestrial environment. In the case of the Lesmahagow Inlier the change is gradational. In the Hagshaw Hills the transition appears abrupt because there is a large hiatus in the sequence, there being no representatives of the strata of the Patrick Burn to Leaze formations inclusive.

The oldest sedimentary rocks of Silurian age in the district are the massive or poorly laminated, somewhat carbonaceous or pyritous, argillaceous rocks of the Smithy Burn Siltstone of the Hagshaw Hills Inlier. The siltstones contain a sparse mainly pelagic fauna which includes graptolites. Deposition may have taken place relatively rapidly under quasi-euxinic conditions (Rolfe, 1961, p.244), presumably at a location some distance from the shoreline of a basin with restricted access to the open sea.

Conditions were significantly different while the succeeding Ree Burn Formation of the Hagshaw Hills Inlier was laid down. The presence of numerous greywacke beds, the composition of the lithic clasts within them and the evidence of their direction of transport imply that the basin was being supplied with sediment from an area of continental crust lying to the south and south-west. Large amounts of arenaceous sediment were deposited in fully marine conditions in shelf seas along the margin of this continental mass, as indicated by the diverse fauna of brachiopods, trilobites, crinoids and molluscs found in the greywacke beds, before being delivered northwards into the basin by turbiditic flows. Within the basin, argillaceous sediment continued to be laid down below wave base in anoxic conditions.

There is a gap in the succession above the Ree Burn Formation in the Hagshaw Hills Inlier. In the Lesmahagow Inlier, however, the rocks of the Ponesk Burn Formation, which bear a close similarity to those of the Ree Burn Formation, are succeeded by the Patrick Burn Formation the greywacke component of which contains a low diversity, mainly molluscan fauna. This suggests that deposition was taking place in a marine evironment which, as a whole, had become more constricted. However, the fish and arthropod fauna found in the laminated carbonaceous siltstones which intervene between the greywacke beds shows little change throughout, suggesting that conditions in the central part of the basin were stable.

During deposition of the Kip Burn Formation, greywacke deposition no longer reached the Lesmahagow area, perhaps because of a failure of the sediment supply, and only carbonaceous mudstones and siltstones were laid down in quiet-water conditions in a greatly constricted basin. Subsequently, water^-depths decreased and, during Blaeberry Formation times, mud and silt were laid down, possibly in the distal parts of deltas. The salinity was less than fully marine as is indicated by a fauna of Lingula, ostracods and molluscs analogous to that found in the Ballagan Formation of the Lower Carboniferous.

As the deltas advanced into the basin, fluviatile deposits appear in the sequence for the first time, represented by the cross-bedded sandstones with rip-up clasts of mudstone in the upper part of the Dunside Formation. Mudstone beds in the lower part of the formation contain a very restricted fauna consisting of species such as Lingula, Modiolopsis, Euomphelites and ostracods which were capable of surviving in the fluctuating salinities of a deltaic or estuarine environment. By the close of the Dunside Formation, silting up of the basin in the Lesmahagow area was complete and the red-brown sandstones and siltstones of the Leaze Formation were laid down in the channels and floodplains of rivers, which were probably sustained by a low to moderate, strongly seasonal rainfall.

The abrupt lithological change that occurs at the base of the Birkenhead Sandstone in the Lesmahagow area probably reflects a non-sequence which is not, however, as marked as that which preceded deposition of its supposed equivalent in the Hagshaw Hills Inlier, that is, the Parishholm Conglomerate. The difference between the sequences in the two areas are considered to result from tectonic activity which caused the Hagshaw Hills Inlier to be uplifted relative to the Lesmahagow Inlier and subjected to erosion prior to deposition of the conglomerate. It is uncertain, however, whether the uplift was preceded by a period during which differential subsidence across a major fault meant that the sequence in the Hagshaw Hills Inlier was already attenuated. The two inliers occur on opposite sides of the NE-trending Kerse Loch Fault and it seems likely that this structure, which had a major effect on sedimentation during the Carboniferous period, was active during the Lower Palaeozoic. There is no evidence in the strata of the Lesmahagow Inlier which would suggest that they were laid down on the neighbourhood of a major growth fault.

Both the Birkenhead Sandstone and the Parishholm Conglomerate are considered to be of fluviatile origin and to have been laid down by rivers which flowed generally towards the north (Jennings, 1961; McGiven, 1968). The assemblages of clasts within the two formations include rock types not presently occurring in the Southern Uplands; hence it has been suggested that they originated in an area of continental crust which formerly lay to the south of the Midland Valley basin and which was subject to island arc volcanism (Heinz and Loeschke, 1988).

In both inliers, sandstones and siltstones, predominantly red-brown in colour, were laid down in fluviatile conditions during most of the ensuing period. Mudcracks in the siltstone beds indicate that the level of the water table fluctuated in response to a highly seasonal rainfall, with consequent drying out of the floodplain deposits. On two occasions, in at least parts of the Lesmahagow area, and once in the Hagshaw Hills Inlier, fluvial sedimentation was interrupted when the drainage was impeded and large lakes formed. These were infilled by generally upwards coarsening sequences typical of deltas. The beds of laminated grey-green and dark grey mudstone and siltstone (rhythmite) in the lower parts of the Dippal Burn and Slot Burn formations of the Lesmahagow Inlier and the Fish Bed Formation of the Hagshaw Hills Inlier were laid down while water depths were at their greatest.

The deposition of conglomerates and pebbly sandstones in the basal parts of the Dungavel Group of the Lesmahagow Inlier and the Monks Water Group of the Hagshaw Hills Inlier marks a significant change in environmental conditions. The existence of the high-energy braided river systems which laid down the large quartzite boulders of the Middlefield and Hareshaw conglomerates reflects either an inceased rainfall or proximity to a source area with considerable relief. It is also noteworthy that for the first time the principal source area lay to the north of the sedimentary basin, as indicated by cross-bedding measurements from the Plewland Sandstone (Jennings, 1961), which was probably also laid down by braided rivers. It is inferred that a major tectonic event preceded deposition of the Dungavel and Monks Water groups.

Chapter 3 Devonian

The Devonian rocks within the district consist of mainly purplish grey cross-bedded sandstones with a conglomerate formation at the base and, in places, a volcanic formation above. The stratigraphical classification adopted, comprising in ascending sequence the Greywacke Conglomerate, the Swanshaw Formation and the Duneaton Volcanic Formation, is that proposed by Smith (1993) for the more complete sequences in the New Cumnock district (Sheet 15W) to the south. The sequence is thin by comparison with adjacent areas and has yielded no fossils within the district. Tectonic activity accompanied by uplift during the Middle Devonian period meant that either no deposits of this age were laid down within the Midland Valley or any such deposits, as well as most or locally all of the Lower Devonian rocks, were removed by erosion. This activity also caused a major NE-trending valley to be incised. As a result of regional subsidence, sedimentation resumed within this valley during Upper Devonian times (Paterson et al., 1990a) but did not extend into the Hamilton district.

Greywacke Conglomerate

The formation consists of conglomerate with a few interbedded sandstone beds. The conglomerate includes cobbles and boulders up to 0.25 m across but mostly less than 0.07 m in diameter. The clasts are predominantly subrounded and most consist of greywacke but quartzite and chert are also present. The rock is clast supported, poorly sorted and roughly bedded (Plate 3). Imbricate structure indicates derivation from the south. Normally the matrix is a dark grey-brown gritty sandstone. In some exposures the voids between the clasts are filled with calcite. Thin beds of sandstone up to 0.2 m thick, which tend to be more common in the upper part of the formation, were noted in some exposures. Near Monksfoot a lenticular body of sandstone was mapped within the conglomerate.

The greatest thickness of conglomerate occurs in the Podowrin Burn area where there is an estimated thickness of 560 m. This is reduced to about 150 m in the outcrop west of Glenbuck Loch. In the absence of any biostratigraphical evidence, the base of the formation is used as the base of the Lower Devonian.

The formation occurs in a NE-trending outcrop on the south side of the Hagshaw Hills Anticline. There are also outcrops on the north side of the anticline near the the west end of Glenbuck Loch, The best exposures are in the Podowrin Burn [NS 795 287], south-west of Low Broomerside, and in the Monks Water near Monksfoot [NS 7856 2850]. The conglomerate on the south side of the anticline is near vertical.

The base of the formation is exposed in the Monks Water [NS 7848 2864], 170 m upstream from Monksfoot. The conglomerate rests upon an eroded surface which can be seen cutting across beds of the underlying Quarry Arenite. The base is also seen in the Ree Burn [NS 7620 2708] just south of the district boundary, resting with apparent conformity on plane-bedded sandstones of the Quarry Arenite.

Swanshaw Formation

This subdivision includes the strata overlying the Greywacke Conglomerate and below the thin development of lavas and volcaniclastic rocks seen in tributaries of the Wood Burn [NS 615 293]. The most extensive outcrops of the formation lie to the west and north-east of the Lesmahagow Inlier. The formation also crops out in the Hagshaw Hills area and west of Nether Whitehaugh in the south-west. The thickness of strata occurring in the district is estimated to be 700 to 800 m.

The rocks consist of medium-grained sandstones, reddish brown or brownish grey in colour. The lower parts of the beds tend to be rather massive and the upper part platy bedded or cross-bedded. Small mudstone clasts are common in the basal parts of the beds. Well-rounded to subangular clasts including quartzite, felsite, chert and lava occur in thin beds of conglomerate at the base of channel-fill sandstones, for example in exposures in the Fairy Burn [NS 726 443], north-east of Sandford, and in the Dipple Burn [NS 618 344]. Typical sandstones of the formation are exposed in the Kype Water [NS 716 433], at Sandford. In the Burnbrae Burn [NS 7110 4245], a little downstream from Burnbrae Bridge, typical sandstones of the formation are overlain by a thin flow of andesitic lava. This is overlain in turn by a bed of soft red-brown siltstone, about 0.5 m thick, which is succeeded by a sequence containing beds of very coarse conglomerate with subrounded clasts of quartzite and basic lava. A fault following the course of the burn, throws down lavas of the Clyde Plateau Volcanic Formation to the west.

Duneaton Volcanic Formation

The rocks of this formation are well developed in the area south of the Hamilton district where they consist largely of andesitic lavas and volcanic conglomerates. Within the district the formation is known only in the extreme south-west where volcaniclastic rocks overlain by andesitic lava are exposed in tributaries of the Wood Burn [NS 615 292].

Chapter 4 Carboniferous

Rocks of Carboniferous age crop out in the low ground in the northern half of the Hamilton district and occur also in small outcrops in the south-east. The district includes parts of three coal basins, namely the Central, Douglas and Muirkirk coalfields, each with its own stratigraphy and local coal seam names. The basins were formerly of great importance as a source of minerals and, especially during the last century, there was intensive extraction not only of coal, but also of ironstone, fireclay, limestone and sandstone. Exploitation of mineral resources continues, employing opencast methods, but attention has now largely shifted to the problems of land stability in areas affected by underground mining.

The lithostratigraphical classification of the Carboniferous rocks (Table 1) accords with that used in the memoir for the Airdrie district (Forsyth et al., 1996). As a result of late-Devonian uplift and erosion no rocks of Upper Devonian age exist in the district and the oldest Carboniferous strata, which are placed in the Inverclyde Group (Paterson and Hall, 1986), rest unconformably on Lower Devonian or older rocks. Tectonic activity resumed during early Carboniferous times causing uplift and erosion of the Inverclyde Group with the result that its strata are now confined to the south-east of the district. If any sediments of this age were laid down in the north, they were uplifted and eroded prior to the formation of the thick pile of mainly basaltic lavas and associated volcaniclastic sedimentary rocks which lies at the base of the Strathclyde Group. The lavas consequently rest unconformably on sandstones of Lower Devonian age. Conversely, there is no representative of the volcanic formation in the south-east, perhaps because this area formed a topographical high.

In the period immediately following the volcanic episode, a regime of cyclic sedimentation was established which persisted throughout the remainder of the Carboniferous. Except for the deposition in the neighbourhood of the lava pile of locally derived volcaniclastic material, sedimentation took place in a fluviodeltaic environment, producing thick sequences composed of grey sandstones, siltstones and mudstones, fossil soil horizons and coal seams. Periodic marine incursions are represented by beds of fossiliferous limestone and mudstone. The most widespread of these constitute marker horizons and have been used in the subdivision of the sequence with the bases of the Hurlet Limestone and the Lowstone Marine Band marking the bases of the Clackmannan Group and Coal Measures Group, respectively.

Differential movement of fault-bounded blocks within the generally subsiding sedimentary basin during the period following extrusion of the lavas led to lateral variation in thickness. An episode of regional uplift approximately halfway through the Upper Carboniferous period is marked by a low-angle unconformity.

Lithostratigraphy

Inverclyde Group

In the western Midland Valley, the group contains three formations: in upwards succession, the Kinnesswood Formation, the Ballagan Formation and the Clyde Sandstone Formation. All of these are represented in the Hamilton district. The known occurrences of Inverclyde Group strata are entirely in the south, where there are outcrops around Middlefield, along the northern margin of the Carboniferous outcrop at Glenbuck, and along the western edge of the Douglas Coalfield between Wedder Hill and North Bankend where the thickest and most complete sequence lies in the area around Burnside. All these outcrops lie to the south of the Kerse Loch Fault, a long-lived fracture-zone which is known to have been active during later Carboniferous times. North of the fault, in the area around Strathaven and Kirkmuirhill, strata of the group are entirely absent and rocks of the Strathclyde Group rest directly on the Lower Devonian. There is no information regarding the Group in the north. The present distribution of the Inverclyde Group in the district (Figure 4) is attributed to mid-Dinantian earth movements, involving northerly upthrow on elements in the Kerse Loch Fault zone, followed by erosion which, at least in the Strathaven–Kirkmuirhill area, eliminated the entire group.

The Kinnesswood Formation, which was previously regarded as part of the Upper Old Red Sandstone and may be partly of Upper Devonian age, is present throughout the Inverclyde Group outcrop. The Ballagan Formation is developed in the Middlefield area but the sequence here does not extend up to the Clyde Sandstone Formation. In Dalfram No. 2 Borehole, sited about 2 km to the south of the district, grey shales and cement-stones of the Ballagan Formation appeared to be directly overlain by a sequence of ash and agglomerate which is presumed to belong to the Strathclyde Group. In the Glenbuck area, the Lower Carboniferous rocks are steeply dipping, much affected by faulting and indifferently exposed. There are exposures of the Ballagan Formation but the presence of the Clyde Sandstone Formation cannot be confirmed at outcrop. However, both formations were proved by the IGS Borehole at Burnside (NS73SE/52) [NS 7862 3373]. In the area east of North Bankend, where there are good exposures of the Kinnesswood Formation, there is no evidence which confirms or denies the presence of the Ballagan or Clyde Sandstone formations.

Kinnesswood Formation

The strata of the Kinnesswood Formation consist of beds of red and white sandstone and red-brown, green-spotted silty mudstone, arranged in upward-fining fluvial cycles. In such cycles, the coarse-grained lower part is considered to have been laid down in the channels of a river system; the fine-grained upper part represents 'overbank' sediment, laid down upon the associated floodplains.

The sandstone beds are mainly fine to medium grained, generally cross bedded and have sharp erosive bases. Commonly in the basal parts of the sandstone beds there are angular clasts of mudstone and limestone derived by erosion from the overbank deposits. Some of the sandstones comprise several channel-fill units, each with an erosive base. Where the cycles contain a single sandstone unit, this is usually from 2 to 3 m thick, but multistorey sandstones may reach a thickness of 10 to 12 m. The red-brown silty mudstone beds range in thickness from a few centimetres to several metres and may contain thin beds of fine-grained sandstone.

Carbonate, mainly calcite, is an integral component of the formation, being precipitated within developing soil profiles generally as nodules scattered through either or both of the lithological members of the cycles. In some cases the nodules are abundant enough to form beds of limestone or calcrete, commonly known as 'cornstone', which may attain a thickness of several metres. In mature pedocals, laminar and pisolitic structures tend to develop, and there is replacement of carbonate by silica. The cornstones have been exploited locally for lime, notably in quarries at Middlefield [NS 680 296] (Plate 4). The formation reaches a maximum thickness of about 140 m. Its base is nowhere exposed but is taken at the unconformable junction with rocks of Lower Devonian or Silurian age. No fossils have been found within the district.

The thickest exposed sequence of the Kinnesswood Formation is in the Back Burn [NS 667 294], about 1200 m west of Middlefield. There are exposures alongside Stottencleugh Burn [NS 737 305] of a thick, well-developed calcrete, which can be traced for up to a kilometre northeastwards into the headwaters of the Galawhistle Burn. Throughout this distance, the calcrete-bearing sandstones are overlain by strata of the Ballagan Formation. However, in the Coal Burn [NS 759 317] a further kilometre to the north-east, dark purplish grey sandstones with carbonate nodules are overlain directly by fine-grained, thinly bedded yellow sandstone of the Lawmuir Formation (Strathclyde Group).

There are exposures of calcrete-bearing sandstones in the area north-west of Wedder Hill. The Burnside Borehole, a little distance to the north-east, was terminated in strata of the Ballagan Formation but, as there is no known locality in Scotland where this formation is not underlain by the Kinnesswood Formation, the presence here of calcrete-bearing sandstones can be assumed. The formation is present in the neighbourhood of North Bankend [NS 786 353], where there are good exposures in the River Nethan. However, the presence of the Ballagan Formation in this area cannot be confirmed.

Ballagan Formation

This formation consists of grey poorly laminated silty mudstones and siltstones with numerous beds of limestone ('cementstone') up to 0.3 m thick, which are usually dolomitic (cf. Paterson et al., 1990a), and thin beds of conglomerate composed of limestone clasts.

Desiccation cracks are common and pseudomorphs after halite have been recorded at localities outwith the district. In the upper part of the formation there are beds of grey fine-grained sandstone up to 1.6 m thick. The base of the formation is transitional from the underlying Kinnesswood Formation and is taken at the base of the lowest bed of grey mudstone. The formation contains a restricted fauna of fish scraps, ostracods and modiolids.

There are good exposures of typical strata of the formation in the Back Burn [NS 670 291], north of Burntfoot, where its thickness is estimated at 150 m. A little south of the district, Da1fram No. 2 Borehole penetrated approximately 45 m of grey siltstone with cementstone beds without reaching the base.

The formation is poorly exposed in the area around Glenbuck, but there are exposures of grey, in places red-mottled, silty mudstones and cernentstones in the Stottencleugh Burn [NS 741 303] and in an unnamed tributary [NS 749 309] of the Galawhistle Burn (Davies, 1972). Strata of the formation were cut by the Burnside Borehole between a depth of 176.50 m and the base of the borehole at 211.93 m. Bivalves were recovered from mudstones at a depth of about 182 m.

Clyde Sandstone Formation: Glenbuck Sandstone Member

The Burnside borehole is considered to have cut strata of the Clyde Sandstone Formation between depths of 148.17 m and 176.50 m. They consist of grey, pale red and greenish grey sandstone with beds of grey-brown mudstone and silty mudstone. Nodules of grey fine-grained limestone occur in places through the sequence, both in the sandstones and the mudstones. The strata are provisionally assigned to the Glenbuck Sandstone Member. Its contact with the underlying Ballagan Formation is taken at the base of a 12 m thick multistorey sandstone. Its top is at the sharp base of the lowest of a series of conglomeratic sandstone beds in the basal part of the Lawmuir Formation.

Strathclyde Group

The Strathclyde Group (Paterson and Hall, 1986) comprises, in upwards succession, the Clyde Plateau Volcanic Formation, the Kirkwood Formation and the Lawmuir Formation (Figure 5). These show marked lateral variation in thickness within the Hamilton district which is reflected in the thickness of the group as a whole, although there is internal compensation to some extent. Thus, where the lavas and associated volcanic detrital sedimentary rocks are well developed, the Lawmuir Formation is thin or absent, as in the area around East Kit-bride and at Drurnclog. Conversely, the Lawmuir Formation tends to be thicker where volcanic rocks are not present, as around the western margin of the Douglas Coalfield.

An exception to this is the area east of Strathaven where the Lawmuir Formation overlying the lavas is relatively thick at about 90 m ((Figure 3), section 2). This area is crossed by the important Strathaven Fault, which may be a continuation of, or a splay from, the Inchgotrick Fault, a major crustal lineament. It is possible that sediment was laid down in a deep valley eroded in fractured rocks along the fault-line or, alternatively, that a narrow graben-like basin developed as a result of active faulting during deposition of the Lawmuir Formation.

Clyde Plateau Volcanic Formation

The main development of the Clyde Plateau Volcanic Formation is in the west, where extensive outcrop of volcanic rocks extends westwards to the Clyde coast. The lavas in general form rounded upland moorland, largely covered by hill and basin peat and extensively afforested. Exposures are scarce, being mainly restricted to stream sections. In ground known to be affected by faulting, it has not been possible to establish a stratigraphy for the lava-pile.

The formation consists mainly of olivine- and feldsparphyric basaltic lavas, with subordinate trachytic flows. Eruption appears to have been subaerial and the tops of some flows are red-stained as a result of weathering, although no boles have been recorded. A few beds of agglomerate, tuff and reworked volcaniclastic material are intercalated in the pile, for example in the Lochar Water [NS 681 406], the Avon Water [NS 694 428] and the Little Calder [NS 645 436], south of Tackhouse. The trachytic lavas are restricted to the western margin of the district, west of Strathaven, where they are associated with a thick deposit comprising trachytic agglomerate and volcaniclastic sedimentary rocks.

To a large extent, the main lava outcrop is fault bounded and only the upper part of the pile is present at surface. The base of the formation is seen, however, in the smaller outcrop east of Sandford, where the lavas rest unconformably on Lower Devonian rocks. This outcrop is separated from the main lava development by a plexus of large faults, which probably represent the northeastwards continuation of the Inchgotrick Fault of Ayrshire. The Clyde Plateau Volcanic Formation is thinner east of the fault zone, and continues to thin eastwards until it is eventually overlapped by strata of the Lawmuir Formation, which accordingly rest upon the Lower Devonian.

Variations in thickness of the Namurian sedimentary formations across the line of the Inchgotrick Fault in Ayrshire have been attributed to contemporaneous movements on the fault (Macgregor and Manson, 1935; Richey, 1937). The rapid south-eastwards thinning of the Clyde Plateau Volcanic Formation around Sandford suggests that movements on the fault-line in Dinantian times may have generated a topographical high which restricted the spread of the lavas. Alternatively, post-extrusion uplift may have led to erosion of the lavas.

Petrography of the lavas

The basaltic lavas are typically porphyritic and, for mapping purposes, they have been classified on the basis of the grain size and mineralogy of their phenocrysts assemblage ((Table 2); MacGregor, 1928). The names given to the lava types refer to localities in central and southern Scotland and the classification is applied only to basic igneous rocks of Carboniferous and Permian age in the Midland Valley of Scotland. From whole-rock geochemical data obtained for other lava assemblages in the Midland Valley ((Table 2); Macdonald, 1975), it is apparent that there is only an approximate correspondence between the petrographical categories and those defined on a petrochemical basis. Thus the very mafic Craiglockhart-type flows (S77550) are ankaramitic, and the feldsparphyric lavas of Jedburgh and Markle type are commonly hawaiites. The basaltic lavas are remarkably fresh in some cases, containing crystals of fresh olivine (for example (S19392)) but, more usually, olivine phenocrysts are pseudomorphed by chlorite, iddingsite, bowlingite and/or opaque oxides. The less fresh material exhibits varying degrees of albitisation, chloritisation, oxidation, hydration and replacement by carbonate.

The most abundant lava types present are of Dunsapie basalt (S19392), (S77528), (S77555) and its fine-grained equivalent Dalmeny basalt (S77574). Basalts or hawaiites of Markle (S77526) and Jedburgh type are also common. The phenocrysts commonly occur as well-shaped crystals but glomeroporphyritic aggregates are also present. In some cases a weak to moderately strong flow foliation is expressed in the alignment of the feldspar laths of the matrix. Many of the rocks are amygdaloidal (S77522), typical amygdale and vein assemblages including chlorite, haematite, calcite and quartz.

Lavas of intermediate composition are volumetrically restricted and include mugearite (S23103), trachybasalt and trachyandesite (S77525), with minor amounts of trachyte also being recorded. The mugearites are typically fine grained and aphyric with a well-developed flow foliation defined by the arrangement of plagioclase laths. The trachybasalts and trachyandesites are fine-grained rocks which contain phenocrysts of plagioclase, clinopyroxene and, in a few cases, hornblende (S77525); (S77599); (S77601). The last are usually pseudomorphed by granular opaque oxide, in some cases with chlorite (S77525). The groundmass of these lavas typically possesses a well-developed flow foliation defined by length-aligned feldspar laths. The more evolved trachytic lavas are usually heavily altered, and are composed of an aphyric groundmass of tightly packed and flow-aligned feldspar crystals.

Kirkwood Formation

The formation contains material derived by erosion from rocks of the Clyde Plateau Volcanic Formation and usually consists of red or purplish brown, poorly bedded, blocky-weathering mudstone and silty mudstone. In places there are beds of sandstone and conglomerate with well-rounded cobbles and boulders of lava. The formation overlies the lavas of the Clyde Plateau Volcanic Formation in the ground north-west and south of East Kilbride and in the Drumclog area. However, north-east of Strathaven the Kirkwood Formation appears to be absent and coal-bearing strata of the Lawmuir Formation rest directly on lavas. There are good exposures of the formation in Coldwakning Burn [NS 639 402] to [NS 648 401], east of Coldwakning. Borehole evidence suggests that the formation may reach a thickness of at least 60 m in the area. However, the BGS Fore Hareshaw Borehole [NS 6137 3988] (Appendix I) proved only about 34 m of volcanic detritus, resting upon basalt lava.

In the Coldwakning Burn section, only a metre or so of strata intervene between the top of the Kirkwood Formation and the base of the Hurlet Limestone. Marine shells obtained from mudstone of the formation at a now-degraded exposure, south of East Kilbride, suggest correlation with the Stonehouse Under Limestone which lies at a high stratigraphical position in the Lawmuir Formation. It would appear, therefore, that deposition of the Kirkwood and Lawmuir formations took place contemporaneously.

Lawmuir Formation

The Lawmuir Formation consists of a variable sequence of sedimentary rocks, including sandstones, siltstones, mudstones, seatrock and coals, arranged in upward-coarsening cycles. A number of marine mudstones and limestones are also present. The most important of these have been correlated throughout the Midland Valley (Wilson, 1989) but their local names have been retained in this account (Table 3).

Glenbuck area

The formation crops out along the northern flank of the area of Lower Carboniferous rocks around Glenbuck, where it is estimated to be about 120 m thick. The lower part of the formation is poorly exposed but appears to consist mainly of sandstone. There are reasonably continuous exposures of the upper part of the formation in unnamed tributaries [NS 752 311] and [NS 755 312] of the Galawhistle Burn (Davies, 1972), where the highest strata consist of white sandstone with beds of calcareous shelly silty mudstone and limestone. The Muirkirk Under Limestone, the local equivalent of the Blackbyre Limestone (Wilson, 1989), is almost 2 m thick and lies about 15 m below the top of the formation.

Dalquhancly area

There are few exposures in the area which flanks the Douglas Coalfield on the west. However, the full thickness of the Lawmuir Formation was penetrated between depths of 9.83 m and 148.17 m in the BGS Burnside Borehole. The lower part of the formation consists mainly of grey and brown sandstone with beds of greenish grey and red-brown siltstone and mudstone. The sandstones in the lowest 10 m are conglomeratic in places, with pebbles and boulders up to 10 cm across of 'felsite', 'porphyrite', greywacke and chert, presumably derived by erosion from Silurian and Devonian rocks. The Douglas Under Limestone, as the Blackbyre Limestone is known locally (Wilson, 1989), is 2.51 m thick at a depth of 34.90 m. The Craigburn Limestone (Lumsden, 1967a), which is regarded as the equivalent of the Hollybush Limestone (Wilson, 1989), is represented by a bed of shelly calcareous mudstone, 0.85 m thick, at a depth of 51.72 m.

The best exposures of the formation are in the River Nethan, 250 m north of Over Stockbriggs [NS 7910 3597], where a 2.2 m thick bed of grey bioclastic limestone is considered to be the Douglas Under Limestone.

Drumclog area

In the faulted outlier at Drumclog, the Lawmuir Formation is represented by an attenuated sequence, little more than 15 m thick at maximum, which rests on reddish brown tuffaccous mudstones of the Kirkwood Formation. A bed of limestone, 0.5 m thick with gigantid productids, seen 550 m west of Snabe [NS 6370 3912] in an unnamed tributary of the Avon Water, was taken by Richey (1946) to be either the correlative of the Black-hall Limestone or of the combined Mid and Main Hosie limestones. However, the limestone and associated shelly calcareous mudstones, with a total thickness of 1.35 m, were penetrated at a depth of 34.91 m by the Fore Hareshaw Borehole and the fauna shows it to be the equivalent of the Blackbyre Limestone. It is red stained and has a nodular appearance, perhaps having been affected by soil-forming processes. The limestone can be seen in Coldwakning Burn [NS 6484 3992], 150 m WNW of Ryelandside, and mudstones with a similar fauna were formerly exposed some 450 m upstream [NS 6466 4014]. At an intervening locality [NS 6475 4002], the limestone is absent and only a thin coal and its seat lie between the top of the Kirkwood Formation and the base of the Lower Limestone Formation.

East Kilbride–Kirkmuirhill area

The Lawmuir Formation has a narrow discontinuous outcrop along the south-western edge of the Central Coalfield. The formation is thin or possibly absent to the west and south of East Kilbride, where the Kirkwood Formation is well developed (Figure 4). It is present in an isolated outcrop north-east of the town and in the valley of the Rotten Calder (Carruthers and Dinham, 1917), where the Basket Shell Bed and the Netherfield Limestone are still visible. In the neighbourhood of Strathaven, the Kirkwood Formation is absent and the Lawmuir Formation reaches a thickness of about 90 m. The formation thins again farther east where it rests unconformably on Lower Devonian rocks, all older Carboniferous strata having been overlapped.

In the area between Strathaven and Glassford, the Lawmuir Formation contains workable seams of coal and ironstone and the sequence is relatively well known from borehole sections. In addition, there are a number of good surface exposures. For example, a good section through the lower part of the formation is available in the Hole Burn [NS 7130 4685] to [NS 7280 4592], where the lowest exposed beds are the sandstones and green mud-stones near Walkerdyke Farm [NS 713 465]. These rocks contain volcaniclastic material washed in from the lava pile and are transitional from strata of the Kirkwood Formation. A little higher in the succession, three coal seams, collectively known as the Walkerdyke Coals, were exploited from crop workings [NS 713 467], north of Walkerdyke Farm. The uppermost of the coals was said to be in excess of 0.3 m thick (Hinxman et al., 1921).

Downstream of Holeburn Bridge [NS 7195 4625], the section is more continuous. Close to Laigh Netherfield, there is a bed of sandstone, more than 20 m thick, with intercalations of mudstone and siltstone. Trial openings were made on the valley sides of the Hole Burn to the Netherfield Coal which lies 3 to 4 m higher in the succession. The coal is now obscured but the doleritic sill intruded into it is still visible.

The horizon known as the Netherfield Limestone, a little above the coal, consists of a series of shelly mudstones with thin lenticular beds of bioclastic limestone, which have been correlated with the Dykebar Limestone of the Central Coalfield sequence (Wilson, 1989) and with the Fraser Shell-Bed of West Lothian (Hinxman et al., 1921). A bed of poorly developed oil shale, 1 m thick, overlying the marine sequence has been equated with the Fraser Shale of West Lothian. The oil shale is overlain by mudstone which passes up into a 6 to 7 m thick sandstone. Farther downstream, trial pits were sunk to an unnamed coal (Hinxman et al., 1921).

Higher beds in the sequence were formerly well exposed in cliff sections along the Avon Water near Cot Castle [NS 7337 4564] to [NS 7380 4617] (Hinxman et al., 1921). The Cot Castle Blackband Ironstone, about 0.3 m thick, near the base of the section, is not now visible. However, the overlying Basket Shell Bed, the probable local representative of the Hollybush Limestone (Wilson, 1989), is still exposed and consists of about 1.7 m of mudstone containing a diverse marine fauna. Above the shell bed are mudstones, 3 to 4 m thick, containing ironstone hands — the Cot Castle Clayband Ironstones. These beds and also the Cot Castle Blackband Ironstone were mined locally. The 0.3 m-thick Tree Coal which lies about 2 m higher in the sequence can still be seen, resting on 0.8 m of seatclay.

A bed of grey bioclastic limestone, 0.5 to 1 m thick, stratigraphically about 17 m above the Tree Coal, is exposed on the eastern side of the Avon Water [NS 7379 4614], south of Braehead. It is proposed that the name given to this horizon in the Stonehouse to Strathaven area be altered from Under Limestone (Hinxman et al., 1921) to the more specific term Stonehouse Under Limestone, in line with the practice adopted in the Muirkirk and Douglas areas. The limestone, which is taken to be equivalent to the Blackbyre Limestone (Wilson, 1989), rests upon about 1 m of fossiliferous mudstone, which in turn overlies a similar thickness of seatrock. The strata between the Tree Coal and the Stonehouse Under Limestone are predominantly arenaceous. Approximately 15 m of mainly arenaceous strata, tuffaceous in places, lie between the top of the limestone and the top of the Lawmuir Formation.

Strata of the formation are exposed in the Powmillon Burn, south-east of Strathaven. The section is affected by faulting but at its western end [NS 7091 4427] the Netherfield Limestone is seen, overlain by fossiliferous mudstone. A fine-grained grey rubbly sparsely shelly limestone, exposed at a point about 30 m downstream, is thought to be at the horizon of the Basket Shell Bed. Still farther downstream [NS 7102 4431], a sequence of abundantly fossiliferous mudstones with thin beds of shelly limestone represents the Stonehouse Under Limestone. The Netherfield Coal was worked in the lands of Netherfield and Floors, east of Strathaven (Hinxman et al., 1921).

Fossiliferous mudstones and limestone beds, exposed in the much-faulted section [NS 725 443] in Fairy Burn south of Whinknowe, are considered to lie at the horizon of the Netherfield Limestone. In the Birkwood Burn [NS 798 420], the formation is only about 15 m thick and rests unconformably upon Lower Devonian sandstones. The White Coral (or Nodular) Limestone, which is taken as the equivalent of the Blackbyre Limestone, is well exposed and contains a rich fauna including colonial corals.

Clackmannan Group

The Clackmannan Group comprises, in upwards succession, the Lower Limestone, Limestone Coal, Upper Limestone and Passage formations. The strata of the group consist of sandstones, siltstones, mudstones, coals, seat-rocks and limestones, arranged in the upward-coarsening cycles characteristic of deposition in a deltaic environment. The division of the sequence into formations reflects changes in the proportion of these lithologies within the cycles, the formation boundaries being taken at marker beds — limestones or mudstones laid down as a result of widespread marine inundations of the delta surface. The group straddles the Dinantian–Silesian boundary, which is taken at a position a little below the top of the Lower Limestone Formation.

All the formations are thicker and more complete in the north, which lies within the Central Coalfield, and the sequences there are taken as the standard.

Lower Limestone Formation

The formation is characterised by the presence of a high proportion of strata of marine origin, mainly mudstone but including a number of laterally extensive limestones. Where the formation is fully developed, its base is taken at the base of the Hurlet Limestone of the Central Coalfield sequence or its correlatives, the Muirkirk Main (or Hawthorn) Limestone in the Muirkirk area, the Douglas Main Limestone in the Douglas Coalfield, the Main Limestone of Drumclog in the Drumclog area and the Stonehouse Main Limestone in the Strathaven area. The top of the formation is defined at the top of the Top Hosie Limestone or its equivalents, the Calderwood Cement in the East Kilbride area and the uppermost of the series of limestones known in the Muirkirk and Douglas areas as the McDonald Limestones (Table 3). Apart from the topmost metre or two, which are of Pendleian (P₁) age, the formation was laid down during the Brigantian (P₂) Stage.

East Kilbride–Hamilton–Kirkmuirhill area

The Lower Limestone Formation is at surface or present at shallow depth in a wide area around East Kilbride, and occurs also in a discontinuous series of outcrops which extends from Glassford to Kirkmuirhill. The best exposures are in the deeply incised valleys of the Avon Water, the Rotten Calder and the Kittoch Water. The formation thins generally eastwards, from 80 to 90 m at East Kilbride to 35 to 45 m around Stonehouse (Goodlet, 1957; Browne et al., 1985).

The Hurlet Limestone, at the base, is usually 2 to 3 m thick, consisting of posts of dark grey, hard, crinoidal limestone interbedded with calcareous mudstone, and has been quarried extensively. Commonly the workings have been continued underground by means of adits driven in from the base of the working faces, as at Cot Castle. The limestone, 1.85 m thick, is exposed on the north bank of the Avon Water [NS 718 445], east of Strathaven. Its full thickness is seen in the Birkwood Burn [NS 798 421], near Kirkmuirhill, where it is almost 5 m thick and has a red-stained rubbly top.

The Wilsontown Smithy Coal, which is up to 0.6 m thick, lies approximately 5 m higher in the succession. It was formerly mined beside the Avon Water below Glassford Bridge [NS 733 455]. A further 5 m higher in the sequence, the Blackhall Limestone, which is known locally as the Foul Hosie Limestone, and the overlying fossiliferous calcareous mudstones of the Neilson Shell Bed are well exposed in the unnamed tributary of the Avon Water [NS 740 456], near Cot Castle, and also in the Avon Water [NS 721 447] south of Waulkmill. The limestone is overlain by a thick sequence of mudstone with numerous thin beds of clay ironstone — the Raesgill (or Crossbasket) Ironstones. These were worked at Earnockmuir, east of East Kilbride.

The upper part of the formation contains four beds of limestone, the Main Hosie (Birkfield), Mid Hosie (Second Kingshaw), Second Hosie (First Kingshaw) and Top Hosie (Calderwood Cement) limestones, the local names being given in parentheses. Known collectively as the Hosie Limestones, all four beds are exposed in the Avon Water between Braehead [NS 738 462] and St Ninian's Church, Stonehouse [NS 747 471], and in Calderwood Glen [NS 659 544] at East Kilbride. Like the Blackhall Limestone, the Main, Mid and Second Hosie limestones are hard, bioclastic limestones, from 0.4 to 1.0 m thick (Hinxman et al., 1921, p.67). The Top Hosie Limestone is commonly more argillaceous and, although usually no more than 0.3 m thick, it has been worked widely, particularly in the East Kilbride area. To the west of East Kilbride, the Main and Mid Hosie Limestone coalesce to form the Hairmyres Limestone. In the east, the Hosie Limestones are seen in the Birkwood Burn, near Kirkrnuirhill.

Glenbuck area

The formation is estimated to have an average thickness of about 30 m in this area. There are good exposures in the Stottencleuch Burn and the headwaters of the Galawhistle Burn, particularly the Coal Burn and the unnamed stream 700 m to the west (Davies, 1972).

The Hawthorn Limestone, at the base, is considered with some reservations to be equivalent to the Hurlet Limestone of the Central Coalfield sequence (Wilson, 1989). It has a pale, nodular, kaolinitic top with large productids and a few rugose corals. It is up to 12 m thick, including beds of calcareous siltstone and mudstone. More than 7 m of fossiliferous strata, mainly limestone, are exposed in Coal Burn [NS 7595 3136].

The Muirkirk Wee Limestone, regarded as the local equivalent of the Blackhall Limestone of the Central Coalfield sequence, is up to 1.3 m thick and lies approximately 10 m higher in the succession. A series of up to four limestones, the base of the lowest of which is from 7 to 9 m stratigraphically above the Muirkirk Wee Limestone, is known collectively as the McDonald Limestones. Individual limestone beds are 0.6 to more than 1 m thick and are separated from one another by calcareous mudstones and siltstones. The uppermost of the limestones is correlated with the Top Hosie Limestone of the Central Coalfield. The McDonald Limestones are well exposed in the Coal Burn and also in the small stream 700 m to the west where, however, there is some complication as a result of faulting.

Dalquhandy area

Grey bioclastic limestone interbedded with fossiliferous mudstone, exposed along the east bank of the River Nethan [NS 791 360], north of Over Stockbriggs, is considered to be the Douglas Main Limestone. At a section farther downstream [NS 793 360], two beds of grey limestone separated by about 2.5 m of shelly mudstone probably lie in the lower part of the McDonald Limestones. The thickness of the Lower Limestone Formation in the area is not known.

Drumclog area

Only the lower part of the Lower Limestone Formation is present in the fault-bounded outlier at Drumclog. The Hurlet Limestone (locally, the Main Limestone of Drumclog), which is up to 5.5 m thick, has been extensively worked, notably in quarries around Meadowfoot [NS 632 394] and WNW of Snabe. The lower part of the limestone is well exposed in a small stream [NS 637 391], north-west of Drumclog, where it was considered by Richey (1946) to be the Top Hosie Limestone.

Two seams of poor quality coal were mined in the neighbourhood of Stobieside. The stratigraphical position of these seams was uncertain though a Limestone Coal Formation age was suggested by Richey (1946). Their position in the Lower Limestone Formation has been demonstrated by the Fore Hareshaw Borehole which penetrated four coal seams, ranging in thickness from 0.06 m to 0.23 m, before encountering fossiliferous mudstones overlying a 3.77 m-thick Hurlet Limestone (Appendix 1). The coal seams are presumed to correspond to the Wilsontown Smithy Coal.

Limestone Coal Formation

Sedimentary strata between the top of the Top Hosie Limestone and the base of the Index Limestone are assigned to the Limestone Coal Formation, which is of lowest Namurian, Pendleian (E₁) age. The rocks are organised into upward-coarsening cycles typical of deltaic sedimentation and consist of sandstones, siltstones, mudstones, seatrocks and the coal seams which characterise the formation. In the north of the district the cycles are thin and there are numerous coals, few of which attain a thickness of 0.3 m. This is in marked contrast with the southern area, where the cycles are twice as thick on average and the coal seams in some cases are more than 2 m thick (Figure 6).

Deltaic sedimentation was interrupted on two occasions by marine incursions, in the course of which widespread marine deposits — the Black Metals and the Johnstone Shell Bed — were laid down. In both cases the marine deposits occur in a relatively thick sequence of argillaceous strata in which beds carrying a marine or quasimarine fauna may alternate with beds containing nonmarine fossils. In such cases, the lowermost of the marine beds generally contains the best-developed marine assemblage (Wilson, 1967).

There is a tendency for the sequence that underlies the Johnstone Shell Bed to be more argillaceous, with few coal-bearing cycles. Strata above the shell bed show a well-marked cyclicity, forming upward-coarsening sequences which usually culminate in seatrock and coal (Figure 4). Although there are exposures of portions of the formation in the main rivers and their tributaries, the bulk of the available information is derived from mine plans and the records of boreholes, only a few of which penetrated the entire sequence.

East Kilbride–Hamilton–Kirkmuirhill area

The formation is present along the axis of a shallow east-trending syncline at East Kilbride and also in a series of disconnected outcrops in the area between Burnbank [NS 699 495] and Kirkmuirhill. Clough et al. (1920), Carruthers and Dinham (1917), and Hinxman et al. (1921) give brief details of the now much-degraded natural sections. The formation varies considerably in thickness from a maximum of about 160 m at Motherwell to 140 m at East Kilbride and to about 90 m near Kirkmuirhill. Most of the thinning east of Motherwell occurs within the area that lies south of Stonehouse, where the formation is estimated to be still approximately 150 m thick.

At East Kilbride the Johnstone Shell Bed lies approximately 40 m above the Top Hosie Limestone, the intervening strata consisting mainly of mudstone in the lower part and predominantly of sandstone above. The interval between the two beds increases eastward and southeastward to around 46 m at Motherwell and to 50 m near Shodinn [NS 710 480] but then decreases abruptly to about 20 to 30 m south-east of Stonehouse (MacPherson, 1992, fig.1). The Flatt (or Heads) Coal, less than 0.3 m thick, is commonly present a few metres beneath the Johnstone Shell Bed. Trials to this coal were made at East Kilbride and Crookedstonemuir [NS 718 490].

A persistent but variable series of coals, approximately midway between the Johnstone Shell Bed and the Black Metals, is named the Lesmahagow Main Gas Coal in the area south of Stonehouse where there are two leaves, each about 0.25 m thick. In the area between East Kilbride and Stonehouse, three seams are present in this position, in upwards succession the Smithy, Jaunt and House coals, which are collectively known as the Crutherland Coals. Mudstones associated with the coals, which are generally from 0.2 to 0.5 m thick, contain Lingula and nonmarine bivalves. The Lesmahagow Main Gas Coal and the House Coal, the upper seam of the Crutherland Coals, have been mined; the Crutherland Coals have recently been worked opencast in the area east of East Kilbride.

The Black Metals lie around 75 to 80 m above the Top Hosie Limestone, with the Black Metals Marine Band in the lower part. Like the Johnstone Shell Bed, the unit is composed predominantly of mudstones but there are several lenticular beds of clayband ironstone, known as the Maggie Bands.

In the sequence above the Black Metals, the sandstones which form the upper part of the cycles are relatively thick and commonly have erosive bases. The coal seams are generally thin and impersistent and detailed correlation within the area is not possible. In the Lanark district around Braidwood [NS 842 477], the Auchenheath Dross Coal, the Wee Gas Coal, and the Auchenheath Smithy Coal have been worked. There is little information on the upper part of the formation in the East Kilbride area but it is now apparent that some of the workings formerly considered to be in the House Coal are actually in a seam higher in the sequence, at a level approximately 97 m above the Top Hosie Limestone. It is proposed to name the seam the East Kilbride Common Coal as an imprecisely located plan shows workings in the neighbourhood of the former common, near Mount Cameron [NS 644 539].

Glenbuck area

The Limestone Coal Formation is approximately 95 m thick near Glenbuck but thickens to around 120 m at Muirkirk in the New Cumnock district (Davies, 1972, fig. 2). The strata are arranged in relatively thick upward-coarsening fluviodeltaic cycles. Two of these involved major incursions of the sea during which the fossiliferous mudstones of the Johnstone Shell Bed and the Black Metals Marine Band were laid down. In a number of cycles, thick coal seams are developed (Lumsden, 1964, pl. II).

The Johnstone Shell Bed, approximately 10 m above the base of the formation, consists of dark grey mud-stones with ironstone ribs. It overlies the McDonald Coal, from 0.25 to 1.82 m thick, which was formerly worked, as also was the Low Band Ironstone within the Johnstone Shell Bed. Two important coal scams, the Six Foot (or Smithy) Coal and the Thirty Inch Coal, lie between the Johnstone Shell Bed and the Black Metals. The Six Foot Coal is about 1.5 m thick, and lies up to 16 m above the Johnstone Shell Bed. The Thirty Inch Coal is from 0.75 to 0.9 m thick, in places developed in several leaves. The interval between the two coals ranges from 6 to 18 m.

The Black Metals Marine Band, which immediately overlies the Thirty Inch Coal, consists of several metres of mudstone. Its fauna is less abundant and diverse than that in the Johnstone Shell Bed. Between the Black Metals and the top of the formation there are several important coal seams. These comprise, in upwards succession, the Nine Foot Coal (up to 3.6 m thick), the Four Foot Coal, (usually in several leaves with an aggregate thickness of up to 2.4 m), the Three Foot Coal (usually in two leaves with a combined thickness of up to 2.6 m) and the Ell Coal (from 0.9 to 1.2 m thick). All the seams have been extensively exploited from underground workings and are currently being extracted by opencast methods.

Dalquhandy area

The formation in the western part of the Douglas Coalfield is about 90 m thick (Lumsden, 1964) and, as at Glenbuck, is composed of relatively thick fluviodeltaic cycles. Deltaic sedimentation was interrupted on two occasions by incursions of the sea in the course of which the fossiliferous mudstones of the Johnstone Shell Bed and the Black Metals Marine Band were laid down. In a number of cycles, thick coal seams are developed which can be correlated in general with the seams in the Glenbuck area. The principal seams are, in ascending order, the Six Foot, the Thirty Inch, the Nine Foot, the Ell, the Dross and the Smithy coals, the last two commonly being close enough to work as one seam. All these seams were extensively worked in the area. The McDonald Coal which lies beneath the Johnstone Shell Bed was little worked underground but recently has been exploited by opencast methods.

There are few exposures of the formation, the best being in the River Nethan [NS 794 365], south of Stockbriggs, where there is a possible representative of the Six Foot Coal in two leaves with an aggregate thickness of about 2 m. A little below the coal are some 4 m of mudstone and silty mudstone with thin beds of clayband ironstone. The coal is overlain by about 1 m of grey mudstone which in turn is succeeded by an upward-coarsening sandstone unit which has siltstone layers in its lower part and is cross-bedded at the top. The Dross and Smithy coals may have been worked at a disused quarry [NS 7965 3614], south-east of Stockbriggs.

Drumclog area

Strata in this area shown as Limestone Coal Group on earlier editions of the Geological Sheet 23 (Hamilton), are now considered to belong to the Lower Limestone Formation.

Upper Limestone Formation

This formation comprises the strata that lie above the base of the Index Limestone. These are considered to have been deposited in a fluviodeltaic environment. As in older formations, the rocks are organised into cycles. Those in the Upper Limestone Formation are relatively thick and consist predominantly of sandstone, coals being few and generally thin. Sedimentation was interrupted by transgressions on a number of occasions during which marine limestones and mudstones were laid down. These consist, in upwards succession, of the Index Limestone, the Lyoncross (or Tibbie Pagan's), the Orchard, the Calmy (or Gair or Blue Tour) and the Plean Nos. 1 and 2 limestones. The entire formation is of Namurian age, the lower part being Pendleian (E_l), the upper part, from the base of the Orchard Limestone upwards, being Arnsbergian (E₉) (Ramsbottom, 1977).

The main development of the formation is in the north. The full thickness of the formation is present also in the Glenbuck area (Davies, 1972) but only the basal few metres of the Douglas Coalfield development of the formation lie within the district. Exposure is poor and the sparse information available is mainly derived from borehole records.

East Kilbride–Hamilton–Kirkmuirhill area

The formation is mainly concealed by younger strata but occurs at surface in a series of disconnected outcrops extending from Torheads [NS 699 533] to Kirkmuirhill [NS 794 435], and in a number of small upfaulted blocks in the neighbourhood of Larkhall. Clough et al. (1920), Carruthers and Dinham (1917), and Hinxman et al. (1921) give details of the natural sections in the area and, although many of these are now considerably degraded, strata of the formation are still visible in the Darngaber, Limekiln and Cadzow burns south of Hamilton, and in the Avon Water near Larkhall.

Within the area, the formation shows considerable variation in thickness, thinning south-eastward from approximately 120 m at Motherwell to 50 m at Kirkmuirhill. This variation is largely attributable to erosion associated with uplift in the period prior to deposition of the Passage Formation. Thus, at Motherwell where the sequence is most complete, the base of the Passage Formation is at a level approximately 5 m above Plean No. 2 Limestone, whereas near Kirkmuirhill it lies about 13 m above the Orchard Limestone. As the interval between the Orchard and Plean No. 2 limestones is approximately 66 m at Motherwell, it may be inferred that some 58 m of strata have been removed by erosion at Kirkmuirhill. The stratigraphical level of the erosion surface cannot be determined accurately everywhere in the area between Motherwell and Kirkmuirhill because of the lack of information. However, in the Darngaber Burn, the Plean No. 1 Limestone is cut out by erosion in places, and in the Larkhall–Stonehouse area the erosion surface lies not far above the Calmy Limestone, at a level some 25 to 30 m higher than at Kirkmuirhill. Although the information is scant, it is possible that not all of the variation in thickness can be accounted for by pre-Passage Formation erosion. Thus the strata between the Index Limestone and the Orchard Limestone are about 45 m thick at Motherwell and are even thicker south-west of Hamilton, but at Kirkmuirhill they appear to be less than 35 m thick.

As in the other parts of the Central Coalfield, the Index Limestone is a hard compact bioclastic limestone, 1.5 to 2 m thick. Algal nodules and brachiopods are fairly common. The limestone is usually overlain by fossiliferous silty mudstone, from 1 to 3 m thick, which is overlain by a persistent sandstone horizon considered to be the correlative of the Bishopsbriggs Sandstone of the Glasgow district. An exceptionally thick sandstone in this position was proved in a borehole (NS75SW/133) [NS 7073 5080], near Thorniehill.

The Lyoncross Limestone, up to 38 m above the Index Limestone, is underlain by a persistent seatrock above which the Lyoncross Coal is developed in places. The limestone is exposed in the Cadzow Burn [NS 7075 5288], ESE of Brackenhill and also in Blackbog Glen [NS 7133 5174], west of the disused railway line, where it consists of fine-grained grey massive sparsely fossiliferous limestone, 0.60 m thick. The limestone is also seen, resting upon pale sandy seatrock, in the Darngaber Burn [NS 7042 5009], upstream of Lochlinn Bridge.

The Orchard Limestone consists of hard bioclastic limestone from 0.4 to 0.8 m thick at the base of a 5 to 6 m thick sequence of shelly mudstone. In some cases a second fine-grained argillaceous or bioclastic limestone is developed, 1 to 2 m above the base of the mudstone. A seatrock, which is locally overlain by a thin coal, occurs immediately below the base of the lower limestone post where it is exposed in the Blackbog Glen [NS 7148 5180]. The limestone and associated mudstones are also exposed in the Limekiln Burn [NS 7152 5124]. A thick development of cross-bedded sandstone lying between the Orchard and Calmy limestones was formerly worked for freestone in a quarry [NS 717 514], north-west of Kilnhill.

The Calmy (or Gair) Limestone is everywhere present except in the neighbourhood of Kirkmuirhill where it is believed to have been eroded prior to deposition of the Passage Formation. It is a fine-grained compact pale grey to white bioclastic limestone, 1 to 2 m thick, usually overlain by up to 7 m of abundantly fossiliferous mudstone which may contain further beds of limestone. The limestone was quarried along the Limekiln Burn [NS 708 509] and also near the Lochlinn Bridge [NS 707 503]. It is also exposed at two places in the Cadzow Burn [NS 7022 5267] and [NS 7036 5273], near Whitecraigs, being repeated as a result of faulting.

The strata above the Calmy Limestone were cut by borehole NS75NW/68 [NS 7374 5622] in the Motherwell area and are exposed along the Darngaber Burn. The limestone seen in this section was shown on the previous edition of the geological map as the Castlecary Limestone but is now considered to be the Plean No. 1 Limestone. Within the exposures the limestone is seen to be cut out beneath an erosion surface which descends stratigraphically towards the east. In the bore, the Plean No. 1 Limestone lies approximately 27 m above the Calmy Limestone and is represented by a thin band of 'earthy' limestone and fossiliferous calcareous mudstone. Immediately beneath the limestone seatrock overlies a siltstone that contains a marine fauna. A bed of crinoidal limestone, 0.5 m thick, seen in a stream section [NS 7223 5007] north of Crookedstone, may be the Plean No. 2 Limestone.

Two thin limestone beds separated by 0.48 m of calcareous mudstone in borehole NS75NW/68 are correlated with the Plean No. 2 Limestone. The upper bed is overlain by mudstones containing marine and quasimarine fauna. Lingula has been reported in a mudstone lying some 4 to 5 m below the Plean No. 2 Limestone.

Strata in the uppermost part of the Upper Limestone Formation, consisting mainly of ripple-laminated and cross-bedded, yellow-weathering sandstone, are exposed in a tributary of the River Clyde [NS 796 502], near Auldton. The rocks here are contained in an upfaulted block, as are strata of the formation inferred on the evidence of poorly located boreholes to crop out in the neighbourhood of Cambusnethan [NS 786 527].

Glenbuck area

The thickness of the formation within the Glenbuck area is estimated to be in excess of 180 m, although reliable information for its upper part is lacking. The Index Limestone is 1.5 to 3 m thick and is overlain by up to 9 m of silty mudstone with limestone ribs. The Birchlaw Limestone, which lies on average 18 m above the Index Limestone, does not appear to have an equivalent in the Central Coalfield area. It is a sandy shelly limestone, 0.6 to 3.6 m thick, and is underlain by a fossiliferous siltstone or a rooty sandstone. The Cokeyard Coal, which lies about 37 m above the Index Limestone and varies from 0.7 to 1.5 m in thickness, was worked locally but is commonly split into several leaves none of which reaches workable thickness.

Tibbie Pagan's Limestone lies approximately 60 m stratigraphically higher than the Index Limestone and is correlated with the Lyoncross Limestone of the Central Coalfield area. It is present throughout the area but has not always been recognised in boreholes because of its sandy nature.

The base of the Orchard Beds, a series of mudstones with limestone ribs, up to 12 m thick, lies 80 to 90 m above the Index Limestone. The Blue Tour Limestone, taken as equivalent to the Calmy Limestone, lies approximately 30 m above the Orchard Beds, with the Blue Tour Coal some 2 m below its base. The coal is usually about 0.9 m thick, but may occur in several leaves with an aggregate thickness of up to 2.1 m. At the Blue Tour Limestone position, there is a series, up to 18 m thick, of alternating beds of limestone and calcareous siltstone or mudstone, usually with a 6 to 7 m thick limestone bed near the base. A limestone which lies approximately 60 m above the Blue Tour Limestone in borehole NS72NW/28 [NS 7442 2842] is possibly the Plean No. 1 Limestone, although there is limited information about this part of the sequence.

Coalburn area

Approximately 55 m of strata in the basal part of the formation crop out within the district.

Passage Formation

Formerly termed the Passage Group, the formation within the district consists predominantly of friable poorly bedded fine-grained yellow-weathering sandstones, with beds of siltstone and scatrock and a few coal seams. Compared with its development in the Stirling district (Francis et al., 1970), the formation is extremely thin, nowhere exceeding about 36 m in thickness. This is attributed to the presence of two major unconformities. The older of the two occurs at the base of the sequence, which rests on strata of the Upper Limestone Formation ranging in stratigraphical position from a little above the Calmy Limestone to above the Plean No. 2 Limestone. The younger of the two unconformities was identified by Ross (1931) in the Douglas Coalfield, in the Lanark district, and considered by Lumsden (1967b) to lie at a stratigraphical level above No. 6 Marine Band Group of the Stirling succession. In the Douglas Coalfield, where the Passage Formation at its thickest exceeds 200 m, all strata of the formation beneath this unconformity have, in places, been removed by erosion and only the uppermost strata of the formation are present. It seems probable that throughout the Hamilton district the very attenuated Passage Formation entirely postdates the younger of the two unconformities. If the Namurian–Westphalian boundary lies near the top of No. 6 Marine Band Group, as Neves et al. (1965) have suggested, then it follows that all the Passage Formation strata in the Hamilton district may be of Westphalian age.

Hamilton–Kirkmuirhill area

The formation is largely concealed by younger rocks, the largest outcrops being around North Crookedstone and in the area west of Kirkmuirhill. There are a number of small outcrops in upfaulted blocks in the neighbourhood of Larkhall. Throughout the area the formation is thin, its thickness being mainly from 20 to 30 m, decreasing to less than 10 m around Larkhall.

The basal beds of the formation are seen in the Darngaber Burn [NS 724 502], east of Burnbrae, where they rest unconformably on seatrocks at a level below the Plean No. 2 Limestone in the Upper Limestone Formation. Sandstones of the formation are also exposed farther downstream [NS 7275 5055], where they are faulted against strata of the Lower Coal Measures, and in the Crookedstone Burn at Wellbog Plantation [NS 7295 5015].

Yellow-weathering cross-bedded sandstones with a bed of seatrock are exposed in a gorge [NS 795 502], east of Auldton. The thickest known development of the formation in the district was cut by NS75SE/333, which penetrated a sequence, about 33 m thick; of grey sandstones with thin beds of rooty siltstone and a 0.6 m thick seam of soft coal, possibly the Bowhousebog Coal, near the base.

The entire formation is exposed in the valley of the Avon Water at Larkhall. Its base is locally marked by a bed of conglomerate containing pebbles of ironstone which rests with unconformity on seatrocks of the Upper Limestone Formation.

Glenbuck area

The small outcrop of the Passage Formation shown in the Glenbuck area is inferred from the depth in underground workings of the highest coal in the Limestone Coal Formation (Davies, 1972).

Coal Measures

Strata of the group occur only in the north-eastern part of the district which falls within the Central Coalfield. The group contains three formations, the Lower Coal Measures, the Middle Coal Measures and the Upper Coal Measures (Figure 7). In all three, the rocks, consisting of mudstones, siltstones, sandstones, seatrocks, coals and ironstones, are arranged in the upward-coarsening cycles characteristic of deposition in a fluviodeltaic environment. There is evidence that, as in earlier periods of the Carboniferous, deposition was influenced by the differential rates of subsidence of fault-bounded blocks.

Systematic differences in the constitution of the cycles justifies the subdivision of the sequence into formations. Thus, in the Lower Coal Measures the coals tend to be thin, being nowhere more than 1 m thick and commonly much less, whereas in the Middle Coal Measures the main seams reach thicknesses in excess of 2 m. In the case of the Upper Coal Measures, the strata have been severely affected by the downwards percolation of oxidising solutions from the pre-Permo-Triassic desert land surface, which removed most of the carbonaceous material and stained the rocks red-brown by converting ferrous oxides to the ferric state. In addition, the cycles tend to be thicker than average and to contain thick bodies of sandstone.

From time to time the delta surface was inundated by the sea, on two occasions as a result of major marine transgressions the effects of which can be recognised in large parts of northern Europe. The deposits laid down in the course of these trangressions, the Vanderbeckei Marine Band and the Aegiranum Marine Band (Queens-lie Marine Band and Skipsey's Marine Band respectively, according to local terminology) have been regarded as marker horizons and used to delimit the formations. Conventionally, the base of the group is taken at the base of the Lowstone Marine Band (also known locally as the Lower Slatyband Ironstone or the Lower Crookedstone Ironstone). The strata are entirely of Westphalian age.

Lower Coal Measures

The Lower Coal Measures are present at depth beneath the entire development of the Coal Measures in the north-east of the district but occur at surface mainly in the area between Quarter and Draffen. In addition there are small outcrops in upfaulted blocks to the east of Larkhall. The best exposures are in the Darngaber and Powforth burns.

Few boreholes have penetrated the entire sequence but an imprecisely located underground borehole (NS75SW/15) [NS 7139 5330] in Eddlewood Colliery suggests that the Lower Coal Measures are about 125 m thick in the west. A borehole at Motherwell Bridge (NS75NW/68) [NS 7380 5622] showed the formation to be at least 113 m thick in that area. No boreholes penetrate the entire formation in the east but its lower part appears to be of similar thickness to the sequence proved at Motherwell.

Lowstone Marine Band to Lower Drumgray Coal

The Lower and Upper Slatyband ironstones, associated with the Lowstone Marine Band and Mill Coal respectively, have been worked in the area around Crookedstone, otherwise this part of the sequence contains no mineral seams of workable thickness and has been penetrated in full by relatively few boreholes. The thickness of the sequence is about 60 m except in an area around Larkhall, bounded on the north by an approximately east–west line through Quarter and on the east by a NE-trending line through Stonehouse, in which the thickness is less by some 20 m. The data are sparse but there is a hint that, during deposition of the oldest strata of the Lower Coal Measures, a fault-bounded block in the Larkhall area subsided less rapidly than adjacent areas.

Few of the boreholes are modern but correlation within the outcrop is relatively straightforward. The position of the Lowstone Marine Band can be recognised but a fauna, including Lingula sp., has been recovered only in the Dykehead Borehole (NS75SE/250) [NS 7564 5267] and at an exposure in the Dalserf Burn [NS 7955 4990], east of Ashgill. The Colinburn Coal is generally thin, its mudstone roof yielding nonmarine bivalves in boreholes at Dykehead and Garriongill. The horizons of the Auldshiels Mussel Band and the Armadale Main and Ball coals can usually be identified, the roof of the last containing Carbonicola pseudorobusta at Dykehead. The Mill Coal, with Carbonicola in its roof at Ferniegair (NS75SE/122) [NS 7525 5441], reaches a maximum thickness of 0.4 m. The Shiels horizon is everywhere recognisable and has yielded a fauna of bivalves at Ferniegair and Dykehead. Similar faunas come from the mudstone above the thin Shott's Gas Coal.

The strata between the Mill and Lower Drumgray coals are seen in exposures along the Crookedstone Burn, where they consist of sandstones and siltstones with seatrocks, the total thickness being about 19 m. The sequence is sandier and may be thicker than in the nearby borehole NS75SW/146 [NS 7359 5009]. In the burn [NS 7303 5019], a 0.16 m thick irony mudstone containing nonmarine bivalves and known locally as the Wellbog or Crookedstone Musselband Ironstone probably marks the position of the Mill Coal. This bed is also well exposed in the Darngaher Burn [NS 7293 5076], near Knowetop, where it rests on pale grey seatrock and is overlain by about 4 m of mudstone with beds of clayband ironstone and fine-grained sandstone. A bed of mudstone, 0.35 m thick, exposed farther downstream in the Crookedstone Burn [NS 7333 5044], resting upon a 0.1 m-thick coal, is probably the horizon of the Shiels fauna. No fossils were obtained here but they have been reported from this level in borehole NS75SW/62 [NS 7488 5443] at Ferniegair. A few metres higher in the sequence, a 1.5 m-thick bed of hard carbonaceous mudstone with fish scraps rests upon a coal, 0.23 m thick, considered to be the Shott's Gas Coal. The Lower Drumgray Coal is not seen in the section.

Strata in the lower part of the formation are exposed in the Avon Water upstream from the Larkhall Viaduct [NS 7543 5030]. Still farther upstream [NS 7584 5000], the Wellbog Musselband (Upper Slatyband Ironstone) at the horizon of the Mill Coal, contains a fauna of nonmarine bivalves. The Wellbog Ironstone was formerly worked under the name Cherryhill Slatyband from mines beneath Cherryhill, south-east of the viaduct (Hinxman et al., 1921).

Lower Drumgray Coal to Kiltongue Coal

This sequence contains four principal seams of which the most extensively worked are the Mid Drumgray (generally under its local name 'Lower Drumgray'), the Upper Drumgray and the Kiltongue coals, which are mainly from 0.6 to 1.0 m thick. The true Lower Drumgray Coal has been worked only in a few places in the north and in the east at Wemysshill. Elsewhere it is thin or absent but, in the Larkhall–Stonehouse area, it is associated with the Watstone Musselband Ironstone which was worked locally. The Larkhall Jewel Coal, a few metres beneath the Kiltongue Coal, reaches workable thickness and has been widely exploited in the area east of Stonehouse. The overall thickness of the sequence is mainly in the range 25 to 30 m but no systematic variation can be detected.

Strata of the sequence are exposed in degraded sections along the deeply incised Avon Water upstream from Millheugh Bridge [NS 7528 5065] to the Larkhall Viaduct [NS 7545 5025]. The Watstone Musselband, at the Lower Drumgray horizon, was formerly seen at several localities [NS 7540 5025]; [NS 7560 5025]; [NS 7552 5013] along the river and has yielded a varied fauna of nonmarine bivalves. The Mid and Upper Drumgray coals were worked from adits on the valley sides of the Avon Water in the vicinity of the Larkhall Viaduct.

The Drumgray coals were recently worked opencast in the Thinacres area [NS 735 504]. At a second site [NS 735 500], a little to the south, the Upper Drumgray was split into three thin impersistent leaves, up to 0.15 m thick, and a thin coal was developed below the upper of the two leaves into which the fossil bed above the Upper Drumgray is characteristically divided.

Top of Kiltongue Coal to Queenslie Marine Band

The strata between the top of the Kiltongue Coal and the top of the Lower Coal Measures, taken at the base of the Queenslie Marine Band, vary from about 37 m to more than 50 m thick. The Airdrie Virtuewell Coal, near the top of sequence, has been extensively worked. The Bell-side Ironstone, which was widely worked in the area east of Wishaw, does not reach workable thickness in the district.

In several boreholes (NS75SW/199; NS75SW/200 and NS75SW/206), fossils including Carbonicola and fish scraps have been reported from a thin bed of mudstone above the Kiltongue Coal. The Kiltongue Mussel Band, a conspicuous horizon recognisable in almost every borehole, lies at a distance above the Kiltongue Coal that varies from less than 10 m to about 18 m. The intervening strata are thickest in a belt extending from Neilsland in the north-west to Thinacres in the south-east, and in a north–south tract east of Stonehouse. Within these zones, the sequence consists largely of sandstone which was probably laid down within the distributary channels on the delta surface. The thickness of the sequence may be a consequence of the relative incompactibility of the sandstone as compared with the mudstones and siltstones deposited in the interfluves. The sandstone is exposed in the Plotcock Burn at Plotcock Castle, where large-scale cross-bedding indicates that it was deposited by a river flowing generally towards the south at this point.

The Kiltongue Mussel Band is well exposed in Thinacre Glen [NS 7391 5062], in the Plotcock Burn [NS 7408 5020] downstream from Plotcock Castle and in the Powforth Burn [NS 7457 5055] to [NS 7497 5064] north of Broomelton. There are good exposures also in the Dalserf Burn [NS 7986 5039], 150 m upstream from the road bridge, where it yielded a varied fauna of bivalves. In all these sections it consists of hard dark grey irony mudstone up to 0.3 m thick, resting on a coal up to 0.2 m thick. The musselband is also well exposed in the Avon Water [NS 7620 4755]; [NS 7620 4785], where the shales associated with it were mined on a small scale for their oil content.

In the sequence above the Kiltongue Mussel Band, the Ladygrange Coal, with a fauna of Naiadites and ostracods in its roof, and the Bellside Ironstone can be recognised in places, neither being present in workable thickness. The strata are laterally very variable, usually with a high proportion of sandstone, as several beds. In some cases the sandstones appear to form lenticular bodies, for example, a sandstone near the base of the sequence in the Allanton–Ferniegair area reaches a thickness of 10 m in places. The most persistent sandstone is that which overlies the Bellside Ironstone. This sandstone is usually 3 to 4 m thick but thickens to 8 m around Fairholm and South Quarter.

Bivalves were found at a horizon between the Kiltongue Mussel Band and the Airdrie Virtuewell Coal in the Dalserf Burn [NS 7987 5044], 120 m upstream from the road bridge. The Airdrie Virtuewell, near the top of the sequence, is up to 0.84 m thick and has been extensively worked except in the area around Hamilton where it is at great depth and in the Coltness–Wishaw area where the structure is complicated. Strata, mainly sandstone, in this part of the sequence are exposed in the glens of Thinacre and Plotcock, upstream and downstream from their confluence [NS 7427 5041]. One 2 m-thick sandstone bed on the left bank of the stream [NS 7412 5028] shows conspicuous large-scale water-expulsion structures.

Some 3 to 5 m of mainly argillaceous strata lie between the Airdrie Virtuewell Coal and the Queenslie Marine Band. In the Dalserf Haugh Borehole (NS75SE/282) [NS 7999 5082], however, this interval increases to about 10 m, mostly of sandstone.

Middle Coal Measures

The Middle Coal Measures, which includes the thickest and most extensively worked coal seams, underlie much of the north-eastern part of the Coal Measures outcrop. The base of the formation is taken at the base of the Queenslie (= Vandcrbeckei) Marine Band. Its position can be proved in only a few boreholes where a fauna has been recovered. Morc commonly, the horizon is taken to lie within a thin development of argillaceous strata some 3 to 5 m above the Airdrie Virtuewell Coal. No borehole penetrates the full sequence but the thickness of the formation appears to increase eastwards from about 175 m to more than 190 m.

Base of Queenslie Marine Band to base of Glasgow Splint Coal

The sequence is about 35 m thick in the west but increases eastwards, being more than 50 m in the area close to Garrion Gill. The Queenslie Marine Band consists of 2 to 3 m of grey mudstone, with the Cleland Roughband Ironstone, at its base. A fauna has been recovered only from borehole NS75SW/200 [NS 7372 5247] at Quarter No. 2 Colliery. The mudstone is succeeded by a sandstone which varies from 8 m to 12 m in thickness, being thickest in the Fairholm area. The Airdrie Black-band Coal is up to 0.75 thick and has been extensively worked in the north-west, around Hamilton. The associated Airdrie Blackband Ironstone has been worked only around Quarter. Fossils were obtained from near the top of the mudstone with ironstone ribs overlying the Airdrie Blackband Coal in the Quarter No. 2 Borehole. A diverse fauna of nonmarine bivalves was obtained from this horizon in a section on the left bank of the River Clyde [NS 7950 5074], 330 m upstream from the Garrion Bridge and a more restricted fauna was found in the Dalserf Burn [NS 7987 5047], 90 m upstream from the road bridge.

The Coatbridge Mussel Band, consisting of up to 4 m of mudstone with pale brown ironstone bands, lies about halfway between the Airdrie Blackband position and the base of the Glasgow Splint Coal. There is no workable coal at this position; nor is there at the horizon of the Virgin Coal, 2 to 4 m below the Glasgow Splint Coal, except in a small area [NS 783 483], near Millburn.

Glasgow Splint Coal to Glasgow Ell Coal

The main seams in this part of the sequence, in ascending order the Glasgow Splint, Main and Ell coals, are everywhere of workable thickness and are separated by approximately constant intervals. The uppermost of the seams was easily recognisable on account of its thickness and many boreholes stopped at this horizon. For this reason, the sequence through the coals is relatively poorly known.

The Glasgow Splint, is up to 2 m thick. The thickness of the sequence from the base of the Glasgow Splint to the base of the Glasgow Main Coal exceeds 25 m in the area south of Wishaw but decreases in all directions to less than 20 m, except in Whistleberry No. 1 Pit [NS 7025 5725] where the sequence is 27 m thick. The strata between the Glasgow Splint Coal and the Humph Coal are mainly arenaceous and vary in thickness from about 6 to 14 m, being thickest and having the greatest ratio of sandstone in a belt extending WSW through South Quarter. The Humph Coal is not present in the south and is best developed in the area around Hamilton, where it reaches a thickness of more than 1 m in places and has been extensively worked. The strata between the Humph Coal and the Glasgow Main Coal, which averages 1.3 m in thickness, consist mainly of sandstone. Fossils have been obtained from mudstones overlying the Glasgow Splint Coal in a number of boreholes and also from the Humph roof. Strata consisting mainly of fine-grained, flat and ripple-laminated, locally cross-bedded sandstone which lie between the Splint and Main coals, are exposed in Skelly Gill between the Lanark–Hamilton road and the M74 Motorway. A thin representative of the Humph Coal is here overlain by a musselband which yielded a fauna of nonmarine bivalves.

The Pyotshaw Coal is everywhere workable, locally reaching a thickness of more than 1 m. It is separated from the Glasgow Main Coal by mainly argillaceous strata which range in thickness from little more than a parting in the area south of Wishaw, where the seam was worked jointly with the Main Coal, to more than 13 m. In the Ferniegair area, the interval consists mainly of sandstone but elsewhere mudstone predominates.

The Glasgow Ell Coal is usually more than 2 m thick, with a local maximum of 3 m (Plate 5). The strata between the Glasgow Main and Ell coals are generally 20 to 22 m thick but are only 17 to 18 m thick in the area south of Wishaw where the sequence between the Glasgow Splint and Main coals is thicker than average. They consist mainly of mudstone and siltstone except in the Ferniegair area. In a number of boreholes fossils have been recovered from mudstone at the position of the Cambuslang Marble horizon, midway between the Pyotshaw and Ell coals.

Glasgow Ell Coal to Skipsey's Marine Band

The strata in this interval are poorly known. They vary from about 90 m to a little over 100 m in thickness and consist in the lower part mainly of silty mudstone and siltstone, commonly with roots. A thick development of pale grey mudstone forming the roof of the Ell Coal is the most distinctive. A number of thick, apparently lenticular beds of sandstone occur, mainly in the upper part of the sequence. Up to eight thin coals may be present, of which the Glasgow Upper, about 29 to 36 m above the Ell Coal, reaches workable thickness in the area east of Larkhall. A group of thin coals about 50 to 65 m above the Ell Coal, have been called the Avonbraes Coal Group (Carruthers and Dinham, 1917). Strata in this part of the sequence are exposed on the steep sides of the Avon Water but the sections are now much overgrown and in general only the sandstones are visible. Two thin coals in Blackbog Glen, east of Blackbog, probably lie only a few metres below Skipsey's Marine Band (Carruthers and Dinham, 1917) .Only one of these coals is still exposed.

Up to five fossiliferous horizons have been recognised in the beds above the Glasgow Upper Coal. Nonmarine bivalves were obtained from the immediate roof of the coal in Thornlie No. 1 (NS75SE/149) [NS 7961 5419], East Thornlie Street No. 1 (NS75SE/408) [NS 7956 5484] and Overtown Colliery (NS75SE/154) [NS 7980 5330] boreholes. The Dalserf Musselband was identified in the Overtown Colliery Borehole and also in Ashgillhead No. 1 (NS75SE/369) [NS 7870 5013], Cornsilloch No. 3 (NS75SE/338) [NS 7801 5053] and Coltness No. 35 (NS75SE/151) [NS 7959 5290] boreholes. The musselband is exposed in the Raegill Burn [NS 7940 4990], east of Ashgill, and was formerly seen in Millburn Glen [NS 7798 5092] (Dinham, in Hinxman et al., 1921) .

Fossils were also found in the Ashgillhead Borehole at three higher levels in the sequence; the lowest was probably the horizon of the Palacecraig Coal (Forsyth and Brand, 1986) and the middle one possibly at the horizon of the Carnbroe Marine Band, although only nonmarine bivalves and Euestheria were found. The Palacecraig horizon also yielded fossils in the Overtown Colliery Borehole. No fauna was found in the Carnbroe position in this borehole but Lingula marked this horizon in the East Thornlie Street Borehole.

A recently drilled series of boreholes (NS75SW/228) (NS75SW/229) (NS75SW/230) (NS75SW/231) (NS75SW/232) near Quarter, recovered fossils from beds above and below a possible correlative of the Glasgow Upper Coal. This evidence resolves the problem raised by Carruthers and Dinham (1917) concerning the stratigraphical level of the rocks in that area. It now seems likely that the area includes not only beds high within the Middle Coal Measures but also strata within the Upper Coal Measures.

A musselband, 180 m upstream from Blackbog [NS 7181 5181], yielded a fauna of nonmarine bivalves which suggests a position above the Glasgow Upper Coal. However, the absence of estheriids means that a level below the coal cannot be excluded.

Upper Coal Measures

The main development of the formation is in the northwest, around Hamilton, where about 150 m of strata are present in a broad syncline. In the lower half of the sequence, the thick Chatelherault Sandstone below and the Hamilton Sandstone above are separated by a variable sequence of seatearths, mudstones and thin coals, termed the Barncluith Coal Group by Carruthers and Dinham (1917) but here referred to as the Barncluith Coals. The uppermost strata are predominantly red-brown sandstones with subordinate seatearth ('fireclay') and mudstone beds. A coal in Skellyton No. 3 Pit, some 30 m above the position of Skipsey's Marine Band, is the only seam from the Upper Coal Measures known to have been worked.

There are good natural sections of the lower parts of the Upper Coal Measures along the incised river valley of the Avon Water and its tributary, the Meikle Burn. Skipsey's Marine Band was formerly exposed in a gully [NS 7349 5297] on the west bank of the river, a little north of Avonbank, where it consists of dark limestone and mudstone with a restricted fauna including brachiopods and orthocones. The horizon was also seen on the east side of the river farther downstream [NS 7360 5355] where it contained a much more diverse fauna.

Strata near the base of the Upper Coal Measures are exposed also in Simpsonland Glen where they consist mainly of silty mudstone and siltstone with seatrocks containing large red-stained ironstone nodules. No coals are present but in a section [NS 7206 5238], about 400 m northwest of Carscallan, a thin, hard bed of carbonate has been formed at the expense of a coal. Mudstones at a similar horizon are currently being quarried for brickclay some 300 m due north of Carscallan. Here the process of conversion of coal to limestone was visible at various stages of transition. Downstream in the Meikle Burn, the argillaceous sequence passes beneath soft yellow sandstone, which is probably at the lowest part of the Chatelherault Sandstone. This sandstone is also developed in the Avon Water below Chatelherault Bridge [NS 735 538] and at Barncluith [NS 725 545], where cross-bedding indicates deposition by generally southward-flowing currents.

Strata low in the Upper Coal Measures are exposed in Stewart Gill, where they consist mainly of sandstone with beds of siltstone and seatearth. At a hitherto unrecorded occurrence [NS 7938 5075], about 80 m upstream from the Lanark Road, Skipsey's Marine Band consists of dark grey carbonaceous mudstone with a fauna which includes Posidonia. Two seams of coal, the lower being about 0.2 m thick, may correlate with the 'Barren' Coal worked at Skellyton. Grey, red-tinged cross-bedded sandstone up to 12 m thick, exposed in a cliff in Hall Gill [NS 7845 5363], may belong to the Chatelherault Sandstone.

Strata above the Chatelherault Sandstone are exposed along the Avon Water near Barncluith. At a point [NS 7310 5396] some 500 m downstream from Chatelherault Bridge, two coals occur in a sequence of grey seatrocks and siltstones; the upper, thicker one is 0.6 m thick. None of the coals in this area is known to have been worked. Beds above the Chatelherault Sandstone are exposed in the Meikle Burn between its confluence [NS 721 527] with a small tributary flowing west from Chapel and the confluence with the Eddlewood Burn, and also in the Eddlewood Burn itself. The sequence consists mainly of siltstone with beds of seatrock and sandstone. There is a good example of conversion of coal to limestone in one place [NS 719 527]. It is probable that a fault throwing down to the north separates this argillaceous sequence from the exposures of the Chatelherault Sandstone to the south.

In the Meikle Burn below its confluence with the Eddlewood Burn, and probably on the downthrown side of another east–west fault, the contact of the argillaceous sequence with the overlying Hamilton Sandstone is seen for a distance of some 900 m. The sandstone is cross-bedded, grey, yellow and red brown and varies from 7 m to 12 m in thickness.

The highest beds of the Upper Coal Measures in the district are less well known but, in the Neilsland Colliery Borehole (NS75SW/3) [NS 7126 5369], they appear to consist of siltstones and seatrocks which are overlain by sandstone. This sandstone, which is mainly red-brown in colour with only very subordinate beds of siltstone, is well exposed in the Earnock Burn and in the Neilsland Burn upstream from the bridge carrying the Neilsland Road. The sandstone is strongly cross-bedded and is thought to have been laid down by rivers flowing generally towards the south-east.

Conditions of deposition

Following a hiatus, sedimentation resumed in the district with the deposition of the calcrete-bearing rocks of the Kinnesswood Formation. These are considered to have been laid down in the channels and floodplains of a mature, eastward-flowing river system occupying a basin approximately as wide as the present Midland Valley. Formation of calcrete, especially in thick continuous beds such as those quarried near Middlefield, takes place within the soil profile in the areas with slow accretion rates found in the floodplains of a stable alluvial system (Marriot and Wright, 1993) . Its development is promoted by a climate having a mean annual temperature of about 16° C and a moderately seasonal rainfall (Allen, 1974). During this early Carboniferous period, calcretebearing sequences were being deposited in Scotland under quiet tectonic conditions in basins as far apart as the Solway Firth and the Moray Firth.

In the succeeding period the tectonic and climatic environment remained unchanged but the nature of the sediments being laid down in the Midland Valley was markedly altered as the continuing regional subsidence allowed the sea to encroach upon the floodplains of the river system. As a result, the Ballagan Formation was deposited in a shallow-water environment with restricted access to the open sea and subject to marked changes of salinity and periodic desiccation. The absence of coarse elastic material indicates that the relief in the source area was subdued. The Midland Valley basins at this time was presumably more extensive than during deposition of the Kinnesswood Formation but there is no known locality where rocks of this age are overlapped by the cementstone-bearing sequences of the Ballagan Formation.

Just as the onset of marine conditions was gradual, so also was the resumption of fluvial sedimentation towards the end of Ballagan Formation times, with deposition of the calcrete-bearing sequences of the Clyde Sandstone Formation. At least initially, the rejuvenation process may have consisted only of elevation of the upland source areas but towards the end of Inverclyde Group times the basin of deposition was also affected. Uplift accompanied by movement on faults, almost certainly in some cases involving the reactivation of more-ancient fractures, induced the removal by erosion of part, and locally all, of the Inverclyde Group strata previously laid down. It is possible that the uplift may be partly attributable to the development beneath the western part of the Midland Valley of the large magma chamber which eventually gave rise to the lavas and associated volcano-detrital rocks of the Clyde Plateau Volcanic Formation. The mid-Dinantian erosion surface marks an important stage in the structural development of the Midland Valley. When sedimentation resumed following eruption of the Clyde Plateau lavas, local control was exerted by contemporaneous movements on faults bounding basement blocks, operating under a right-lateral strike-slip regime (Read, 1988).

The lavas of the Clyde Plateau Volcanic Formation were extruded subaerially and generated a relief which probably amounted to several hundred metres in places. During later Strathclyde Group times, the volcaniclastic detritus of the Kirkwood Formation accumulated on the low ground within and adjacent to this lava pile, encroaching farther on to the lavas as subsidence continued. The lavas continued to contribute material to the basin almost until the end of Strathclyde Group times, as shown by the occurrence within the detritus of a possible correlative of the Blackbyre Limestone, and in adjacent districts the lava pile was not completely buried until well into Clackmannan Group times.

While the Kirkwood Formation was being deposited, non-volcanic sedimentary rocks of the Lawmuir Formation were being laid down in adjacent ground. The relationship of the two formations with one another is strongly diachronous and in the west, for example at Fore Hareshaw, the oldest strata of the Lawmuir Formation are at a level only a little below the horizon of the Blackbyre Limestone. The strata of the Lawmuir Formation represent the earliest deposits in the southwestern part of a large delta which was advancing southwards in the area now occupied by the North Sea and extending into the Midland Valley.

Once established, the deltaic environment persisted until the end of Carboniferous times. The internal organisation of the deposit not only reflects the complexities intrinsic to sedimentation in a deltaic environment but also betrays the influence of eustatic changes of sea level and the fact that subsidence of the Midland Valley basin was complicated by differential movement of fault-bounded blocks (e.g. Read and Dean, 1976; Read, 1988) . Interaction of basin subsidence with global changes of sea level, possibly of glacial origin, meant that the strata of the Lawmuir Formation and the later Carboniferous as a whole were laid down under conditions of oscillating local sea level. When eustatic rise of sea level combined with basin subsidence, this resulted in rapid and widespread marine incursion. Conversely, eustatic falls of sea level were counterbalanced by basin subsidence and may in some cases have failed to register on the record of local changes of sea level, particularly in those parts of the basin subsiding relatively rapidly under structural control. At times when there was local uplift within the basin or when eustatic falls of sea level outpaced subsidence, the rejuvenated drainage incised channels in the delta surface.

At times of rising sea level, space was generated within which sediment could accumulate, typically in the form of a generally upward-coarsening sequence or cycle. In distal parts of the basin, the cycle in some cases takes the form of a delta deposit, commencing with a marine limestone or mudstone, passing up by way of siltstone into sandstone, to upward-fining fluvial sequences deposited in the channels and floodplains of the delta-top distributary system. In cycles deposited in more proximal parts of the basin or during periods of less marked rise of sea level, the initial marine element of the cycle is not developed and the fluvial component dominates, in some cases constituting the entire cycle. During the final stages of the cycle, soils developed on the interdistributary floodplains under the hot, wet to humid climatic conditions which were initiated in late Inverclyde Group times (Paterson et al., 1990a, p.30) , and layers of vegetable material were laid down.

There seems little doubt that the cycles that contain the most prominent and widespread of the marine horizons owe their origin to major eustatic events (Read, 1988, p.228). If this were true for all cycles, their number should remain relatively constant over the whole of the basin. However, analysis of sequences in the Central Coalfield (e.g. Read and Dean, 1976) has revealed some correlation between the thickness of strata and the number of cycles. This suggests that local sedimentation processes predominated at times, perhaps during periods when relative sea level was stable and even more so when the drainage system, rejuvenated as a result of falls of relative sea level or uplift of the source area, reworked previously deposited sediment.

Chapter 5 Carboniferous biostratigraphy

The Muirkirk Basin, described by Davies (1972), lies partly in the Hamilton district and partly in the New Cumnock district (Sheet 15W) to the south. The Douglas Basin, described in a series of papers by Lumsden (1964; 1967a; 1967b), extends eastwards into the Lanark district (Sheet 23E). The Carboniferous strata of the Drumclog Outlier and the East Kilbride area continue into the Kilmarnock district (Sheet 22E); no recent account of these areas exists.

The local names that have been used for the various fossiliferous beds in the different areas described below and their standard Central Coalfield equivalents are given on (Table 3).

Dinantian

? Tournaisian

The oldest fossiliferous Carboniferous rocks exposed in the Hamilton district are to be found on the western margins of the Douglas (Coalburn) Basin and in the Harwood Burn [NS 677 288] on the northern edge of the Muirkirk Basin. Much-faulted exposures of cementstone facies have yielded fish remains in the Scots Burn [NS 7825 3381] and, in the BGS Burnside Borehole [NS 7862 3373], Modiolus sp. was recovered from beds of cementstone facies at a depth of 182 m. No fossils of undoubted Tournaisian age have been recovered.

Viséan

Viséan rocks of various ages rest on either the volcanic detritus overlying the Clyde Plateau Volcanic Formation, for example south-west and west of East Kilbride and in the Strathaven and Drumclog areas, or on an unfossiliferous sequence of siltstones and sandstones of uncertain age, as in the Coalburn and Muirkirk areas. A marked non-sequence exists in the more north-easterly areas and the most complete sequences above the lava pile are those north-east of Strathaven [NS 720 460] and around Earnockmuir [NS 720 485].

Kirkwood Formation

Calcareous mudstone and limestone in an ashy sequence in the Shield Burn, south-west of East Kilbride [NS 6202 5065], may be the Stonehouse Under Limestone. The fauna obtained at this locality consists of Crurithyris?, Productus cf. concinnus, Schizophoria resupinata, Nuculopsis gibbosa and Pernopecten sowerbii.

Lawmuir Formation

Dykebar Limestone

In the Strathaven area, this bed is represented by the Netherfield Limestone which in the Netherfield Borehole [NS 7278 4524] has yielded a fauna including Crurithyris urii, Productus sp., Euphemites sp., Actinopteria persulcata, Modiolus sp., Palaeolima cf. simplex, Pernopecten sp., Posidonia becheri, Streblochondria sp., orthocone and goniatite fragments. Exposures in the Hole Burn [NS 7259 4601] and in the Powmillon Burn [NS 7091 4427] have yielded similar faunas with the addition of Glabrocingulum beggi?, and Clinopistha parvula from the Hole Burn. The overall aspect of the faunas is similar to that recorded from the Raeburn and Fraser shell beds of the West Lothian Basin with which the Netherfield Limestone has been correlated (Wilson, 1989, fig. 4). The bed has also been recognised in the Earnockmuir [NS 6940 5292] and Maxwellton [NS 6452 5480] boreholes and in the former contains Serpuloides sp., Lingula spp., Productus sp., Straparollus sp., Limipecten sp, Pernopecten sowerbii, Streblochondria sp. and goniatite fragments. It is possible that the Craigburn Shell Bed, the local correlative of the Netherfield Limestone in the Douglas Basin, may underlie the eastern parts of the Coalburn area as the bed was proved in the BGS Auchmeddan Borehole [NS 8435 3959] in the adjacent Lanark district (Sheet 23E).

Hollybush Limestone

In the area east of Strathaven this limestone is represented by the Basket or Cot Castle Shell Bed, still visible in the River Avon at Cot Castle [NS 7398 4575]; [NS 7380 4576]. Here the fauna includes Alitaria cf. panderi, Crurithyris urii, Linoproductus sp., Martinia sp. nov., Pugilis pugilis, Tornquistia cf. polita, Euphemites urii, Actinopteria persulcata, Cypricardella rectangularis, Pernopecten sowerbii, Posidonia becheri, Sanguinolites costellatus, S. variabilis? and Beyrichoceratoides truncatus amongst other forms. A similar fauna was present at this horizon in the Netherfield Borehole. The corresponding shell bed in the West Lothian Basin contains a very similar assemblage (cf. Wilson in Mitchell and Mykura, 1962, p.100). In the Coalburn area the lowest marine band recognised, at a depth of 51.72 m in the BGS Burnside Borehole is known as the Craigburn Limestone, which has been correlated with the Holly-bush Limestone (Wilson, 1989, fig. 4). The limestone is thin and poorly fossiliferous, containing Gigantoproductus sp., Pleuropugnoides sp. and Productus sp. No surface exposures of the limestone have been recorded in the district.

Blackbyre Limestone

In most exposures in the district this limestone, known locally as the 'Under Limestone', is in part leached or affected by roots. The fauna includes Antiquatonia spp., Avonia youngiana, Kochiproductus sp., Megachonetes sp., Pugilis cf. pugilis, Tornquistia spp., Straparollus carbonarius, Actinopteria persulcata, Limipecten dissimilis, Pernopecten sowerbii, Sanguinolites sp. and Beyrichoceratoides sp.

In the Coalburn area the bed is known as the Douglas Under Limestone. It is up to 2.5 m thick at a depth of 34.90 m in the Burnside Borehole and contains Saccaminopsis fusulinaformis, Aulophyllum sp., Dibunophyllum?, Lonsdaleia cf. caledonia, Siphonodendron junceum, S. pauciradiale, Alitairia cf. panderi, Echinoconchus elegans, Kansuella sp., Krotovia?, Pugilis sp. and Rugosochonetes sp. In the underlying shales Tornquistia sp., Actinopteria persulcata, Limipecten sp., Myalina peralata?, Schizodus sp. and Wilkingia sp. are present. An exposure of the bed occurs in the River Nethan near Over Stockbriggs [NS 7903 3591], and contains a similar fauna.

In the Drumclog area the local 'Under Limestone' rests on volcanic detritus and is generally thin. From exposures in the Coldwakning Burn [NS 6484 3992]; [NS 6466 4014] and in the Snabe Burn [NS 6364 3911], a fauna comprising Antiquatonia hindi, Gigantoproductus giganteus, Productus cf. concinnus, Pugilis?, Schizophoria cf. resupinata, Semiplanus sp. and Pernopecten sowerbii has been obtained. A comparison may be drawn with the Stonehouse Under Limestone of Cot Castle in the Strathaven area [NS 734 461].

At the north end of the Muirkirk area the Muirkirk Under Limestone rests on unfossiliferous strata considered to belong to the Inverclyde Group. The limestone is exposed in the Galawhistle Burn and its tributaries [NS 7524 3113]; [NS 7565 3114] and in the Coal Burn [NS 7595 3138] and contains Siphonodendron junceum, Syringopora sp., Eomarginifera sp., Gigantoproductus giganteus group, Productus concinnus, Pugilis sp., Leiopteria sp., Myalina sp. and Wilkingia sp.

Lower Limestone Formation

Hurlet Limestone

The base of the Lower Limestone Formation has been taken at the base of the Hurlet Limestone which to the west and north-west of East Kilbride may rest on volcanic detritus. Here the Hurlet Limestone is characterised by a variety of coral genera including Aulophyllum fungites and Siphonodendron junceum. They are associated with a predominantly brachiopod assemblage, including Antiquatonia spp., Avonia youngiana, Echinoconchus spp., Krotovia aculeata, Latiproductus sp., Martinia sp. nov., Productus concinnus, Pugilis pugilis and Rugosochonetes celticus. Only a few bivalve genera are associated, Limipecten dissimilis being the most common. The overlying mudstones are variable in thickness, and usually contain Curvirimula cf. scotica, at least in the upper part. Around Strathaven the equivalent Stonehouse Main Limestone has been extensively quarried and commonly contains colonies of Siphonodendron junceum or S. pauciradiale and a varied fauna, chiefly of brachiopods, including Angiospirifer sp., Brachythyris sp., Pugilis spp., Spirifer bisulicatus group and Edmondia cf. senilis . The underlying shale contains Productus sp., Rhipidomella cf. michelini, Schizophoria cf. resupinata, Actinopteria persulcata, Pernopecten sp. and Streblochondria sp. Wilson (1989, p.104) referred to this fauna as being a characteristic assemblage in the sub-Hurlet mudstones in the Midland Valley of Scotland.

In the western outlier at Drumclog the Hurlet Limestone, known locally as the Main Limestone, was extensively quarried. It occurs in the Coldwakning Burn [NS 6484 3992]; [NS 6483 4001]; [NS 6460 4014], at Meadowfoot [NS 6128 3938] and near Snabe [NS 6364 3911]. The fauna is more varied than that occurring in the Blackbyre Limestone and includes, amongst other species, Dibunophyllum muirheadi, Angiospirifer cf. trigonalis, Antiquatonia hindi, Beecheria sp., Eomarginifera spp., Gigantoproductus giganteus, Latiproductus latissimus, Pleuropugnoides pteurodon, Productus cf. concinnus, Pugilis pugilis, Rugosochonetes celticus, Aviculopecten spp., Leiopteria cf. hendersoni, Limipecten sp., Myalina cf. flemingi, Pernopecten sowerbii and Streblochondria sp. Although not containing any distinctive forms, the fauna compares with that obtained from the 'Main Limestone' of Carluke in the adjacent Lanark district (Sheet 23E).

In the Coalburn area the limestone known as the Douglas Main Limestone contains algal haloes, Siphonodendron junceum, Acanthoplecta mesoloba„Antiquatonia spp., Crurithyris urii, Echinoconchus spp., Eomarginifera spp., Krotovia aculeata, K spinutosa, Pugilis pugilis and Pernopecten sp. Although the fauna is locally distinctive, it is not in itself characteristic of the Hurlet Limestone, nor is the fauna found in the underlying shales comparable with that from the corresponding position below the Hurlet Limestone of the Glasgow district. Nevertheless comparison with sections in the part of the Coalburn area around the Boghill Borehole [NS 8372 3800] in the Lanark district, point to this bed being the correlative of the Hurlet Limestone.

In the Muirkirk Basin the base of the formation has been taken at the base of the Hawthorn Limestone in accordance with Wilson (1989, fig. 4), although he expressed doubts concerning the identity of the Hawthorn Limestone with the Hurlet Limestone of the Central Coalfield (Wilson, 1989, 1998). A large fauna has been recorded from this horizon by Wilson (in Davies, 1972, pp.30–31) , of which Siphonodendron junceum, Gigantoproductus giganteus group, Latiproductus cf. latissimus, Pleuropugnoides sp., Productus concinnus, Spirifer spp. and Pernopecten sp. are the most common.

Near Cot Castle [NS 7354 4549], south-west of Stone-house, Lingula sp. has been recorded in workings from the mudstone above the Wilsontown Smithy Coal but elsewhere the bed appears barren. However, at Drum-clog, in the BGS Fore Hareshaw Borehole, the roof of the seam above the local 'Main Limestone' carries a small marine fauna consisting principally of poorly preserved productoids, and may represent the position of the Craigenhill Limestone of the Lanark district.

Blackhall Limestone

The Blackhall Limestone is thin and may lie close to the Hurlet Limestone in the area to the west of East Kilbride around Thorntonhall, in the Kilmarnock district (Sheet 22E). The overlying mudstones, which have been termed the Neilson Shell Bed, contain a typical fauna with Crurithyris urii, Tornquistia youngi, Glabrocingulum atomarium, Retispira spp., Nuculopsis gibbosa, Anthraconeilo spp., Pernopecten fragilis, Posidonia corrugata, Catastroboceras sp. and goniatite fragments. As Wilson (1966, pp.119–120) pointed out, this fauna characterises the least calcareous facies of the Neilson Shell Bed. He suggested that the sea-bed sediment, though mud, was not anaerobic and any decaying vegetable matter did not inhibit the bottom dwellers. Much of the fauna may have either floated or lived attached to vegetable matter enabling them to live without danger of being engulfed by the mud. Currie (1954) recorded species of Girtyoceras and Sudeticeras from this bed at Thornton Quarry, Thorntonhall [NS 5954 5480].

Around Strathaven the limestone may be recognised by the accompanying Neilson Shell Bed which has a characteristic fauna including Fenestella sp., Crurithyris urii, Eomarginifera spp., Rugosochonetes speciosus, Tornquistia politer, T youngi, Euphemites sp., Glabrocingulum atomarium, Straparollus carbonarius, Strobeus spp., Euchondria sp., Nuculopsis gibbosa, Phestia attenuata, Reticycloceras sp. and Epidomatoceras sp. Farther south, in the Coalburn area, the Blackhall Limestone is known as the Douglas Wee Limestone, and in the Lanark district is associated with a Neilson Shell Bed fauna.

In the Muirkirk area the corresponding Muirkirk Wee Limestone also contains Gigantoproductus sp., together with Composita sp., Rugosochonetes sp. and Limipecten sp. In this area, unlike much of the rest of the Midland Valley of Scotland, the Neilson Shell Bed cannot be recognised (Wilson, 1989, p.98).

Hosie limestones

Historically, the East Kilbride area has attracted fossil hunters since the early parts of the 19th century. Ure (1793) described material collected from here, whilst Patton (1885) and Young and Robertson (1873) all examined the quarries and exposures in the Hosie limestones around the town.

The lower Hosie limestones (Main and Mid Hosie) are separated from the upper Hosie limestones (Second and Top Hosie) by seatrock in the west and more coally strata in the east around Earnockmuir. The fenestellid phase (Wilson, 1989, pp.105–107) in the Main Hosie Limestone is well developed at East Kilbride, a locality at Hairmyres [NS 6045 5452] having produced well-preserved examples of the genus. In the Maxwellton Borehole [NS 6452 5480] the Main Hosie Limestone contains Siphonodendron junceum, which is not recorded elsewhere at this level. The faunas of the lower Hosie limestones are, in general, similar to those found elsewhere at this level along the southern margin of the Central Coalfield with, in particular, Angiospirifer trigonalis, a variety of species of Eomarginifera, Pugilis pugilis, Pernopecten sowerbii and Sanguinolites costellatus present in considerable numbers. The upper Hosie limestones are individually recognisable by their faunas. The Second Hosie Limestone contains a variety of brachiopod species including Eomarginifera lobata, Pugilis sp., Spirifer sp. and Spiriferellina sp., together with Euphemites urii, Pernopecten sowerbii and Streblochondria sp.; the Top Hosie (Calderwood Cement) Limestone is associated with overlying mudstones which contain an abundance of Posidonia corrugata. Currie (1954, p.575) recognised Cravenoceras scoticum in material collected by Wilson approximately 1 m below the Top

Hosie Limestone in the Gill Burn at Jackton [NS 5871 5338] in the Kilmarnock district. She drew attention to the similarity of this species to Cravenoceras leion the first appearance of which has been taken to mark the base of the Namurian in the British Isles (Ramsbottom et al., 1978, p.2). Thus the base of the Namurian should be drawn below the Top Hosie Limestone if these species of Cravenoceras are coeval.

The Hosie limestones are present, though not well known, in the area around Stonehouse. Exposures of the lower pair, the Main and Mid Hosie limestones, include those in the River Avon at Avonholm [NS 7401 4650] where the combined fauna consists of Fenestella?, Actinoconchus?, Avonia youngiana, Composita sp., Eomarginifera sp., Phricodothyris cf. lineata and Pterinopectinella? Either the Second or Top Hosie limestone is exposed in the River Avon [NS 7461 4696] and here the fauna includes Brochocarina sp., Eomarginifera cf. longispina, Liralingua wilsoni, Productus sp. and Posidonia corrugata.

In the Coalburn and Muirkirk areas the Hosie limestones are collectively known as the McDonald Limestones. On the western margin of the Coalburn area, in the discontinuous section [NS 7928 3604] in the River Nethan near Over Stockbriggs, beds probably within the lower portion of the McDonald Limestones contain Angiospirifer cf. trigonalis, Buxtonia sp., Composita ambigua, Eomarginifera lobata, Krotovia sp., Rugosochonetes celticus, Euphemites urii and Pernopecten sowerbii. The abundance of specimens of Eomarginifera and the considerable numbers of Pernopecten sowerbii obtained recall faunas from the lower Hosie limestones of the southern part of the Central Coalfield (cf. Wilson 1989, p.105).

In the Muirkirk area the McDonald Limestones are exposed in the Ponesk Burn [NS 7288 2940], the Stottencleugh Burn [NS 7421 3014] and the Coal Burn [NS 7594 3133]. In an exposure near Spireslack [NS 7555 2963] Siphonodendron junceum was recovered but elsewhere this species is absent. The fauna is principally composed of brachiopod species of which Avonia youngiana, Beecheria cf. hastata, Composita cf. ambigua, Eomarginifera cf. lobata, Latiproductus sp., Pleuropugnoides sp., Productus sp., Promarginifera trearnensis, Rugosochonetes cf. celticus, Schizophoria cf. resupinata and Spirifer bisulcatus group occur most extensively. Pernopecten sowerbii, Prothyris scotica, Pterinopectinella sp., Streblopteria ornata and Wilkingia sp. are also present, together with trilobite remains. The fauna as a whole is characteristic of calcareous strata throughout the Dinantian of the Midland Valley. The uppermost bed of the McDonald Limestones has been correlated with the Top Hosie Limestone of the Central Coalfield.

From this account it will be clear that despite the lithological and stratigraphical distinctions between the successions in the Central Coalfield, Coalburn, Drumclog and Muirkirk areas the faunal distinctions are not well marked. Wilson has already noted (1989, p.98) that it was not until the Hurlet Limestone transgression that the region was wholly covered by sea. The faunas of earlier incursions depend, at least in part, on the relationship of the present outcrop to the Carboniferous shorelines at the time of the incursion. Even at the time of the Hurlet transgression faunas differed between geographical locations, with an increase in the variety of corals and in the presence of giganteid productoids in the limestone in a south-westerly direction. This would appear to be related to the diminution of mud content, and possibly to a harder substrate resulting from such a lithological change.

Namurian

Limestone Coal Formation

The successions in the Muirkirk Basin, the Coalburn area and the Central Coalfield show marked differences, particularly in the number of faunal markers present. Coals in the Central Coalfield are, in general, thinner than in the areas to the south. At most locations the Top Hosie Limestone is overlain by a considerable thickness of mudstone. The lower part of this bed is often highly fossiliferous with many examples of Posidonia corrugata and a few other fossils. The upper part contains Lingula squamiformis, with alternating beds containing Naiadites sp. In both the Coalburn and Muirkirk areas the mudstones overlying the McDonald Limestones are thin with Lingula sp. and brachiopod fragments.

Johnstone Shell Bed

In the Central Coalfield this bed contains a varied fauna including Serpuloides sp., Pleuropugnoides cf. greenleightonensis, Productus concinnus, Aviculopecten sp., Myalina sp., Nuculopsis sp., Pernopecten sowerbii and Streblopteria ornata. In the mudstones forming the upper part of the bed Paracarbonicola pervetusta also occurs, as in the adjacent Glasgow district (Sheet 30E). Along the southern margin of the Central Coalfield, as in the Birkwood Burn [NS 7980 4201], the Slingstane Limestone of Carluke is developed at the position of the Johnstone Shell Bed; it contains abundant Productus concinnus.

In the Coalburn and Muirkirk areas the Johnstone Shell Bed immediately overlies a coal above the McDonald Limestones. It contains an abundant but restricted assemblage consisting of Pleuropugnoides sp., Productus concinnus, Schizophoria cf. resupinata and Pernopecten sowerbii.

Black Metals Marine Band

This marine band is poorly developed in the west near East Kilbride; in East Kilbride No. 22 Borehole [NS 6264 5317] it contains Buxtonia sp., Lingula sp. and Productus? Eastwards it is more fossiliferous and in the Motherwell Bridge Borehole contains Serpuloides sp., Buxtonia sp., Lingula squamiformis, Orbiculoidea craigi, Productus sp., rhynchonellid fragments and Sedgwickia sp. The presence of Buxtonia in the Black Metals Marine Band is a feature of the productoid distribution in these two marine bands. Wilson (1967, p.457) suggested this was related to differing facies of deposition. In the Coalburn area the Black Metals Marine Band is poorly developed and in general only Lingula sp. has been recovered from the horizon.

In the Muirkirk Basin the same marine band overlies the Thirty Inch Coal and contains Lingula sp. and brachiopod fragments. In the Central Coalfield a number of variably developed beds with Lingula and Naiadites are known in the sequence beneath the Index Limestone. In both the Coalburn and Muirkirk areas these beds are much reduced. In Coalburn, Linguta-bearing beds are known only above the Coalburn Nine Foot Coal and a nameless seam just below the Index Limestone. In Muirkirk only the band just below the Index Limestone has been recognised.

Upper Limestone Formation

The formation comprises four major marine cycles in the Hamilton district: the Index, Lyoncross, Orchard and Calmy limestones. Minor cycles such as the Huntershill Limestone and the Plean limestones are poorly developed, particularly in the south. They are, however, better developed towards the north and east. The faunas of all these beds have been discussed by Wilson (1967, pp.460–464) who drew attention to the geographical variations apparently related to the thickness of sediment in the interval between the Index and Calmy limestones. In general the faunas are those which characterise the individual horizons throughout the Central Coalfield. Goniatites of zonal significance are rare in the Scottish Carboniferous but sufficient have been discovered to demonstrate that the boundary between the Pendleian and Arnsbergian stages must lie between the Lyoncross and Orchard limestones.

In the Coalburn area the Index Limestone is the only horizon in the formation which has been examined; around Auchenbeg [NS 798 361] the limestone and its associated mudstones contain Gigantoproductus cf. irregularis, Latiproductus cf. latissimus, Pleuropugnoides sp., Schellwienella sp., Myalina sp. and Polidevcia attenuata. In exposures in the adjacent Lanark district the limestone contains algal nodules, a feature which distinguishes the Index Limestone from most of the other limestones in the formation throughout much of the western part of the Midland Valley of Scotland. In the Limekiln Burn [NS 7156 5125] an exposure of mudstone has yielded a typical Orchard Beds fauna including Serpuloides carbon-anus, Fenestella sp., Eamarginifera sp., Linoproductus sp., Latiproductus cf. latissimus, Glabrocingulum sp., Straparollus carbonarius, Phestia attenuata, Posidonia corrugata, Streblochondria sp. and nautiloid fragments. A similar, though less extensive, fauna was collected from an exposure in the River Avon at Clocksy Mill [NS 7660 4916].

Above the Calmy Limestone, mudstones with Leptagonia smithi, Pugnax cf. pugnus, Rugosochonetes caledonicus, Euphemites ardenensis, Glabrocingulum sp., Nuculopsis gibbosa, Anthroconeilo cf. laevirostrum and Tylonautilus? occur at a number of locations around Limekilnburn [NS 713 512]. This association is typical of the roof of the Calmy Limestone over a wide area of the Central Coalfield. The overlying Plean limestones (Nos. 1 and 2) are variably present. In the Darngaber Burn [NS 7142 5022] a mudstone bed has Lingula mytilloides, Curvirimula sp. and Sanguinolites clavatus, afaunal association of frequent occurrence at these positions. The Castlecary Limestone at the top of the formation appears to be absent throughout the district. Stratigraphical breaks occurring around its position account for the variability of the succession above the Calmy Limestone.

The Upper Limestone Formation in the Muirkirk area has been described in some detail by Davies (1972, pp.23–28) and the contained faunas discussed by Wilson (in Davies 1972, pp.29–38) who drew attention to the similarities of the faunas with those in the adjacent Douglas Basin. No higher strata are exposed in the Coal-burn and Muirkirk areas within the district.

Passage Formation

In the area flanking the gorge of the River Avon [NS 759 495] much of the formation is absent and the Lowstone Marine Band, the local base of the Coal Measures, lies stratigraphically close to the Calmy Limestone. Elsewhere little is known of the marine bands which commonly occur in this formation in the Central Coalfield.

Westphalian

Lower Coal Measures (Langsettian)

Exposures of the Lower or Crookedstone Slatyband Ironstone (Lowstone) occur in the River Avon and its tributaries [NS 7594 4945] where the bed contains Lingula mytilloides and Carbonicola? In Stonehouse No. 77 Borehole [NS 7683 4579], the corresponding position contains sponge spicules, Serpuloides sp., Lingula cf. mytilloides and a bellerophontid gastropod fragment. Characteristic bivalves of the lenisulcata Biozone have not been recognised from this interval in the Hamilton district. Poorly preserved Carbonicola aff. pyramidata occurs in mudstones at the top of what are collectively known as the Overwood (Top) Claybands in the Burnhead Borehole [NS 7953 4656], whilst in the corresponding position in the Motherwell Bridge Borehole Carbonicola of the C. browni group are present. These forms are elements in a widespread basal communis Biozone fauna throughout Scotland. Rare specimens of Carbonicola sp. have been recorded from above the Mill Coal in the southern part of the outcrop, but to the north nothing has been recovered from this horizon.

The principal musselband in this part of the sequence around Stonehouse and Larkhall is the Watstone Musselband, a highly fossiliferous canneloid horizon containing, amongst other forms, Anthraconaia?, Carbonicola cf. communis, C aff. polmontensis, C. cf. pseudorobusta (large form), C. aff. rhindi, C. cf. rhomboidalis, C. cf. robusta, Curvirimula cf. candela and Naiadites spp. The bed occurs in the River Avon [NS 7540 5025] where most of these forms may be found. The position has been correlated with the Lower Drumgray Coal of the Central Coalfield. Above the local Lower Drumgray Coal Curvirimula sp. occurs and the seam has been correlated with the Mid Drumgray seam in other parts of the Central Coalfield. The rich bivalve fauna of the Watstone Musselband is geographically confined and a much reduced fauna occurs at this level in the more easterly parts of the district.

The musselband above the Upper Drumgray Coal, commonly highly fossiliferous in the area around Shotts in the adjacent Lanark district, is here poorly fossiliferous, but the characteristic Carbonicola cf. pseudorobusta (small form) is present in the fauna from this horizon in Stonehouse No. 77 Borehole and around Canderside [NS 777 463]. The Kiltongue Musselband, containing Anthraconaia sp., Anthracosia spp., Carbonicola oslancis and Geisina arcuata, is widespread and has been taken as the base of the modiolaris Biozone. There are a few records of nonmarine bivalves at various horizons up to the Virtuewell Coal.

Middle Coal Measures (Duckmantian)

The Vanderbeckei (Queenslie) Marine Band is poorly developed in the Hamilton district and only foraminifera and Lingula mytilloides have been recovered from boreholes and field exposures. The band is exposed in the banks of the River Avon [NS 7661 4902] where Naiadites quadratus occurs in the mudstone above it. Brand (1977) has given an account of the geographical distribution of the band. The roof of the Airdrie Blackband Coal contains a limited fauna including Anthracosia aquilina, A. beaniana, A. ovum and A. phrygiana. Anthraconaia williamsoni occurs in this bed in the Motherwell Bridge Borehole and in Hall Gill [NS 782 533]. In the Quarter area [NS 726 515] the coal is replaced in part by blackband ironstone from which amphibian remains have been recorded. Surface exposures of the various beds up to the Aegiranum (Skipsey's) Marine Band are uncommon and only the Dalserf Musselband fauna is well represented. Exposures in the Mill Burn [NS 7798 5092], [NS 7816 5107] provided a typical fauna which included Anthraconaia cymbula, A. oblonga, Anthracosia acutella?, A.atra, A. fulva and Naiadites alatus. Forsyth and Brand (1986, p.8, fig. 5) described the marine beds in the upper part of the sequence in some detail. Fuller discussion of the faunas of the Middle Coal Measures is contained in Forsyth and Brand (1986).

Upper Coal Measures (Bolsovian)

The Aegiranum Marine Band at the base is found at a number of locations, notably in Stewart Gill [NS 7938 5075], the River Avon at Hamilton High Parks [NS 7349 5297]; [NS 7360 5355], the River Clyde [NS 7976 5075] and the railway cutting near Parkhead, Motherwell [NS 7510 5640]. Lachrymula pringlei, Reticulatia? craigmarkensis, Rugosochonetes skipseyi, Dunbarella macgregori, Pernopecten carboniferus, Posidonia sulcata, Metacoceras sp., Donetzoceras sp., Homoceratoides jacksoni and various conodont form genera characterise the band at these locations. Westwards, however, towards the BGS Craighead Borehole [NS 7008 5735], the black 'cank' facies with which this fauna is associated dies out and the fauna is replaced by Planolites ophthalmoides, foraminifera, Serpuloides sp., Lingula mytilloides and Phestia sp. The change was discussed by Forsyth and Brand (1986, p.17, fig. 2) who concluded that there was a shoaling towards the west or north-west during deposition of the marine band.

Few faunal bands occur at higher levels. The Bothwell Bridge Marine Band, so named from an exposure in the River Clyde [NS 7085 5766] on the adjacent Airdrie district (Sheet 31W), is present in the Craighead Borehole at a depth of 72 m. The bed contains Lingula mytilloides associated with Euestheria sp. The position has been correlated with the Shafton Marine Band of the Pennine succession (Anderson, 1955, p.55). Stratigraphically higher faunas are unknown in the Hamilton district, although in the BGS Hallside Borehole [NS 6694 5975], and in exposures in the Rotten Calder [NS 6777 6002] in the Airdrie district, Anthraconauta aff. phillipsii and A. aff. tenuis were present, indicating the presence of the phillipsii and tenuis biozones.

Chapter 6 Intrusive igneous rocks

Bodies of intrusive igneous rock in the Hamilton district range in composition from basalt to dacite. The oldest and most numerous were probably emplaced during the Lower Devonian, around 400 million years ago, into the Silurian and Lower Devonian rocks of the Lesmahagow Inlier. A number of bodies of mainly basic, in some cases heavily altered, material, cutting Carboniferous rocks, are considered to have been intruded at various periods during the Carboniferous. The youngest intrusions date from the Tertiary period, about 50 million years ago.

The intrusions vary greatly in form and size but only the Caledonian granodioritic–dioritic plutonic complex of Distinkhorn is large enough to be classified as major. Most of the minor intrusions are sheet like and approximately concordant with the bedding of the country rock. This is especially true of the numerous bodies of mainly acid composition, ranging in thickness from less than a metre to more than 100 m, which are intruded into the Silurian and Devonian rocks in the south. The largest of these bodies extend for several kilometres. Near-vertical, cross-cutting dykes are also common. These are mainly small but the Tertiary dykes are thick enough to have been quarried and are part of a complex of dykes which extends for several hundred kilometres. A few bodies are of approximately cylindrical shape and are interpreted as the infilling of a volcanic pipe, the most notable being that which forms Loudoun Hill.

Major intrusions

The north-eastern part of the Distinkhorn Plutonic Complex, the only major Caledonian pluton in the Midland Valley of Scotland, lies within the district, cropping out in poorly exposed ground centred on Cairnsaigh Hill [NS 610 362]. As a whole the pluton includes rocks of granodioritic and dioritic composition, but only the latter are known within the district. The outcrop appears to be bounded at its northern and eastern margins by faults which throw down to the north and east respectively. Magnetic and gravity highs over the area to the east of the surface outcrop suggest that dioritic rocks of the complex continue at depth for a considerable distance in that direction. An Rb-Sr isochron of age 412.8 ± 5.6 Ma, determined for the body from fresh biotite and plagioclase separates and a whole rock granodiorite by Thirlwall (1988), suggests that it was emplaced during the earliest Devonian.

Minor intrusions

Lower Devonian

The most prominent intrusions are the suite of lenticular, sheet-like bodies of mainly acid intermediate composition which cut Silurian and Lower Devonian rocks in the south of the district. One of the largest, forming the ridge extending north-east from Black Hill, is exposed in a roadside quarry [NS 698 291], where it consists of a somewhat fluxioned microgranodioritic felspar-porphyry. There are further exposures on the slopes [NS 718 302] east of the Ponesk Burn, where the mass appears to be displaced by a fault, and also farther upstream [NS 726 308]. A thinner sheet of similar composition is emplaced in steeply dipping Silurian siltstones in the Ponesk Burn [NS 721 309], a little upstream from its confluence with the Patrick Burn. Other large sill-like bodies of felsitic composition occur near Auchrobert [NS 764 384], form the ridge [NS 795 390] connecting Black Hill and Warlaw Hill and have been worked extensively at Dunduff Quarry, near Kirkmuirhill [NS 779 410] (Plate 6). An–Sm–Nd isochron of 411.9 ± 1.9 Ma has been determined using garnet separates and garnet-free whole rock powders for a member of the suite at Tinto, east of the district (Thirlwall, 1988).

In addition there are numerous lesser intrusions, mostly concordant but cross-cutting relationships also occur. Where thin sill-like bodies are intruded into grey Silurian turbidites they may be difficult to distinguish in the field from beds of greywacke. They range in width from 0.5 to 6 m and can rarely be traced for more than a few metres. The rock types are andesite and microdiorite, generally porphyritic to some extent. The intrusions are fine grained, grey coloured when fresh, but all show alteration of groundmass and phenocrysts to a greater or lesser degree. The phenocrysts are commonly whitish or pink. Good examples of these small intrusions occur in the Patrick Burn [NS 7185 3120], where two thin sheets of felsite are emplaced concordantly in the greywacke–siltstone sequence.

Carboniferous

Sills of olivine-dolerite crop out extensively in the area between Nethershields [NS 700 488] and Craigthorn [NS 735 484], intruding strata of the Lawmuir and Limestone Coal formations. Exposure is poor with many of the small quarries recorded during the previous surveys now being infilled or degraded. The crop of the sills is therefore largely conjectural, walkover magnetometer traverses at Craigthorn proving inconclusive. To the southeast of Stonehouse, in the neighbourhood of Draffen [NS 794 453], doleritic sills have been encountered in underground workings in the Lesmahagow Main Gas Coal. The sills are generally less than 2 m thick but in places coalesce to form bodies up to 5 m thick. The dolerite may be altered to 'white trap' where intruded at the position of a coal seam. A good example of this is exposed on the east bank of the Cadzow Burn [NS 7070 5284] where a 1.15 m-thick sill of altered dolerite interverses between a seatrock and its overlying coal. A second sill, 0.4 m thick, is emplaced at a somewhat lower level.

An east–west-trending dyke composed of heavily altered dolerite, exposed [NS 7472 4930] south of Corslet, in an unnamed tributary of the Darngaber Burn, probably belongs to the extensive suite of dykes of this trend which was emplaced during the late Carboniferous or early Permian.

A sub-cylindrical body of trachyte, which forms the conspicuous skyline feature of Loudoun Hill (Frontispiece), is considered to be a plug infilling the feeder pipe of a vent active during eruption of the Clyde Plateau Volcanic Formation. In the south-west, four bodies consisting mainly of basaltic breccia, cutting strata ranging in age from Silurian to early Namurian (Limestone Coal Formation) are regarded as vents associated with late Carboniferous to Permian volcanicity. The breccia in the poorly exposed vent at Middlefield Law [NS 681 307] appears to be intruded by a body of basalt of Hillhouse type.

Tertiary

Dykes of tholeiitic basalt and tholeiitic andesite with a north-westerly trend, in the south of the district, belong to a suite of particularly long dykes which are associated with the Mull Tertiary igneous centre. A member of the suite, which has been traced as far as Hart Fell to the south-east, is 22 m wide where it crosses the Ponesk Burn [NS 7160 3070], south-west of Priesthill, and was at one time worked to a small extent for road metal at Catcraig Quarry [NS 7492 2866]. A second dyke of the suite was worked in a quarry [NS 696 299], near Linburn. The rock is commonly dark grey, fresh and unaltered and the dykes locally form small crags. Other dykes of the suite are thinner and cannot be traced so far.

Petrography

The classification of the intrusive igneous rocks adopted for the Hamilton district accords with the scheme proposed by Le Maitre (1989) and Cameron and Stephenson (1985).

Devonian intrusions

The Distinkhorn Plutonic Complex is made up bodies of granodioritic and dioritic composition. The minor intrusions of Devonian age can be divided into three broad groups: felsites, granitic quartz-porphyries and a suite of porphyritic microgranodiorites to microdiorites.

The dioritic rocks which lie at the eastern margin of the Distinkhorn Plutonic Complex are typically medium to coarse grained, inequigranular and holocrystalline, consisting of anhedral to weakly subhedral plagioclase feldspar grains with intergranular hornblende, biotite and quartz (S18760, (S18761), (S18762). Alkali feldspar forms a minor phase occurring intergranular or interstitial to the plagioclase (S18761), (S23370). Hypersthene, where present, forms small rounded anhedral crystals intergranular to plagioclase (S23384). Accessory minerals include opaque oxides, apatite and sphere. The ferromagnesian minerals are variably altered to or pseudomorphed by a fine-grained aggregate of chlorite, secondary ?actinolitic amphibole (S 18762) and carbonates (S23370). A sample of a hypersthene-quartz-diorite from the Distinkhorn Complex has been analysed (Radley, 1931, anal. 136) .

The felsites are typically fine-grained, cryptocrystalline to microcrystalline, aphyric to weakly porphyritic rocks which characteristically contain irregular patches of a partially recrystallised alkali feldspar–quartz mosaic. Turbid albitic plagioclase occurs in variable amounts, usually extensively altered to secondary carbonates and clay minerals. Feldspars within the groundmass may have a weak prismatic form but more commonly occur as patches of small flow-aligned laths. Accessory minerals include opaque oxides and apatite, with rare muscovite and biotite flakes. Typical examples include the large felsite mass (S 62830) exposed to the north of Eaglinside [NS 7619 3473] and a 1 m-thick sill (S62860) in the River Nethan, west of Black Hill [NS 7579 3382]. At Hare Craig [NS 7406 3175], the felsite exhibits extensive patchy recrystallisation (S63033). The quartz-porphyries are of generally granitic composition and consist of scattered phenocrysts of quartz and alkali feldspar within a microcrystalline groundmass of quartz and feldspar. Feldspar phenocrysts include orthoclase and perthite and are usually turbid and sericitised. Euhedral to extremely corroded and embayed quartz phenocrysts are common, possibly of xenocrystic origin, and in some cases have a cryptocrystalline reaction rim against the groundmass. Typical examples are to be found south-cast of Nutberry Hill [NS 7527 3320] (S63043) and on the west side of Mannoch Hill [NS 7474 3245] (S63051).

The quartz-porphyries exposed to the south-west of South Brackenridge [NS 7670 3876] (S62867) and on the southwestern side of Sclanor Hill [NS 7380 3069] (S64479) have a much finer-grained groundmass and contain a higher proportion of alkali feldspar phenocrysts together with sparse biotite flakes. The porphyritic microgranodiorites are composed of relatively abundant phenocrysts (up to 3 mm in length) of euhedral sericitised plagioclase (albite to oligoclase) together with lesser amounts of microporphyritic quartz and biotite, set in a cryptocrystalline to microcrystalline quartofeldspathic groundmass. Minor textural variations within the groundmass include slightly coarser grain size (S62833), (S63028), (S64478), feathery fine-grained feldspar laths (S23327), (S23329), (S62843), and a locally developed flow foliation comprising slightly coarser laths of albitic plagioclase feldspar. Phenocrysts of prismatic ? hornblende locally occur, pseudomorphed by secondary carbonates and opaque oxides. Disseminated, fine-grained to dusty secondary' iron oxides give this suite of rocks a characteristic reddish colour. The porphyritic microgranodiorites grade into the quartz-porphyries, by an increase in alkali feldspar and quartz phenocrysts, and into the more basic porphyritic microdiorites by an increase in modal plagioclase feldspar, ferromagnesian minerals and opaque oxides.

A number of dykes occur which are of generally andesitic composition. They are commonly highly altered rocks, in some cases containing phenocrysts of albitised feldspar. The groundmass of these typically fine-grained rocks consists of felted flow-aligned plagioclase grains, with variable amounts of interstitial quartz and alkali feldspar. Original ferromagnesian minerals are pseudomorphed in calcite and chlorite. Examples of these dykes occur to the SSW of Auchrobert [NS 7617 3756] (S 62842) and in the headwaters of the Eaglin Burn [NS 7542 3525] (S62838).

The porphyritic microdiorites are composed of variable amounts of euhedral, locally zoned plagioclase (albite to oligoclase) phenocrysts, up to 3 mm in length, within a groundmass of fine-grained feldspar laths, altered pyroxene and interstitial quartz. Chlorite and carbonate pseudomorphs after pyroxene phenocrysts are common (S24698), (S65393). The intrusions are commonly considerably altered, resulting in the development of secondary carbonates, chlorite, biotite and the sericitisation of feldspar (S64241), (S64247), (S64253).

Carboniferous intrusions

The rock forming the Loudoun Hill intrusion consists of fine-grained microporphyritic trachyandesite and hornblende-trachyandesite (S24005), (S24006). Subhedral phenocrysts of hornblende are pseudomorphed by granular aggregates of opaque oxides, pyroxene and biotite. The phenocryst assemblage also includes plagioclase and locally glomerophorphyritic titaniferous augite. The groundmass comprises fine-grained flow-aligned plagioclase feldspar laths with granular pyroxene, alkali feldspar, opaque oxide and intersertal glass. Samples of trachyandesite from the marginal facies and central portion of the Loudoun Hill intrusion have been analysed (Radley, 1931, anal. 164 and 165).

Tertiary intrusions

The Moneyacres (S24594, (S62859, (S63039) (S63040), (S64387), (S64391), (S64393), Barrmill–Hartfell (S25807), (S63433), (S64263), (S64392) and Dalraith–Eskdalemuir dykes (S23330), (S23346), (S23372), (S25810), (S63427), (S65364), (S65371), (S65397), of tholeiitic andesite and quartz-dolerite composition, belong to the suite of WNW-trending Tertiary dykes which cross the district. In general, these rocks are composed of randomly oriented, zoned plagioclase (labradorite) laths with granular to locally subophitic augitic pyroxene and a partially devitrified mesostasis. Minor pigeonite occurs as subhedral prisms which in a few cases are enclosed within augite. A few elongate prisms of hypersthene may occur as partly corroded microphenocrysts. Abundant interstitial material varies from partially devitrified glass, composed of aggregates of feathery to spherulitic feldspar, pyroxene microlites and magnetite-ilmenite needles, through partially crystalline pockets of quartz and feldspar to completely recrystallised micrographic quartz and alkali feldspar.

The tholeiitic basalts typically occur as narrow dykes, and consist essentially of calcic plagioclase and pyroxene with devitrified intersertal glass, minor magnetite-ilmenite and accessory apatite. Plagioclase forms random, stellate or radiating laths of labradorite, and in some dykes abundant large bytownitic phenocrysts are present. Pyroxene is predominantly augite, with subhedral microphenocrysts of orthopyroxene and rare pigeonite. The overall texture of these rocks varies from very fine grained containing weakly subhedral pyroxenes (S65377) to generally coarser grained with subophitic pyroxene (S24679), (S62171), (S63048) depending on the position within the intrusion. Olivine may occur within these rocks forming scattered microphenocrysts which are typically pseudomorphed by chlorite and serpentine. The interstitial glass contains a high proportion of very fine-grained magnetite-ilmenite. In the coarser-grained olivine-tholeiites the mesostasis is more crystalline, with magnetite-ilmenite dendrites, hollow skeletal feldspar and aggregates of pyroxene and felspar microlites (S63042), (S64239), (S64289). The tholeiitic basalts grade into the quartz-dolerites, with the more evolved lithologies being transitional into the tholeiitic andesites.

Chapter 7 Structure

The Hamilton district lies close to the southern margin of the major NE-trending graben-like structure that underlies the Midland Valley of Scotland. Information relating to the earliest development of this ancient feature, and in particular of the fault system which forms its southern boundary, is provided by the Silurian rocks of the Lesmahagow and Hagshaw Hills inliers. Reactivation of these faults at various times had an especially marked effect on the distribution of the Lower Carboniferous.

Late Caledonian and early Hercynian earth movements

During the Silurian period, closure of the Iapetus Ocean by subduction processes brought the Eastern Avalonian plate into oblique collision with the Laurentian plate (e.g. Soper et al., 1992). Sinistral strike-slip displacements occurred on the set of NE-trending faults, which includes the Great Glen and Southern Uplands faults as well as a major fracture between the Lesmahagow and Hagshaw Hills inliers. This fracture, which lies along the line of the later Kerse Loch Fault, may have operated during the Silurian, possibly as a growth fault with a northerly downthrow but more probably causing previously deposited Silurian strata in the Hagshaw Hills on its south side to be upthrown and eroded.

The structural setting of the Midland Valley Silurian depositional basin is obscure but it has been proposed on geophysical grounds that it is underlain by crystalline basement at relatively shallow depth. Many of the rocks in the Silurian inliers, including the greywackes in the early part of the sequence (Jennings, 1961; Rolfe, 1961) and the conglomerates (McGiven, 1968; Heinz and Loeschke, 1988) , appear to have been derived from the south. As they cannot have had their source in the greywackes which form the Southern Uplands, it has been suggested (Bluck, 1983) that an acidic to basic volcanic terrane, which also included quartzite conglomerate, formerly lay to the south but has been tectonically overridden by, and lies concealed beneath, the Southern Uplands greywackes. More probably, this source terrane has been displaced by movements on strike-slip faults during the late Silurian.

In the Lesmahagow and Hagshaw Hills inliers, it is probable that a stratigraphical break occurs between the Silurian rocks and those assigned to the Devonian, although there is no marked angular unconformity corresponding with that in the Pentland Hills. The Lower Devonian sandstones and conglomerates were laid down by SW-flowing rivers in an elongate basin at least as wide as the present Midland Valley. Emplacement of the Distinkhorn granodioritic plutonic complex, dated at 412.8 Ma (Thirlwall, 1988), and numerous sheet-like bodies of felsite took place during this period. It has been argued that development of the basin during early Devonian times took place under a sinistral strike-slip regime (e.g. Bluck, 1984) but there is reason to believe that lateral displacements on at least the northern boundary fault in post-Lower Devonian times cannot have exceeded a few tens of kilometres (Trench and Haughton, 1990).

During the Middle Devonian, NW–SE compressional earth movements took place, possibly representing the first effect of the northward impingement of the Armorica–Iberia block against the Laurentia–Baltica–Eastern Avalonia continent (Soper et al., 1992). These caused the Silurian and Lower Devonian rocks of the Lesmahagow and Hagshaw Hills inliers to be deformed into asymmetric anticlinal folds, faulted in their axial regions. Almost certainly, there was movement on fractures along the line of the Kerse Loch Fault, as well as on many other fractures of similar trend. Some important faults, such as the Logan Water Fault and the Wedder Hill Fault, with trends between north–south and NNW–SSE, may date from this period. The earth movements were accompanied by persistent regional uplift with the result that most and, in places, all of the Lower Devonian rocks were removed by erosion and in the northern part of the Midland Valley, a deep NE-trending basin was eroded (Paterson et al., 1990a).

The left-lateral strike slip stress regime of the Lower Devonian period was replaced in the Middle Devonian by a regime mainly involving thermal subsidence following lithospheric extension (McKenzie, 1978), with some degree of superimposed right-lateral strike slip (Leeder and McMahon, 1988; Dewey, 1982; Read, 1988). When sedimentation in the Midland Valley resumed, in Upper Devonian times, the Hamilton district apparently lay outwith the depositional basin. The onset of sedimentation was delayed until early Carboniferous times when the cornstone-bearing sandstones of the Kinnesswood Formation were laid down under quiet tectonic conditions by NE-flowing rivers in a subsiding basin at least as broad as the present Midland Valley. Continuing subsidence for a time allowed deposition of the Ballagan Formation in a coastal sabkha or restricted marine environment subject to fluctuating salinities and periodic desiccation before uplift of the upland source areas restored fluvial conditions towards the end of the Inverclyde Group period.

In mid-Dinantian times, deposition of the Inverclyde Group in the Midland Valley was halted by a renewal of tectonic activity which had the effect of exposing the strata of the group to erosion. The evidence is sparse but in places removal of the Inverclyde Group was mainly or entirely complete before extrusion of the Clyde Plateau

Volcanic Formation lavas, especially over a tract in the west-central part of the Midland Valley which appears to have been bounded in the south by the Inchgotrick Fault, and in the west by the Largs Ruck (Figure 8). The eastern margin of the uplifted area may lie at the north-trending Bo'ness Line (Read, 1988, fig. 16.1) between Salsburgh, where lavas of the Clyde Plateau Volcanic Formation are considered to rest upon Lower Devonian rocks (Falcon and Kent, 1960), and the West Lothian–Edinburgh area, where lavas of equivalent age overlie a sequence up to 1120 m thick of early Dinantian corn-stone- and cementstone-bearing strata (Cameron and Stephenson, 1985, p.60). The block thus corresponds to the North Ayrshire and Airdrie blocks of Read (1988).

It has been suggested that the uplift was due to magmatic updoming prior to extrusion of the lavas of the Clyde Plateau Volcanic Formation (cf. Monro, 1982) . However, variation in the preserved thicknesses of Inverclyde Group strata suggests that ancient fractures of Caledonoid trend, such as the Kerse Loch and Ochil faults, were reactivated at this time also and it is possible that the block movements are a response to east–west compression during a transpressional episode when dextral shearing occurred on the fault-systems which bound the Midland Valley. The conjugate north–south tensional forces may have caused fissure systems to form from which the lavas of the Clyde Plateau Volcanic Formation were extruded. Alternatively, fissure formation may reflect a return to a mainly transtensional regime.

Following extrusion of the lavas, cyclic sedimentation in a delta prograding towards the south-west became established and continued throughout the late Dinantian and Silesian. Lateral variation in facies and thickness indicates that deposition was influenced by differential subsidence of fault-bounded blocks in a dextral transpressional regime. Some of the bounding faults, such as the Kerse Loch and Inchgotrick faults, clearly follow pre-existing fractures; others may be of more recent origin. One such is the NW-trending fracture zone, represented by the post-Westphalian Dechmont Fault which forms the eastern boundary of the North Ayrshire Block (Read, 1988). This block, which contains the thick lava pile of the Clyde Plateau Volcanic Formation, remained relatively buoyant through much of the later Carboniferous, possibly as a result of isostatic response to crustal thickening caused by emplacement of a high-level magma chamber. In consequence, the successions of both the Lawmuir and Lower Limestone formations are thin over the block as compared with adjacent districts, even after allowance is made for a delayed start to deposition because of the topographical high formed by the lavas.

During deposition of the Lawmuir Formation, movements on the Inchgotrick and Strathaven faults, at the southern margin of the North Ayrshire Block, may account for the unusually thick sequence contained in a narrow basin at Strathaven. The Kerse Loch Fault played an important role in controlling sedimentation at various times during the Carboniferous, forming the northern boundary of rapidly subsiding basins in the Muirkirk and Douglas areas (Lumsden, 1967b; Read, 1988). In the Douglas Basin especially, thicker sequences of the Lawmuir, Upper Limestone and Passage formations were laid down than occur elsewhere in the Hamilton district (Browne et al., 1985) .

Basin subsidence in the district was halted by uplift, presumably as a result of transpressional movements, late in Upper Limestone Formation times and during deposition of the Passage Formation. At Kirkmuirhill, at least 45 m of Upper Limestone Formation strata were removed by erosion and even greater thicknesses of the Passage Formation were eroded in the Douglas area (Lumsden, 1967b). Significantly, at about this time, right-lateral strike slip on the Ochil Fault led to the formation of the rapidly subsiding Westfield Basin in western Fife (Read, 1988).

It is probable that most of the extensive system of faults of east–west and NW–SE trend (Figure 9), which cut the youngest Carboniferous rocks in the district, date from the latest Carboniferous or early Permian. Notable examples of these are the Dechmont Fault, with an easterly downthrow of up to 1000 m in places, and the faults which form a graben-like structure in the Wishaw area. Open folds with north–south to NNW-trending axes were also formed during this general period.

Chapter 8 Quaternary

Evidence from ocean floor sediments indicates at least 16 cold events during the last 1.6 million years (Bowen, 1978; Price, 1983). Many of these caused central Scotland to be covered by a thick ice sheet which eroded the underlying rocks or sediments and deposited lodgement tills. During warmer episodes interglacial deposits developed. In the Hamilton district almost all evidence of earlier glaciations and interglacials was removed by erosion during the last main late Devensian (Dimlington) glaciation which lasted from about 28 000 to about 13 500 years before present (BP). Possible interglacial deposits and an older till remain in the north but the bulk of the superficial deposits in the district date from this late Devensian glaciation and from the period since then up to the present.

Late Quaternary drift deposits blanket much of the area of the Hamilton district, particularly on the low ground, masking a land surface which had evolved during the Tertiary and the early Quaternary. In the Larkhall–Hamilton area, buried drift-filled channels mark the course of former drainage into the valley of the River Clyde (Figure 10). Extensive drift deposits cover the less resistent Devonian and Carboniferous sedimentary rocks in the northern half of the Hamilton district except where these deposits have been cut through by later erosion in the incised valleys draining into the River Clyde or by the recent activities of man. The drift deposits are thin or absent in the rounded Silurian uplands in the south of the district and in the west where there are the craggy knolls of the Clyde Plateau lavas and associated volcanic plugs. The distribution and nature of the drift, or superficial, deposits are shown on the 1:50 000 Geological Drift Sheet 23W (Hamilton), published in 1993.

Summary of Quaternary history

The oldest Quaternary deposits in the Hamilton district, preserved as part of the infill of buried channels in the north, are thought to pre-date the Dimlington ice sheet (Table 4). This started to develop at about 28 000 BP as the onset of colder climatic conditions permitted ice caps to build up in the areas of highest precipitation — in the western Highlands and possibly also the south-west Southern Uplands. The ice of these centres subsequently coalesced to form an ice sheet which, at its maximum extent at about 18 000 BP, covered not only the Midland Valley of Scotland but most of the north British landmass.

As the ice sheet expanded and subsequently decayed, its centre migrated, thereby altering the direction of ice flow across the Hamilton district. At an early stage of the development of the ice sheet, the Highland ice centre was dominant and south-easterly flowing ice crossed the central Lowlands to encroach upon the Southern Uplands (Geikie, 1894; Sutherland, 1984). Later the Highland ice was forced back as the Southern Uplands centre gained strength and the zone of confluence of the Highland and Southern Uplands ice was displaced northwards until it lay across the centre of the district. The resultant ice flow directions, as inferred from the alignments of drumlins and glacial striae and the analysis of till fabrics, were from the WNW in the north of the district and from the west and south-west in the south, although there are indications in the south of southeasterly flow also, reflecting the earlier period when the Highland ice centre was dominant (Figure 11). The smooth, rounded landforms, particularly in the upland areas in the south, are probably the result of erosion by successive ice sheets and only the few examples of glacial striae can with confidence be attributed to the last ice sheet in the area. An extensive, mainly thin layer of lodgement till, moulded into drumlins in the north, was laid down by the ice sheet.

During the period between about 17 000 and 13 500 BP, the eastern margin of the ice sheet retreated westwards. To some extent this may have reflected westward migration of the centre of the ice mass but all the members of staircases of shoreline features in eastern Scotland (Cullingford and Smith, 1966), which date from this period, have been tilted and uplifted as a result of isostatic recovery and thus suggest that considerable ice melt was also taking place. Following about 13 500 BP, however, the rate of withdrawal was accelerated in response to climatic amelioration. As the ice surface downwasted, the higher hills in the district emerged and the ice sheet became confined to the lower ground. Meltwater cut glacial drainage channels and laid down deposits of sand and gravel in ice-contact kames and eskers, and as outwash spreads. Retreat of the ice margin towards the west and north-west caused the valley of the River Clyde and its principal tributaries to be occupied by a large lake, in which were deposited laminated clays, silts, sands and gravels in the upwards coarsening sequences characteristic of deltaic sedimentation. The occurrence of ice-wedge casts penetrating the sand and gravel forming the upper part of the delta deposits is an indication that periglacial conditions prevailed in the district during this period. This may mean that the sand and gravel deposits were laid down during the Dimlington Stadial when periglacial processes were active over a longer period than during the Loch Lomond Stadial (Armstrong et al., 1985). The water levels of the lake fell as continuing retreat of the ice uncovered progressively lower spillways.

Final melting of the ice in the lower River Clyde valley eventually allowed the sea to inundate the Glasgow area, possibly to a level 40 m higher than at present (Browne et al., 1983). It is uncertain, however, whether the sea gained access to the Hamilton district. Sea level continued to fall as a result of isostatic recovery and by 11 000 BP, lay at a level equivalent to its present position or even lower. A return of very cold climatic conditions to Scotland in the period of the Loch Lomond Stadial,  between 11 000 and 10 000 BP, caused expansion of the Highland ice sheet which, however, did not extend into the Hamilton district.

The Flandrian Stage commenced at 10 000 BP, with a rapid rise in temperature. Although sea levels rose again around 8000 BP, reaching a maximum at about 6500 BP, the sea inundating the valley of the Clyde failed to reach the district. Warmer, wetter conditions prevalent at times during the Flandrian initiated growth of peat as extensive spreads on the higher ground in the south and west of the district and as more restricted deposits of lowland basin peat covering lake and river alluvium. Much peat has since been removed by erosion or by man.

Following deglaciation, the modern drainage pattern developed. Many streams and tributaries, particularly those flowing into the River Clyde, cut or exhumed incised valleys and formed spectacular rock gorges mainly in Carboniferous sedimentary rocks. Where the valleys are cut into the infill of buried channels, landslips are common in the thick drift deposits forming the valley sides. Lakes occupying hollows in the glaciated landscape gradually filled up with laminated silt and clay, commonly overlain by peat. Extensive alluvial deposits were laid down, as along the valley of the River Clyde and its tributaries the Avon Water and the River Nethan. Terraces formed in these reflect adjustments of river levels in response to changing base levels. Finally, man has left his imprint on the landscape during the last two centuries of industrial activity, particularly in the areas underlain by coal-bearing Carboniferous rocks in the northern half and the extreme south-east of the district.

The following description of the Quaternary sediments and landforms takes account of the stratigraphical framework established for the Clyde valley area by Browne and McMillan (1989a; 1989b) and which was expanded and developed in the full and wide-ranging description of the Quaternary of the Airdrie district by McMillan (in Forsyth et al., 1996). The stratigraphical framework defines formations on the basis of their lithology and principal provenance (Table 4). Deposits elsewhere in the district have not been assigned to formations.

Pre-Dimlington Stadial

Topography

The smoothly rounded uplands in the south of the district are typical of glaciated terrains. The broad east–west-trending valleys of the Avon Water between Loudoun Hill and Strathaven and of the Greenock Water in the south are also likely to have been affected by glacial erosion, probably during many glacial episodes. The most detailed information on topographical features in existence prior to the Dimlington Stadial relates to deeply incised, drift-filled valleys associated with the River Clyde and its tributaries.

From borehole evidence, the rock floor of the smooth-sided, NW-oriented buried valley of the River Clyde is known to lie at a level close to OD within the district and to descend north-westwards to depths well below present sea level. Part of the infill of the valley consists of till, overlain by what is believed to be interglacial sediment. It would appear that the buried valley lies along the line of the present valley and that this had been occupied and over-deepened by a glacier in pre-Dimlington Stadial times.

The former pattern of tributaries of the River Clyde to north and south differs markedly from that of the present-day system (Figure 9). For example, the Avon Water formerly flowed almost due northwards instead of following its present sinuous course, incised deeply into rock gorges except to the west of Larkhall where it is cut in drift deposits infilling the abandoned channel. A drift-filled channel near Wishaw shows that the South Calder Water used to flow directly into the Clyde at Upper Carbarns [NS 780 532] but is now diverted northwards and takes a longer route to the west. There is borehole evidence also of a drift-filled channel running east–west under the former Ravenscraig Steel Works [NS 770 570]. Deposits up to 50 m thick, composed mainly of sand and gravel, overlie till in the channel, the base of which drops from 62 m above OD in the east to 35 m above OD in the west (McMillan in Forsyth et al., 1996).

Deposits

Deposits which predate the last glaciation to affect the district are most likely to have been preserved within hollows in the rock surface, as is the case in the Airdrie district (McMillan in Forsyth ct al., 1996). Some of the infill of the buried channels in the north-east of the Hamilton district may include pre-Dimlington Stadial material but there is no direct evidence of this. Elsewhere the drift deposits are relatively thin and the till which rests on rockhead is presumed to relate to the last glacial episode in the district.

Dimlington Ice Sheet

The last ice sheet to cover the whole of northern Britain is believed to have been initiated at about 28 000 BP and to have reached its maximum extent at about 18 000 BP. Initially, an ice cap is likely to have developed in the south-west Highlands — an area of high precipitation facing the prevailing wind direction. This view is consistent with south-easterly flow of ice in the area around the Firth of Tay (Armstrong et al., 1985, fig. 14) and southerly flow in the Greenock area (Paterson et al., 1990a, fig. 9), indicated by glacial striae on hilltops. The orientation of some directional features, that is glacial striae and the long axes of drumlins, suggests that the entire Hamilton district was at some time during the final glaciation covered by south-eastwards flowing ice. Whether this period of dominance by ice from a Highlands' centre represents a stage in the build-up or the decay of the ice sheet cannot be determined from evidence within the district. However, at localities in the Lothians, east of the Pentland Hills, till with a clast content and fabric consistent with deposition from eastwards flowing Highland ice is overlain by till laid down by ice flowing northwards from the Southern Uplands (Kirby, 1968; 1969) .

Throughout the Midland Valley, there is abundant evidence from glacial striae and drumlins of eastwards directed ice-flow across the full width of central Scotland and also of a north-eastward flow. These flow directions must postdate coalescence of the Highland ice cap with ice masses which had developed over the western

Southern Uplands, the Lake District and Northern Ireland to form an ice sheet which expanded to cover most of Britain and part of the adjacent continental shelf, reaching its maximum extent at about 18 000 BP. The easterly flow direction, epitomised by the Lennoxtown erratic train (Peach, 1909) , may have related to a configuration of the ice sheet during a period when the Highlands and the southern centres were of comparable size and the ice shed lay to the west of the Ayrshire coast. The north-easterly flow, which transported shelly till containing material dredged from the sea bed in the Firth of Clyde to localities in Ayrshire and south Lanarkshire at heights of at least 300 m above OD (Smith, 1898), and also carried Strathmore drift to Aberdeen, must date from a period when ice of the southern centre was dominant within the Midland Valley.

During the period following 18 000 BP, the eastern margin of the ice sheet withdrew slowly westwards while the climate continued to be Arctic, as indicated by the faunas recovered from contemporaneous marine sediments (Peacock, 1975; Armstrong et al., 1985). By the time sea level had dropped to the position of the Main Perth Shoreline, a feature which may represent a response to accelerated melt as a result of marked amelioration of climate in the period 13 500 to 13 000 BP (Ruddiman and McIntyre, 1973; Paterson et al., 1981), the ice margin in the Forth valley is believed to have lain close to Stirling (Sissons and Smith, 1965) .

As deglaciation proceeded, draw-down of the ice-surface as a result of iceberg calving in the deepwater estuaries of the Tay and Forth and the emergence of the high ground of the Pentland Hills may have promoted the separation of the Highlands and Southern Uplands ice masses. The indications of south-easterly and north-easterly flow directions apparent in the north and south respectively (Figure 11) and (Figure 12) suggest that, prior to the decoupling, the Hamilton district straddled the zone where a balance between the Highlands and Southern Uplands ice was maintained. That decoupling of the two ice masses eventually took place along a line which traversed the district is implied by the evidence of the east–west-trending series of esker ridges at Chapelton. Thus, the western part of the esker complex lies on the north-facing slopes of the ridge which extends eastwards from Corse Hill [NS 598 465] and requires the presence of impermeable ice on the north side of the ridge at least as far east as Chapelton (Figure 13). At Chapelton, the esker complex crosses the crest of the Corse Hill ridge. Continuation of the eskers towards Glassford demands the existence of impermeable ice to the south to prevent the meltwaters escaping by way of the Powmillon Burn, as happened at a later stage of the deglaciation when the esker at locality A (Figure 13) was laid down. Separation of the ice masses along the Chapelton line would have been completed when the high ground around and to the east of Corse Hill emerged above the surface of the downwasting ice.

Glacial Till (Wilderness Till Formation)

The Dimlington ice sheet deposited a layer of lodgement till over the whole district, except for the high ground, and infilled hollows in the preglacial surface. The deposit is a continuation of the Wilderness Till Formation (Rose, 1980; Browne and McMillan, 1985), named after the Wilderness Plantation north of Bishopbriggs in the Glasgow district. It is a variable diamicton, generally a stiff to hard, silty or sandy clay containing rounded and striated pebbles, cobbles and boulders consisting mostly of the local country rock but including a significant proportion of far-travelled erratics such as Highland metamorphic rocks. In places, the till contains contorted lenses of sand and gravel, for example in the River Irvine, south of Loudoun Hill.

The composition of the till matrix and of the included clasts is strongly influenced by the nature of the adjacent bedrock. In areas of Carboniferous sedimentary rocks the till is a dark grey silty clay with pebbles and boulders mainly of Carboniferous sandstones and limestones. Over Devonian outcrops, the till consists of reddish sandy clay with predominantly red sandstone clasts. There are few exposures of till in the mainly peat-covered ground over the outcrop of the Clyde Plateau Volcanic Formation but here the deposit tends to be variegated with mainly igneous pebbles and boulders. In areas of Silurian rocks, where the till is commonly confined to the lower valley-sides and valley-bottoms, there is a predominance of tabular greywacke and siltstone clasts in an assemblage which also includes clasts of quartzite, chert, jasper and the various Lower Devonian intrusive rocks.

At several localities in the south of the district, such as in the Dippal Water, shells were recorded in the till (Smith, 1898, p.80). This has been taken (Eyles et al., 1949) as evidence of the extent of Ayrshire ice which dredged the shells from the floor of the Firth of Clyde and carried them eastwards up the valley of the River Ayr. Local ablation till of less-consolidated material formed in places from the wasting ice.

Till lies at surface on half the district area. It has been proved to be in the order of 2 to 5 m thick but where infilling channels it can be 30 m or more thick. Till is normally the deposit lying immediately on the bedrock and underlies most of the other Quaternary deposits. It is commonly formed into drumlins, which are a characteristic feature of the Midland Valley landscape and provide most of the evidence of ice-flow direction (Figure 10) and (Figure 11). These consist of oval mounds formed under the ice sheet and elongated in the direction of flow. They are particularly common in the northern half of the district, representing part of a larger field extending to the north and west. The drumlins vary greatly in size and shape. Larger drumlins in and to the south of East Kilbride may be up to 500 m long by 200 m wide and 20 m high; the one surmounted by the Dollan Baths in the town centre [NS 633 543] is a fine example. Smaller elongate drumlin ridges, forming a swarm to the southeast of the town, are as much as a kilometre long but less than 100 m wide and only a few metres high. Some are more or less surrounded by a blanket of peat. Glacial drainage and, subsequently, alluvial drainage have at times eroded, cut through and variously modified the original shape of many of the drumlins. Some drumlins are formed entirely of till, for example, up to 20 m of till is present in the 'Dollan Baths' drumlin; others are likely to be rock-cored.

Early Deglaciation Phase

Itis considered that during the early stages of deglaciation, the Dimlington ice sheet divided into masses centred over the Highlands and the high ground on either side of the Scottish Border. The decoupling process is thought to have been initiated and sustained by drawdown of the ice surface over the Midland Valley in response to iceberg calving in the deep-water embayments of the Tay and Forth estuaries at a time when relative sea level was high as a result of isostatic depression of the land. Because of its position in relation to the zone of decoupling, emergence of the Pentland Hills above the downwasting ice surface played an important role in separating the two ice masses. For a time, the margin of the Highland ice must have lain against their northern slopes, perhaps while the ice front in the Forth valley was stabilised at the Queensferry constriction. An ice margin at this position must considerably predate the climatic amelioration of 13 500 BP, as the fauna found in the basal division of the Late-glacial marine sequence laid down in the Grangemouth area while the relative sea level was at least 44 m higher than at present shows that this area was deglaciated while the climate was still arctic (Browne et al., 1984). Sea level at the time of the amelioration is believed to be marked by the prominent Main Perth Shoreline (Paterson et al., 1981; Boulton et al., 1991, p.531).

Meltwaters from the west flowed along the interface between Southern Uplands ice and the south-facing slopes of the Pentlands, laying down deposits of ice-contact sand and gravel at Carstairs (Goodlet, 1964) and Dolphinton before passing into the catchment of the River North Esk by way of the spillway at a height of about 268 m above OD near Carlops (Figure 12) and (Figure 14).1. This could only happen while alternative lower spillways to the south-east, the west and the north were blocked by impermeable ice. Subsequent lowering of the ice surface north of the Pentland Hills allowed meltwaters to escape north-westwards into the Forth valley by way of the col at Cobbinshaw Reservoir (Figure 12) and (Figure 14).2. At about 265 m above OD, the present height of the col is similar to that of the Carlops spillway but during the Dimlington Stadial, while isostatic recovery was incomplete, the difference in height was considerably greater. The well-defined esker ridges, which pass along the northern margin of the Carstairs sand and gravel complex and continue northwards to Couthalley Castle [NS 972 482] and Woodend Moss [NS 977 503] (McLellan, 1969; Laxton and Nickless, 1980), were doubtless formed at this time.

As deglaciation proceeded, with ice of the Highland and Southern Upland centres retreating to north and south respectively, new low-level routes became available for meltwater escaping eastwards through, and laying down ice-contact sand and gravel deposits within, areas of stagnant ice in Avondale and, to the east of the district, in the Clyde valley around Lanark. The earliest of these, which operated while ice blocked alternative routes at Dolphinton, approximately 224 m above OD, and Biggar, a little below 200 m OD, was the east–west valley which passes eastwards into the valley of the Breich Water. Here, on the south side of the valley a little to the east of the district, a linear series of ridges and mounds of sand and gravel extends for a distance of 350 m through a point, 1000 m NNW of Spoutscross [NS 881 563], reaching at a height of about 253 m above OD. This deposit records the first eastwards seepage of meltwater through crevassed ice of the Highland centre. As downwasting of the ice continued, the meltwater laid down the train of ice-contact sand and gravel along the valley floor past Stane before escaping into the Forth valley over a col at a height of about 245 m above OD into the Breich Water. At a somewhat later stage, meltwater may have passed into the catchment of the River Almond through the col at Shotts at a height of about 235 m above OD.

Diversion of the flow from the Cobbinshaw col to the Stane spillway would have brought to a close deposition of the north-trending esker system at Carstairs. Down-wasting of Southern Upland ice in the neighbourhood of Dolphinton, in conjunction with the unblocking of the lower Tweed valley, would subsequently have allowed meltwater to flow north of Black Mount and access the catchment of the Tweed by way of the channel near Garvald House [NS 488 095] at about 224 m above OD. At this time the meltwater channel system and associated ice-contact deposits of sand and gravel along the northern flank of the Tinto massif were formed, while ice still blocked the Biggar Gap (Sissons, 1961, p.189).

At about this stage of the deglaciation, the summits along the SE-trending ridge that extends from Hawk-wood Hill [NS 687 379] to Hareshaw Hill [NS 764 296] in the south of the Hamilton district (Figure 11) and (Figure 12) emerged above the surface of the downwasting southerly derived ice. For a time, ice continued to flow north-eastwards through breaches in the ridge (McLellan, 1969, fig. 2) but, as the lines of supply over the ridge were cut, ice in its lee gradually became stagnant. Meltwaters also utilised the breaches and cut the large channels now occupied by the Kype and Logan waters, the Birkenhead Burn, the River Nethan and the Pockmuir and Scots burns. The sequence in which the breaches became operative cannot be determined as it depends on an exact knowledge of the form of the ice surface. The effect of the emergence of the ridge was to divide the Southern Uplands ice into two streams, one flowing north-eastwards around its northern end, the other entering the valley of the Douglas Water.

There are a number of points of interest concerning the meltwater channel system. Meltwater that passed between Side Hill [NS 680 370] and Harting Rig [NS 695 363] to form the headwaters of the Feeshie Burn may for a time have encountered an ice margin at Feeshie Moss and been diverted into the channel occupied by the Long Knowe Burn. The Blaeberry Burn may originally have been supplied by waters escaping through the breach between Auchengilloch [NS 705 356] and Goodbush Hill [NS 715 347]. The case of the River Nethan is particularly instructive. Here the meltwaters eroded a deep, virtually straight channel, parallel to the present course of the river but up to 300 m to the south-east and at a level about 20 m higher, before dying out in low ground occupied by an ice-contact deposit of sand and gravel. The eastern edge of the channel and part of the sand and gravel deposit which infills it are exposed, 200 m SSW of Over Stockbriggs [NS 788 354], in a small tributary of the River Nethan. If there was a pre-existing valley along the line of the present river, it must have been occupied by ice or, possibly, by glacial sediment.

As the ice passing around the northern end of the Hawkwood–Hareshaw ridge withdrew towards the west, meltwaters flowing along its south-eastern margin cut extensive systems of marginal channels on the north- and north-west-facing slopes of the ridge, the channels on the lower slopes of Hawkwood Hill [NS 680 390] and Middle Rig [NS 715 405] being especially prominent ((Figure 11); (Plate 7)). For the most part the channels lie at heights between 215 m and 300 m. They are commonly associated with deposits of sand and gravel laid down within crevassed ice along the margin of the ice lobe, generally as kame terraces but also as esker ridges, the most notable example being the system known as the Kaims of Avon [NS 605 350] ((Plate 8)), the extreme eastern part of which lies within the district. The terraces mostly lie at heights below 300 m but degraded examples are associated with meltwater drainage channels at a height of about 320 m in the valley of the Powbrone Burn [NS 678 340]. As a result of a further westward withdrawal of the ice, a lake was for a time impounded in the Glengavel Burn around Laigh Plewland [NS 654 353], as shown by the occurrence of laminated silts and clays interbedded in the sands and gravels (Nickless et al., 1978) .

As deglaciation continued, the high ground around Blackside [NS 305 590], to the south-west of the district boundary was the next to emerge above the surface of ice entering the district from the south. It is possible that this ice was part of the mass that was responsible for the abundant NE-trending striae and drumlins prevalent in coastal districts of Ayrshire and also for depositing the Shelly tills found there and in the south-west of Arran (Sissons, 1967, fig. 33). If so, the ice shed at this time, possibly along a saddle linking ice centres over the Mull of Kintyre and the western Southern Uplands, would appear to have lain between Arran and Ailsa Craig, otherwise erratics of the distinctive microgranite would surely have been reported from coastal areas in Ayrshire. The drumlins of NE–SW trend in the Lochwinnoch area (Paterson et al., 1990a, fig. 9) may have been formed at this time also.

North-east of a line connecting the Renfrewshire Hills to the high ground over the Clyde Plateau Volcanic Formation west of Strathaven, glacial striae and drumlins are aligned NW–SE whereas to the south-west they have a SW–NE trend. It is considered that this line marks the contact of north-easterly flowing Southern Uplands ice with south-easterly flowing ice from the Highlands centre. No deposits containing a fauna of arctic affinity have yet been discovered in the Firth of Clyde, implying that an ice lobe continued to occupy the firth until the period of climatic amelioration at about 13 500 BP. The ice in the outer firth, as far north as the islands of Bute and the Cumbraes, may have originated in the south. For a considerable period during the deglaciation, ice from this lobe occupied the valley of the River Irvine at Darvel.

Later deglaciation and glacial lake formation

During a stage in the deglaciation when ice in the Irvine valley, west of Loudoun Hill, prevented westwards escape of meltwater and Highland ice filled the lower parts of valleys of the Avon Water and the River Clyde, a lake, which has been given the name Lake Clydesdale (Bell, 1874), developed at the expense of the residual ice left to stagnate by separation of the Highland and Southern Upland ice centres ((Figure 14).3). The lake was rapidly infilled with sediment, characteristically consisting of laminated silts and clays (Bellshill Formation of Browne and McMillan, 1989a and see below). In areas where there was a copious supply of outwash from a nearby ice front, these silts and clays form the basal unit of upwards coarsening sequences typical of deposition in Gilbert deltas. There are excellent sections in the coarser deposits of the upper part of the delta (Ross Formation of Browne and McMillan, 1989a and see below) in sand and gravel quarries in the area east of Loudoun Hill (Frontispiece: (Plate 9)). Outwash sand and gravel, introduced into the Avon valley by the Calder Water, also overlies glaciolacustrine silts and clays.

The former extent of the lake in the valleys of the River Clyde and its main tributaries and the identification of its spillways, all of which must lie to the east of the district, is to some extent speculative ((Figure 14).4). Laminated silts and clays of the Bellshill Formation (Browne and McMillan, 1989a) occur to a height of 205.7 m above OD in borehole NS63NW/61, south-east of Loudoun Hill (Nickless et al., 1978) , and provide a minimum level for the lake surface in that area. The age of the deposit is not known but ice-wedge casts have been recognised in sand and gravel spreads in the area suggesting that they were laid down while the climate was still arctic prior to the amelioration of about 13 500 BP. Raised shorelines of this age or older have gradients of more than 0.4 m per km towards the ESE as a result of isostatic recovery and a tilt of this order must be assumed for lake levels of the period also. The Biggar Gap (Figure 12), some 30 km to the SSE of Loudoun Hill, is at a little below 200 m above OD and appears to be the only escape route for the lake water which is at the appropriate height. Note that the colat Hillend Reservoir [NS 835 675], which delivers water past Blackridge ultimately into the Avon Water, is at a similar level at present but must lie close to the isobase which passes through Loudoun Hill and was therefore too low at the time of Lake Clydesdale. If the Biggar Gap did act as the outlet, then the lake, together with any surviving dead ice within it, must have occupied thevalleysof the Avon Water and theClyde, downstream from Loudoun Hill and Biggar respectively, almost to their confluence.

As north-westward retreat of the Highland ice front continued, escaping meltwater may have exploited, in turn, the spillway at Hillend Reservoir (Figure 12), then a col at around 165 m above OD between Langdales and Fannyside Reservoir, and subsequently the deeply incised Red Burn, which has its intake at a height a little over 100 m above OD, thus progressively lowering the lake level. Finally, the col at about 85 m above OD at East-field, near Cumbernauld, may have been the last major spillway to operate before dissolution of the ice dam that had maintained the lake in lower Clydesdale and denied access of the sea to the Glasgow area.

The infill of the lake northwards is represented by the copious, generally upward-coarsening deposits in the Avon valley and along both sides of the Clyde valley from Larkhall to Motherwell. The lower part of the deposit consists of laminated clay and silt assigned to the Bellshill Formation, the upper part, composed mainly of sand with gravel in places, is referred to the Ross Formation. The rejuvenation by stages of the rivers entering the lake caused them to rework the sediment infilling the lake and form the well-marked sand and gravel terraces seen, for example, along Avondale between Drumclog andStrathaven. In the Larkhall to Motherwell area, the deposit has been heavily dissected.

The height at which the lake deposit occurs diminishes northwards partly because of lowering of the lake level as northwards retreat of the ice made available new outlets at lower levels and partly as a result of reworking of the existing deposit as the rivers adjusted their courses in relation to these new outlets. Thus, in the Avon valley at Laigh Netherfield [NS 727 454], a small deposit of brown clay survives at a height of more than 150 m above OD whereas laminated clay and silt, exposed in an excavation [NS 724 524], north of Carscallen reaches a height of only 142 m above OD. A widespread clay deposit south-west of Wishaw reaches a height of about 120 m above OD but, in general, the upper limit of the main lacustrine deposit preserved in the Clyde valley is lower, being at about 75 m above OD near Merryton [NS 758 531], about 63 m above OD at Greenfield, Hamilton and 60 m above OD in Motherwell.

At a late stage in the lifetime of Lake Clydesdale, perhaps while the ice dam lay at the suggested terminal moraine of Blantyreferme ridge [NS 673 595] a short distance north of the district boundary (Clough et al., 1911), a thick body of sand and gravel of the Ross Formation was contributed to the lake infill in the neighbourhood of Ferniegair by the northward flowing Avon Water. The height of the eroded surface of the deposit, which has the characteristics of a Gilbert delta (Plate 9), reaches a little above 80 m above OD, compatible with a lake level that was controlled by the Eastfield outlet. The relationships are not entirely clear but the sand and gravel, which occupy the high ground, appear to overlie silts and clays, inferred to the of the lacustrine Bellshill Formation, which occur on the lower slopes. Representatives of both formations were identified in the BGS Ross House Borehole (NS75NW/88) below a depth of about 17 m (Browne et al., 1989a).

At Greenoakhill [NS 675 625], north of the district boundary, a body of sand and gravel, assigned to the Broomhouse Formation by Browne and McMillan (1989a), shows evidence of deposition by east-flowing currents in association with ice presumed to be at the south-east margin of the mass impounding Lake Clydesdale.

Drainage of Lake Clydesdale

Deposits known from faunal evidence to be of marine origin are widely distributed in the area around the Firth of Clyde. The oldest of these, composed of laminated silts and clays, have been referred to the Paisley Formation by Browne and McMillan (1985). Deposits of generally similar lithology but with exceedingly sparse faunas were penetrated in the Hamilton area by the BGS Ross House Borehole and by commercial boreholes at Carbarns [NS 774 540] and assigned to the Paisley Formation (Browne and McMillan, 1989a). The top of the deposit in both cases reaches heights of about 45 m above OD. If the deposits are indeed of marine origin, this implies a minimum level of the sea during deposition that was well above the level of the Main Perth Shoreline, which is estimated by extrapolation of the isobases determined by Smith et al. (1969) to be at a height of about 35 m above OD at Hamilton. The height of the marine limit indicated by the deposits at Ross House and Carbarns is, on the other hand, comparable to that pertaining while the Loanhead Beds were being laid down in the Grangemouth area (Browne et al., 1984) , during the Dimlington Stadial, prior to the climatic amelioration of 13 500 BP.

If its level was significantly more than 45 m higher than at present, the sea could have entered the Hamilton district by way of the valley now occupied by the westward flowing River Kelvin, west of the 45 m high watershed at Kelvinhead, and by the eastward flowing Bonny Water and the River Carron, while the lower Clyde was still occupied by ice. However, no marine deposits have been reported from the Kelvin valley. It is possible that the very sparse fauna in the silts and clays near Hamilton has been derived by reworking from Shelly tills within the catchment area of the Avon Water and that the silts and clays in the upper part of the sequences at Ross House and Carbarns are actually part of the deposit infilling Lake Clydesdale and should be assigned to the Bellshill Formation. The position of the Ross House and Carbarns boreholes in relation to the known lake infill deposit would tend to support this suggestion. This would mean that the Kelvin valley became ice-free after sea level had fallen below 46 m above OD and acted as an outlet for Lake Clydesdale until final collapse of the ice dam in the lower Clyde valley ((Figure 14).5)

The events associated with the final melting of the ice impounding Lake Clydesdale are obscure. When the Firth of Clyde ice began to break up, in part because of the calving of bergs at its western margin, lake waters then for the first time able to escape westwards would have laid down the ice-contact sands and gravels of the Bridgeton Formation (Browne and McMillan, 1989a).

Glacial drainage channels

The numerous glacial drainage channels throughout the district are an integral feature of the deglaciation process (Figure 11). The most important to the interpretation of the deglaciation pattern are those cut by meltwater flowing along the interface between the surface of impermeable ice and the ground surface. These marginal channels tend to be subparallel to the contours and now occur as dry valleys perched along hillsides. Such channels are found all along the north side of the Silurian hills from Burnside [NS 666 384] to the edge of the district at Ladehead [NS 792 413], south of Kirkmuirhill. A particularly fine set of nine channels is incised into the hillside above Hawkwood [NS 685 395], the upper ones being over 10 m deep, the lower ones very shallow but still discernible (Plate 7). The numerous deeply incised tributary valleys such as the Logan Water and the River Nethan, which flow north-east into the Clyde, are believed to have been cut by meltwater flowing along the ice margin.

Other channels found downhill escape routes, as did the many of the contour channels for the latter part of their route. Typical of these are the channels flowing east off the Clyde Plateau lavas around the Calder Water to the north of Drumclog and west from Strathaven. Other channels mark spillways where glacial or lake water overflowed across a watershed. Principal of these are the saddles between the summits of the Dungavel ridge (Figure 11) and (Figure 12) and the col at Loudoun Hill.

Glacial drainage channels in upland areas are mainly cut into the bedrock. In lowland areas they may be cut entirely in drift deposits or, in the case of many of the larger channels, they may cut through thin drift into the underlying rock. Many of the present streams and rivers carried meltwater during the deglaciation period and have subsequently been partly infilled with alluvial deposits, which are commonly overlain by peat.

Glacial meltwater deposits

Material eroded by the meltwater, derived both from the till and the underlying rock was laid down as extensive deposits in contact with decaying ice and as outwash in front of its retreating margin. Characteristically, the deposits consist of sand and gravel that has been carried only short distances. The finer material has commonly been transported long distances before being laid down in a low-energy lake or river environment as laminated silts, clays and fine sands. Glaciofluvial, glaciodeltaic and glaciolacustrine deposits are undoubtedly present within the district but these deposits grade into each other and their distinction is commonly not clear and often arbitrary.

Glaciofluvial sands and gravels (Broomhouse Formation)

Sands and gravels deposited from glacial meltwater at or near the edge of the ice sheet have been assigned to the Broomhouse Formation in the adjacent Glasgow district (Browne and McMillan, 1989a). Typically they rest directly on the till of the Wilderness Till Formation as shown by the evidence of boreholes and field relationships. In many cases, the sands and gravels are associated with glacial drainage channels.

The deposits may be in the form of mounds, esker ridges or kame terraces, all indicating more or less proximity to the ice. They are discontinuous and vary greatly in thickness, up to 20 m or more. Where seen, the deposits consist of varying proportions of bouldery to fine gravel and coarse pebbly to fine sand, with even some silt and clay in places. Bedding is always present, tending to become better developed the finer the grain size. Planar bedding, cross-bedding, trough cross-bedding and ripple-lamination all occur (Plate 10a)(Plate 10b)(Plate 10c)(Plate 10d). Deformation of the finer beds and faulting, both normal and reverse, indicate proximity to the ice; the deposits were disturbed either by movement of the ice or removal of support on melting of the ice. The beds were frozen where fracturing occurred rather than plastic deformation.

Glaciolacustrine silts and clays (Bellshill Formation)

This formation is named from the Bellshill area to the north in the Airdrie district (Browne and McMillan, 1989a). Typically the lithology is a brownish grey silty clay with wisps, laminae and bands of silt and sand. Drop-stones are present and some of the silt/clay couplets may be true varves. In the Ross House Borehole [NS 7390 5504] east of Hamilton, well-laminated clays and silts of this formation, containing dropstones and a 9 cm till band suggesting the proximity of the ice, were recorded between 19.53 and 22.19 m depth and, as suggested here between surface and a depth of 17.20 m. Silts and clays of the formation are exposed in a small stream near Millheugh Bridge [NS 7525 5062] and closely laminated, possibly varved, silty clay was seen resting upon till at a small quarry, south-east of Hamilton [NS 7237 5237]. Landslip deposits containing masses of Bellshill Formation occur along the west bank of the Avon Water, downstream of Millheugh Bridge.

Glaciolacustrine sands and gravels (Ross Formation)

The type locality of this formation is in the Ross House Borehole where, between depths of 17.20 and 19.53 m, it overlies the silts and clays of the Bellshill Formation (Browne and McMillan, 1989a). The lithology is flat and ripple-bedded, medium- to fine-grained sand with laminae and thin bands of silt. When seen in sand pit sections as in the Loudoun Hill area, the deposit is up to 20 m thick and demonstrates delta foreset bedding, as well as containing beds up to gravel grade. These features are consistent in this area with a delta building north-eastwards in the ice-dammed Lake Clydesdale. A very similar deposit (Plate 9) was quarried adjacent to Chatelherault [NS 733 542] in the Ferniegair area, the large-scale cross-sets indicating deposition by northwards flowing rivers.

Loch Lomond Stadial

The Loch Lomond Readvance (Simpson, 1933) occurred between 11 000 and 10 000 BP when there was a general return to Arctic conditions in Scotland. Glacier ice which developed at several localities in the western Highlands did not reach the Hamilton district. Periglacial conditions may have been re-established within the district at this time but it is not known whether ice wedges, formerly exposed in the sand and gravel deposits of South Torfoot Sand Pit [NS 637 383] (Plate 10a), date from this period or are earlier. The thin gravel that overlies the sand and gravel and infills the ice wedges indicates a brief spell of high-energy drainage. Screes, such as the one on Loudoun Hill, may have been initiated by during periglacial conditions but continued to form throughout the Flandrian.

Flandrian

The Flandrian Stage commenced at 10 000 BP, with a rapid rise in temperature putting an end to the periglacial conditions. Initially sea level fell as a result of continuing isostatic recovery but a subsequent rise, culminating at about 6500 BP, was insufficient to permit the sea to reach the Hamilton district. Woodlands of birch and hazel dominated the land from about 9700 BP, alder increased around 7000 BP and elm decreased about 5000 BP (Price, 1983, pp.165–170). Peat growth occurred during much of the Flandrian. Most of the geological processes that shaped the landforms throughout the Flandrian probably had started in the Late Devensian: screes, incised gorges, landslips, lacustrine deposits and fluvial deposits. Most recently man has had the greatest impact on the land surface.

Peat (Clippens Peat Formation)

After glaciation, peat growth was initiated whenever the appropriate location and wet climate combined with a suitably poorly drained and acid soil. Such conditions occurred several times throughout the Flandrian. In the Glasgow area peat of Flandrian age has been assigned to the Clippens Peat Formation (Browne and McMillan, 1989a) from the type section in the Linwood Borehole. There the peat occurs in two beds; the base of the lower has been radiocarbon dated at 9540 ± 50 BP,early in the Flandrian, whereas the base of the upper peat has been dated at 7110 ± 50 BP, the start of the main episode of peat growth.

Hill peat (blanket bog) formed on all the high ground including the slopes. Generally there is only a metre or two of peat but it can be 3 m or more thick in hollows. Natural erosion along gullies and at the edge of the peat forms peat hags and, over the centuries, man has worked the peat extensively for fuel. Despite these depredations, hill peat still covers much of the Silurian hills in the south and Clyde Plateau lavas hills in the west of the district, notably at Mossmulloch [NS 635 415]. Basin peat grew widely in lowland areas on lake alluvium and to a lesser extent on river alluvium. It forms flat heather-covered moors or 'mosses' commonly with a thickness of 3 to 5 m. These deposits have mostly been removed for fuel or for horticultural use and during agricultural improvements or opencast coal exploitation. Isolated patches of basin peat remain particularly in the area between East Kilbride and Chapelton, as at Blantyre Muir [NS 665 525], Cladance Moss [NS 665 490] and Rigfoot Moss [NS 675 472] and also at Cander Moss [NS 782 460] north of Blackwood. As well as forming the surface deposit, peat can be interbedded with fluvial and lacustrine deposits.

Scree

Weathering of cliffs on rockier hills has given rise locally to screes. The most notable scree deposit is on the south side of Loudoun Hill [NS 608 378] where it consists of large boulders of trachyte (Frontispiece). Small screes have developed on some of the Silurian hills such as Dungavel Hill [NS 675 354].

Gorges

In establishing a more permanent drainage, many streams and tributaries, particularly those flowing into the River Clyde, cut incised valleys. These form spectacular rock gorges, such as those in the Avon Water and in the Rotten Calder near East Kilbride, cut mainly in lower Carboniferous sedimentary rocks with normally only a thin cover of superficial deposits (Figure 10) and (Figure 11).

Landslips

Where incised valleys cut thick drift deposits, such as where they intersect buried channels, landslips have commonly occurred on the valley sides. There are many examples of landslips at various places along the Avon Water, notably north of Millheugh Bridge where there are large slipped masses of laminated silts and clays of the Bellshill Formation. Other valleys where landslips have been recorded include the tributary of the Avon Water at Strathaven, the Calder Water southwest of Strathaven, the Calder Water/Rotten Calder at East Kilbride and the Earnock Burn south-west of Hamilton.

Lacustrine deposits (Kelvin Formation)

Lakes occupied hollows left in the glaciated landscape, between drumlins or alongside gravel ridges and mounds. These lakes gradually filled up with sediment, mainly laminated silt and clay, which is commonly overlain by peat. There are many examples of lake deposits among the drumlinoid till landscape in the northern half of the Hamilton district. Most, however have outlets leading into the fluvial drainage system and are not mapped separately from the fluvial deposits. In age the lacustrine deposits probably range from the earliest Flandrian almost to Recent and may even date back to the latest Devensian. McMillan (in Forsyth et al., 1996) assigned such deposits formed during the Flandrian to the Kelvin Formation. Any earlier lacustrine silts and clays formed during the Windermere Interstade of the Devensian would be assigned to the Bellshill Formation (Browne and McMillan, 1989a).

Fluvial deposits (Law Formation — sands and gravels, Kelvin Formation — silts and clays)

Throughout the Flandrian extensive alluvial deposits built up as the post-Devensian drainage system developed. Major deposits in the Hamilton district occur in the valley of the River Clyde and along its main tributary, the north-flowing Avon Water/Glengavel Water. Almost all the smaller rivers, tributaries and streams have more or less continuous strips of alluvium along the bottom and sides of their valleys. Worthy of mention are the Kype Water and the Calder Water, tributaries of the Avon Water, the NE-flowing River Nethan and its tributary the Logan Water in the east, and to the south the Greenock Water and Whitehaugh Water both flowing south towards the River Ayr. Terraces reflect the manner in which river levels changed as erosion lowered valleys. Terrace systems are particularly well developed in the upper reaches of the Avon Water and along the Glengavel Water.

River alluvium consists in places of intermixed clay, silt, sand and gravel and elsewhere of sorted coarser and finer fractions. Sands and gravels predominate in the higher terraces and along the steeper headwaters, whereas silts and clays are commoner in the lower reaches and along floodplains. Peat may overlie or be interbedded with alluvial deposits. Coarse-grained Flandrian sediments (sands and gravels) deposited in fluvial environments, including present-day river courses, were assigned by Browne and McMillan (1989a, 39–40) to the Law Formation. This formation is named from the Law Borehole on the alluvial flat of the Garrion Burn in the Lanark district (Sheet 23E). Fine-grained sediments were assigned to the Kelvin Formation named from bore-holes in the alluvial plain of the River Kelvin in the Glasgow district (McMillan in Forsyth et al., 1996).

Man-made deposits

Man-made deposits are most widespread in the industrialised and built-up parts of the Hamilton district. These correspond to the areas underlain by coal-bearing Carboniferous rocks in the northern half of the district and in the extreme south-east. Extensive areas of waste ground are associated with the steel industry around Motherwell, particularly at the Ravenscraig Steel Works. Embankments were constructed for railways, for roads such as the M 74 and for the numerous small dams in the district. Spoil was dumped from quarries, excavated mainly for limestone and for igneous rocks. Bings contain waste from the numerous large and small coal mines, although increasingly these bings are being smoothed off and their area landscaped. Opencast coal workings, mainly in the East Kilbride–Chapelton and Glenbuck areas, have been back-filled and mostly restored to their original agricultural use. Refuse has been dumped in old quarries and even disused reservoirs. Mining areas are characterised by worked and disturbed ground and subsidence (Plate 11). Landscaping has occurred during reclamation and development, particularly that associated with East Kilbride New Town.

Chapter 9 Economic geology

Mineral deposits

The Hamilton district has a long history of mineral extraction and, although the activity has declined from its peak during the early part of the century, many minerals, particularly coal, are still exploited on a considerable scale.

Peat

Peat, which may be used as a fuel or in horticulture, occurs in extensive but generally thin deposits of hill peat in the area to the west and south-west of Strathaven and Lesmahagow. Much of this ground is currently given over to forestry. Basin peat deposits, such as Blantyre Muir [NS 665 528], Cladance Moss [NS 662 493] and Rigfoot Moss [NS 675 471], which lie to the south-east of East Kilbride, are more restricted but tend to be thicker. Little information is available but it is improbable that thicknesses exceed 5 m. Cander Moss [NS 781 461], to the east of Stonehouse, has been designated a Site of Special Scientific Interest.

Brickmaking materials

Within the superficial deposits, clay and silt, known locally as 'brickclays' or 'brickearths', are interbedded with the sand and gravel of the valley floors or occur as dissected or featureless spreads on higher ground. Brickclay workings operated at Stonehouse [NS 755 464] but are not now active. Colliery tips ('bings') that contain a high proportion of unburnt mudstone may provide suitable material for brick-making but their composition is highly variable. Beds of mudstone within the bedrock offer more uniform material, possible horizons being the Black Metals and the mudstones that overlie the Top Hosie, Index, Orchard and Calmy limestones (Elliot, 1985). Thick mudstones also occur above the Glasgow EllCoal. Within the Upper Coal Measures mudstones associated with the Chatelherault and Hamilton sandstones have been worked at Carscallan [NS 723 523], near Hamilton. Seatclays and seatearths have also been mined and quarried locally, in conjunction with coal workings, as a source of 'fireclay'.

Sand and gravel

Resources of sand and gravel are mainly found within the fluvioglacial ice-contact and terrace deposits along the valleys of the River Clyde, and the Avon Water and its tributaries. Fluvioglacial sand and gravel deposits are also present along the south-western edge of the district and eskers and mounds of sand and gravel occur in the area west of Chapelton. Pits have been dug in many of the deposits, for example at Ferniegair [NS 740 545], south-east of Hamilton, at Glassford [NS 714 473] and at Chapelton [NS 683 476]. Current extraction is concentrated in the valley of the Avon Water, south-west of Strathaven, for example at Snabe [NS 650 390] (Plate 12) and Loudounhill [NS 605 375] (Frontispiece). Details of this resource which includes areas of alluvium, glacial sand and gravel and glacial lake deposits have been are published (Nickless et al., 1978).

Sandstone

The Chatelherault and Hamilton sandstones of the Upper Coal Measures in the Hamilton area were formerly an important source of building stone (Cover). A sandstone lying a little above the Kiltongue Musselband was quarried and mined for building stone at Overwood [NS 771 461], south-east of Stonehouse. The thick sandstone which lies between the Orchard and Calmy limestones was worked west of Quarter [NS 717 514].

The highly siliceous sandstone beneath the Calmy Limestone was formerly mined for refractory use in the Avon Water gorge [NS 764 494] at Larkhall. It is unlikely that the sandstones of the district now represent a significant resource for silica sand (MacPherson, 1986a).

Limestone

The thicker limestones in the Upper and Lower Limestone formations were quarried and mined for agricultural lime and for use in the iron and steelmaking industry. Workings in the Hurlet and Calmy limestones were particularly common. The thickest beds of cornstone in the Kinnesswood Formation were also exploited.

The Top Hosie Limestone, although relatively thin, was mined on account of its properties as a natural cement, in the East Kilbride area [NS 660 545] under its local name the Calderwood Cement. Although small areas of limestone remain in the Hamilton district these are unlikely to offer a significant resource (MacPherson, 1986b).

Bedded ironstone

Ironstone developed at about the position of the Crutherland Coals was worked around Earnockmuir [NS 688 526], south-east of East Kilbride. The Maggie Bands, the collective name for the clayband ironstones at the position of the Black Metals in the Central Coalfield, have been worked sporadically in association with coal extraction. The Crossbasket Ironstones of the Lower Limestone Formation were mined around Auchentibber [NS 668 552] and near Craigendhill [NS 692 513], east of East Kilbride. Clayband and blackband ironstones in the Lawmuir Formation were worked beside the Avon Water at Cot Castle [NS 739 458] near Stonehouse.

Two ironstones seams, known as the Mid and High Band ironstones and lying at a similar stratigraphical position to the Maggie Bands of the Central Coalfield, were of the considerable economic importance in the Glenbuck–Muirkirk area, as was a lower ironstone, known as the Low Band, which lies just above the Johnstone Shell Bed. Ironstones at other positions, such as the Cleland Roughband Ironstone, Watstone Musselband Ironstone, Upper and Lower Slatyhand Ironstones and the Airdrie Blackband Ironstone, have also been exploited to some extent.

It is unlikely that any of the ironstone deposits constitute a resource suitable for modern needs.

Haematite

Haematite in veins cutting Silurian strata was formerly mined at Auchenlongford and Garpel mines [NS 603 297] on the Pennel Burn, west of Muirkirk (Macgregor et al., 1920).

Oil shale

The 'shale' which commonly occurs at the Kiltongue Musselband position has been worked for oilbeside the Avon Water at Stonehouse [NS 760 476].

Hard rock aggregate

The Clyde Plateau Volcanic Formation is composed predominantly of basaltic lavas, with occurrences of trachytic lava in western parts of the district. Individual flows tend only to be a few metres thick and often have weaker, less competent material in their upper and lower parts. Although the lavas have been exploited on a small scale mainly for fill and road bottoming, they are unlikely to constitute a significant aggregate resource. Intrusions of igneous rock may provide better materials for aggregates, the rocks being generally more uniform and fairly fresh. Several trachytic pipes cut the lavas, notably at High Huntlawrig [NS 610 483], North Brownhill [NS 642 425] and Loudoun Hill[NS607 380], the last forming a distictive local landmark. Several doleritic sills which lie to the east of Chapelton were quarried locally, but are probably too thin to offer a modern resource. Doleritic and felsitic dykes have been exploited in the past, but the restricted size of these intrusions means they are unlikely to afford sufficient reserves for a modern quarry.

Large intrusions of felsite and porphyritic microgranodiorite occur in the southern half of the district. These felsitic aggregates were once a popular surfacing material for Lanarkshire roads, but are now generally considered too prone to polishing to be used on other than minor roads. Felsitic aggregate is used mainly for coated macadams in basecourses and as granular subbase in road construction. Their red to red brown colour offers some potential as a coloured aggregate. They may also be usable as low-shrinkage' concreting aggregate and as rail ballast (Merritt and Elliot, 1984). Dunduff Quarry [NS 778 408], near Kirkmuirhill (Plate 6), operates partly in a porphyritic microganodiorite silland partly in the associated indurated sandstones.

Part of the Distinkhorn Complex (Richey et al., 1930) lies on the the south-west edge of the district. It is the only noteworthy granitic intrusion within Central Scotland but, due to its poor exposure and remoteness, has received little attention as a source of aggregate.

Baryte

Baryte veins have long been known to occur in the Silurian sediments and Telsitic' sills of the Lesmahagow area (Wilson and Flett, 1921; MacGregor, 1944). Revision mapping of the inlier has revealed 29 new baryte veins having a thickness greater than 0.1 m. The veins lie in a 1 km-wide zone extending for 3 km from Meikle Auchinstilloch to Nutberry Hill, approximately at right angles to the strike of the sedimentary host rocks and to the trend of the faults associated with the Kerse Loch Fault system. The veins in the sedimentary rocks usually occupy small faults, whereas those in the igneous intrusions are fissure veins, swelling and pinching along strike. The association of galena or haematite with the baryte appears to be related to wall-rock composition. Traces of sphalerite and chalcopyrite have been observed in the spoil heaps of former lead mines in the baryte veins. Gallagher (1984) gives details of the mineralisation and estimates reserves of some tens of thousands of tonnes.

Coal

Coal occurs at two main levels in the succession, the Limestone Coal Formation and in the Lower and Middle Coal Measures. The thickest seams, in some cases exceeding 2 m, are in the upper of the two developments. However, the workable reserves have been greatly depleted as a result of intensive mining during the last 150 years. Much of the shallowest, and often earliest, extraction was by 'stoop-and-room' methods whereby only about half of the coal was taken, Despite the practices of 'stoop-splitting' and 'stoop-robbing', in places there remains sufficient coal to justify opencast working.

Many of the thinner coals in the lower half of the Lower Coal Measures have been exploited by underground working to a limited extent only. In areas where several seams lie close together within the sequence, as in the case of the Drumgray Coals, there may be potential for opencast extraction.

The Limestone Coal Formation contains few thick coals but the relatively close spacing of seams, such as the Crutherland Coals in East Kilbride area for instance, means that opencast extraction may be economic. The seams in the Glenbuck and Dalquhandy areas are thicker and some are being mined opencast near Muirkirk.

Groundwater

Most of the district lies within the catchment of the River Clyde but the extreme south-west corner drains to the River Ayr. Mean annual rainfall varies from less than 900 mm in the Clyde valley at Motherwell to over 1400 mm on the higher ground adjacent to Glengavel. Potential evapotranspiration is about 400 mm/annum giving an effective rainfall ranging from 500 mm on low ground to 1000 mm on the hills. Approximately two-thirds of the effective rainfall contributes to run-off but the remainder is available for potential recharge to groundwater.

The regional shallow groundwater flow pattern for the Midland Valley was described by Robins (1990). Much of the groundwater flow is towards the Clyde — from beneath high ground at the periphery of the catchment towards the low ground occupied by the river where it discharges as baseflow. However, none of the bedrock of the district offers any significant intergranular permeability and most groundwater flow is limited to fissures at shallow depth. Groundwater flow is, therefore, essentially only local and flow paths are generally only a few kilometres in length. Deeper groundwater circulation is present only in the Carboniferous rocks.

Groundwater has never been widely exploited in the district because sustainable yields from boreholes and wells in bedrock are invariably less than 1 1/s.

Modest supplies for domestic use have been obtained from the Lower Devonian rocks and from the Lower Carboniferous sequence, including the lavas. Typical supplies include a 20 m deep borehole in the Lower Limestone Formation near Chapclton [NS 672 504] which sustained 0.1 1/s, a 30 m deep borehole in till overlying lava at Loudoun [NS 6083 3864] yielding 0.5 1/s and a well in Coal Measures at Stonehouse [NS 753 483] which also provided 0.5 1/s. There were numerous wells around Strathaven and East Kilbride, including the Trumpeter's Well near Calderbank, so called because Covenanters put a trumpeter from the Dragoons into the well. The water from the bedrock is of variable quality, being weakly mineralised in some places, hard and iron rich in others.

Boreholes and shallow wells have only a small area open to an aquifer whereas a coal mine with all its adits and driveways has a much greater access to the water-bearing fissures. In addition, the action of mining tends to enhance the local hydraulic conductivity. Dewatering rates and pumping depths are shown for a number of collieries in (Table 5). The West Machan Colliery was remarkably dry, whereas Bankend No. 13 Shaft was one of the wettest in Scotland. For the most part, water quality is reported to have been adequate for raising steam, although Canderrig discharged water with 32 mg/1 of iron in solution, and Overtown discharged water with an alkalinity of 610 mg/l.

Deposits of sand and gravel are present along most valley bottoms, although in places they are partly concealed by alluvium. Wherever the deposits are saturated they can be exploited with a shallow well, for example at Netherburn [NS 7950 4450] where a windpump formerly supplied the farm. Springs at Loudounhill [NS 6083 3864] flow at 20 1/s to supply the village of Loudoun but a nearby borehole [NS 598 374] yielded only 6.5 1/s with a drawdown of 17.5 m. Water is also present within the peat deposits of the district. However, the hydraulic conductivity of the peat is characteristically very low.

Disposal of trade effluent to disused mine workings has been carried out at Cleland [NS 797 598] and at Ravenscraig [NS 770 565]. At Cleland, up to 770 m³/d (9 1/s) of alkaline fluid with a high biological oxygen demand was discharged into workings of the Glasgow Main Coal, the workings in the Glasgow Ell Coal, at a shallower depth, being dry. At Ravenscraig, oil-rich coke oven effluent and pickling wastes were pumped down boreholes but emergence into the nearby South Calder Water caused the disposal licence to be revoked in 1983.

Landfill disposal of domestic and industrial waste has occurred in a number of quarry and other derelict sites. Leachate disposal has progressed in recent years to containment and treatment.

Geothermal energy

The geothermal potential of the Midland Valley is summarised in Browne et al. (1985; 1987), Rollin (1987) and Evans et al. (1988). The average geothermal gradient is 22.5°C km^-1 and the heat-flow 54.5 mW m^-2 (Browne et al., 1987).

Only two formations above the basement within the district have geothermal potential — sandstones of the Kinnesswood Formation at the base of the Carboniferous and sandstones of the Namurian Passage Formation. In the Douglas Basin the base of the Lower Carboniferous may reach a depth of at least 1 km with an estimated temperature of 30°C, but is not considered a geothermal resource (Browne et al., 1985). Geophysical and geothermal modelling suggest that the area of the Hamilton gravity low may represent a geothermal prospect, with the base of the Lower Carboniferous reaching at least 2 km depth (60°C) (Browne et al., 1987). However, the geological evidence suggests that the Lower Carboniferous is unlikely to reach this depth and overlies Lower Devonian rocks with little geothermal potential.

Basement rocks may themselves have geothermal potential as 'hot dry rock' sources in the area of the Hamilton gravity low, where calculations using the LISPB seismic crustal model suggest their top could reach 7 km depth (175°C) (Anonymous, 1987) but this prediction depends upon the structural model used. The MAVIS and quarry blast seismic model predicts that the basement here is at about half the depth suggested by the older and less precise LISPB model and the temperature would be proportionately less.

Chapter 10 Geophysics

Some information on the significance of structures at surface and at depth in the Hamilton district is provided by the results of geophysical surveys. The geophysical data available comprise those obtained from regional gravity and aeromagnetic surveys and from various seismic surveys.

The Bouguer gravity anomalies for the district (Figure 15) are referred to the 1973 National Gravity Reference Net, calculated using the 1867 International Gravity Formula and reduced to sea level using a density of 2.75 Mg M^-3. The total field aeromagnetic map (Figure 16) is based on data collected along east-west flight lines with a mean terrain clearance of 305 m. A linear regional field for the UK has been removed from the observed magnetic data. The data shown on (Figure 15) have been analytically continued upwards to a surface at a height of 1.3 km above ground level. This transformation reduces the effect of high-frequency anomalies due to near-surface sources and enhances the effects of larger, or deeper, structures.

The earliest refraction surveys carried out in the Midland Valley, LOWNET and LISPB, were designed to examine crustal-scale structure and had low resolution in the upper few kilometres (Crampin et al., 1970; Bamford et al., 1978). Subsequent refraction surveys across the Midland Valley using quarry blasts as sources provided information on crustal structure to depths of about 6 km (Davidson et al., 1984), while the longer, controlled source MAVIS refraction experiment yielded structural information to a depth of about 10 km (Conway et al., 1987; Dentith and Hall, 1989). The locations of the refraction lines are shown on (Figure 14) and (Figure 15). One of the lines ((Figure 14), line QB-3) traversed the line of the Distinkhorn Plutonic Complex which was interpreted as a steep-sided body intruded only in the Lower Palaeozoic rocks of the Lesmahagow Inlier (Davidson et al., 1984). The Lower Palaeozoic sequence was considered to have a maximum thickness of about 3 km. The refraction line MAVIS-S passes a little north of the district boundary ((Figure 14), line M-S). Where the line crossed the eastern margin of the outcrop of the Clyde Plateau Volcanic Formation, a 6.0 km s^-1 refractor was identified at a depth of about 4 km.

Physical properties

The Ordovician and Silurian greywackes of the Southern Uplands have a mean saturated density close to 2.72 Mg m^-3 (Bott and Masson Smith, 1960) but this figure can be expected to increase slightly with depth. Ordovician rocks, similar to those of the Highland Boundary Complex and having a density of 2.68 Mg m^-3, may underlie parts of the Midland Valley (Bluck et al., 1984; Dentith and Hall, 1990). The density of the greywackes forming the lower part of the Silurian sequence in the Lesmahagow and Hagshaw Hills inliers is comparable with that of similar rocks in the Southern Uplands. Lithologies in the upper part of the Silurian sequences in the inliers, however, have greater affinities with those of the Lower Devonian sedimentary rocks of the Midland Valley, which have a mean saturated density of 2.62 Mg in^-3 (McLean, 1961).

There is no evidence that Upper Devonian rocks are present in the district but, in the Midland Valley as a whole, rocks of this age have been given a mean saturated density of 2.41 Mg m^-3 (McLean, 1961). The carbonate-bearing sandstones of the Kinnesswood Formation, which are mainly, if not entirely, of lower Carboniferous age, are assigned the significantly higher value of 2.58 Mg m^-3. Olivine-basalts of the Clyde Plateau Volcanic Formation have a density of 2.87 Mg m^-3 but this is reduced by weathering and McLean (1961) suggests a mean value of 2.72 Mg m^-3. The Dinantian sedimentary sequence has a mean density of about 2.58 Mg m^-3; the Namurian and Westphalian sedimentary rocks have a density of about 2.55 Mg m^-3.

Olivine-basalts have a high mean magnetic susceptibility (0.06 SI units) and at outcrop individual flows may have susceptibility values in excess of 0.1 SI units. Many of the Carboniferous extrusive rocks in the Midland Valley have a low intensity of Natural Remanent Magnetism (NRM) and show no marked preferred orientation of the remanence (Palmer et al., 1985). The aeromagnetic anomalies associated with rocks of the Clyde Plateau Volcanic Formation can be interpreted in terms of induced magnetism within the Earth's present field. The NRM of late-Carboniferous intrusive rocks characteristically has a shallow inclination and a mean declination of about 180°. Minor intrusions of gabbroic or dioritic composition into the Clyde Plateau Volcanic Formation have susceptibilities at outcrop up to 0.04 SI units, similar to that of the Distinkhorn Plutonic Complex.

Further details of the physical properties of the main rock types are given in Forsyth et al. (1996).

Remanent magnetism has been used to date the baryte haematite mineralisation in the Pockmuir Burn [NS 757 329]. The NRM direction obtained from haematite is consistent with mineralisation during the Lower to Middle Jurassic (Evans and El-Nikhely, 1982).

The quarry blast refraction studies and the MAVIS experiment provided average compressional (P-) wave velocity estimates for components of the upper crust in the Midland Valley. The suggested values are 2.5 to 3.5 km s^-1 for the Carboniferous and Upper Devonian sequence and 4.0 to 5.5 km s^-1 for the Lower Devonian and Lower Palaeozoic sequences (Davidson et al., 1984; Conway et al., 1987). Velocities obtained from commercial seismic reflection investigations in the region tend to be slightly higher than those inferred from refraction studies. Also, sequences of extrusive rocks can be expected to have a significantly increased P-wave velocity. The northern part of the north–south MAVIS line indicated first arrival P-wave velocities for the Lower Devonian volcanic rocks of about 5.0 km s^-1 (Dentith and Hall, 1989). Penn et al. (1984) suggest a mean P-wave velocity of 3.77 km s^-I for the Upper Devonian, of 3.42 km s^-1 for Carboniferous sedimentary sequences and of 4.17 km s^-1 for the Clyde Plateau Volcanic Formation.

Geophysical interpretations

Gravity and aeromagnetic

The main features of the Bouguer gravity and aeromagnetic anomaly maps are:

• An extensive gravity low centred near Hamilton and bounded on the west by the zone of steep gradient along the Dechmont Fault.
• A large gravity high over the North Ayrshire Block and the associated magnetic magnetic anomalies over the Clyde Plateau Volcanic Formation.
• To the north-east of the district, an extensive gravity high and associated magnetic high centred on the Bathgate area.
• Gravity and magnetic anomalies associated with the Distinkhorn Plutonic Complex.
• Linear gravity anomalies in the south of the district.

The interpretations of several of these anomalies have been discussed in regional assessments of the Midland Valley (for example, Davidson et al., 1984; Evans et al., 1988).

Hamilton Gravity Low

Alomari (1980, as discussed in Davidson et al., 1984) suggested that the gravity low of the Hamilton–Wishaw area was related either to a granite which extended from depths of 4 to 12 km or to an 8 km thick Lower Devonian basin with its base at a depth of 12 km. Refraction surveys suggest that the basement is too shallow here to accommodate such a deep basin. On the other hand, the surveys failed to reveal the inferred granite. Alternative interpretations attributing the anomaly to a thick sequence of low density post-Lower Devonian rocks appear to be supported by the close correspondence of the trend of the Bouguer anomaly contours with the boundaries of the Carboniferous outcrops and with mapped faults. Browne et al. (1987) and Evans et al. (1988, fig. 4.10) interpreted the low as being due to a 2.1 to 2.5 km thick sequence of Upper Devonian and Carboniferous rocks, possibly containing 500 m of Lower Carboniferous lavas, and bounded to the west by the Dechmont Fault. The steepness of the gravity gradient on the south side of the low suggests that this side of the basin is also fault bounded, possibly by a continuation of the Inchgotrick Fault.

It should be noted, however, that the geological evidence available in the Hamilton district suggests that the aggregate thickness of post-Lower Devonian rocks is not likely to exceed 1.5 km and is probably significantly less. It is possible therefore that the observed gravity anomaly is due partly to a low-density body beneath the Carboniferous basin.

Other, more local gravity lows are associated with the Carboniferous basins of Douglas and Muirkirk.

North Ayrshire block anomalies

The large gravity high over the North Ayrshire Block is associated with outcrops of the Clyde Plateau Volcanic Formation and also with high amplitude magnetic anomalies (Figure 15). The high-frequency, large-amplitude magnetic anomalies in these areas correspond with the outcrop of the Clyde Plateau Volcanic Formation and are bounded on the east by the Dechmont, Stonehouse, Strathaven and related faults, and by the Inchgotrick Fault in the south. However, interpretation of the magnetic anomalies suggests that they cannot be caused by the inferred thickness of lavas alone but are probably also partly due to underlying penecontemporaneous basic, probably gabbroic, intrusions about 1.5–2.0 km thick emplaced within high density Lower Palaeozoic rocks (Evans et al., 1988) and/or to relatively dense, strongly magnetic rocks within the basement.

Other more local magnetic anomalies are associated with basalts of the Clyde Plateau Volcanic Formation basalts south-east of Strathaven and strongly point to their probable continuation at depth north-eastwards into the Stonehouse area. A magnetic anomaly with a centre coinciding with that of the Douglas Coalfield, a little to the east of the district, suggests a possible occurrence of lavas at depth.

The North Ayrshire gravity high is bounded on the east by a zone of steep gravity gradient which, in the neighbourhood of Hamilton, is subparallel to the line of the Dechmont Fault but from Strathaven southwards has a SSW trend approximately parallel to the Strathaven Fault and related fractures. Farther west, the southern boundary of the anomalies associated with the North Ayrshire Block coincides with the Inchgotrick Fault. It would appear that the movements on the Dechmont, Strathaven and Inchgotrick faults during the late Carboniferous result from the re-activation of more ancient lineaments which bounded the North Ayrshire Block.

Although the pronounced gravity and magnetic highs of the Bathgate area are centred outside the district, their source may extend at depth within it. Interpretations indicate only that the cause of these anomalies lies between depths of 2 and 17 km. Evans et al. (1988) suggested that the source of the anomalies was a composite volcanic centre of Devonian–Carboniferous age with superimposed Dinantian extrusive activity.

Distinkhorn anomalies

In the south-west corner of the district, a significant gravity high and an associated aeromagnetic anomaly are centred close to the outcrop of the Caledonian intrusive body known as the Distinkhorn Plutonic Complex. At outcrop, the pluton consists mainly of granodiorite but the southeast part within the district consists only of diorite. The outcrop and aureole of the intrusion lie within sedimentary rocks of Lower Devonian age and, to the south-east, Silurian age. The granodiorite has a density similar to that of the host rocks and consequently has little effect on the gravity field. The existence of the gravity high probably indicates that the concealed part of the intrusion consists mainly of diorite but part of the anomaly could be due to the presence of relatively high density Lower Palaeozoic sedimentary rocks. The ridge in the Bouguer gravity anomaly field near Linburn [NS 698 298] could be related to the Lower Palaeozoic greywacke and siltstone sequence which occupies the core of the anticlinal structure of the Lesmahagow Inlier. Rock samples from the Distinkhorn Plutonic Complex are known to have high magnetic susceptibilities (0.02 SI) and the intrusion is therefore believed also to be responsible for the pronounced aeromagnetic anomalies in the area.

Southern Upland Fault

Significant short wavelength gravity, and especially magnetic, anomalies occur on either side of the Southern Upland Fault. They have mainly been attributed to serpentinites, and to basic lavas and intrusions of Caledonian age, though some of the magnetic anomalies may arise from Lower Palaeozoic sedimentary rocks which contain magnetic minerals eroded from the adjacent Caledonian igneous rocks (Powell, 1970; 1978).

Tertiary dykes

Members of the NW-trending tholeiitic Tertiary dyke-swarm, which extends from Mull to the north-east coast of England, give rise to high frequency aeromagnetic anomalies clearly visible on maps covering larger areas (BGS aeromagnetic maps; Kirton and Donato, 1985).

Within the district, four of these dykes can be traced across the Lesmahagow Inlier but processing of the aeromagnetic data has failed to show any additional bodies. Accurate mapping of the dykes in Northumberland has been carried out by means of ground magnetic surveys (Titman et al., 1989) but there is no information regarding any such surveys in the Hamilton district.

Seismic

The acoustic impedance of the Clyde Plateau Volcanic Formation is generally much higher than that of sedimentary rocks with the result that its boundaries have high reflection coefficients, preventing much energy from reaching greater depths. The usual velocity inversion at its base means that this surface is difficult to detect by conventional refraction exploration.

Deep reflection surveys in the Glasgow area (Penn et al., 1984) show coherent reflectors only to 1 to 2 s two-way travel time, equivalent to a depth of 1.5 to 3.0 km. These are seldom below the base of the Clyde Plateau Volcanic Formation, interpreted as being 500 m thick. The deepest reflector, at about 2 km, was interpreted as the top of the Lower Devonian.

The LISPB experiment has interpreted the upper crustal structure of the Midland Valley in terms of an upper 5.8 km s^-1 refractor at a depth of about 3 km identified as the top of the Lower Palaeozoic (Bamford et al., 1978; Barton, 1992) and a lower 6.4 km s^-1 refractor at about 7 to 8 km, regarded as the top of the crystalline basement. However, Hall et al. (1983) have suggested that the top of the basement was much shallower in the southern part of the Midland Valley, being represented by a 6.0 km s^-1 refractor at a depth of about 3 km.

The MAVIS and quarry blast surveys have been interpreted in terms of a four-layer upper crust:

• 0.5–3.0 km thick Carboniferous and Upper Devonian strata
• 1.0–5.0 km thick Lower Devonian and Lower Palaeozoic strata
• gneissose crystalline basement at depths of 3.0–6.0 km
• high-grade metamorphic basement of pyroxene granulite facies at depths greater than 8.0 km.

Crucial to this view of the upper crustal structure in the Midland Valley is the identification of the 6.0 km s^-1 refractor seen on the QB and MAVIS lines with crystalline basement. The identification is based primarily on the hiatus in the observed surface p-wave velocity spectrum. There are no crystalline basement rocks exposed at surface. On the other hand, there are no sedimentary rock sequences exposed at surface with measured p-wave velocities in the range 5.7 to 5.9 km s^-1. The only exposed formation with p-wave velocities close to 6.0 km s^-1 is the Devonian volcanic sequence.

A notable feature of the QB and MAVIS refraction lines is the uniform nature of the 6.0 km s^-1 refractor. Across much of the southern part of the Midland Valley, the surface of this layer lies at a depth close to 4 km with a gentle dip towards the south-west. The uniformity of the refractor, together with fault and fold pattern analysis, have led Dentith and Hall (1989; 1990) to suggest that decollement occurred at the top of the crystalline basement and possibly also at the base of the Upper Devonian/Carboniferous sequence.

References

Most of the references listed below are held in the Library of the British Geological Survey at Murchison House, Edinburgh and Keyworth, Nottingham. Copies of the references can be purchased subject to current copyright legislation.

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Appendix 1 Data sources

a. 1:10 560 and 1:10 000 maps (solid)

The maps at 1:10560 or 1:10 000 scale covering wholly or in part the solid rocks in 1:50 000 Sheet 23W are listed below with the surveyors' initials and the date of survey. The surveyors were M Armstrong; A Davies; I B Cameron; J D Floyd; M J Gallagher; I H Forsyth; D N Halley; G I Lumsden; K A T MacPherson; A D McAdam; A A McMillan; S K Monro and I B Paterson.

The maps are not published but are available for consultation in the library, British Geological Survey, Murchison IIouse, West Mains Road, Edinburgh, EH9 31.A. Dyeline copies can be purchased from the Sales Desk.

b. 1:10 560 and 1:10 000 maps (drift)

The maps at 1:10560 or 1:10 000 scale covering wholly or in part the drift deposits in 1:50 000 Sheet 23W are listed below with the surveyors' initials and the date of survey. The surveyors were M A E Browne; I B Cameron; P M Craig; J M Dean; J D Floyd;

M J Gallagher; I H Forsyth; D N Halley; K A T MacPherson; A D McAdam; A A McMillan; S K Monro; I B Paterson and P Stone.

The maps are not published but are available for consultation in the Library, British Geological Survey, Murchison House, West Mains Road, Edinburgh, EH9 3IA. Dyeline copies can be purchased from the Sales Desk.

c. Geological survey photographs

Several hundred black and white and colour photographs illustrating aspects of geology of the Hamilton district are deposited for reference in the library at the British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA. Many are also deposited in the library at the BGS, Kingsley Dunham Centre, Keyworth, Nottingham NG12 5GG; and in the BUS Information Office at the Natural History Museum Earth Galleries, Exhibition Road, London SW7 2DE. They belong to the B, C, 1) and MNS Series and were taken between about 1900 and the present during various surveys. The photographs depict details of the rocks and general views of the district. A list of titles can be supplied. The photographs can be supplied, on request, as black and white or colour prints or transparancies, at a fixed tariff.

d. Petrography

Thin sections referred to in the text (e.g. S 18760), and many others collected during various surveys but which are not specifically mentioned, are archived at the British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA. They, and accompanying rock samples, are available on short-term loan for further research.

Appendix 2 Lithology and faunal content of the BGS Fore Hareshaw Borehole

NS63NW/83 Fore Hareshaw Borehole [NS 6137 3988]

Palaeontology

• Strata between 17.24 and 19.85 m:  Setpuloides?, Buxtonia sp., Lingula sp., cf. Pleuropugnoides, Produclus concinnus?, rhynchonellids, Sanguinolites?, ostracods?, fish material.
• Brigantian: possibly the horizon of the Craigenhill Limestone of the Carluke area.
• Strata at 19.91 m:
• ?Curvirimula scotica.
• Strata between 21.95 and 22.74 m:
• Plant material, Lingula squarrtiformis, productoids, Retispira sp., Edmandia sulcata, Myalina?, Sanguinolites abdenensis?, crinoid columnals, fish material.
• Strata between 22.94 and 26.50 m:
• Solitary corals, bryozoa, Angiospirifer sp., Buxtonia sp., Eomarginifera?, Composita ambigua, Gigantoproductus sp., Latiproductus latissimus?, Lingula. sp., Orbiculoidea?, orthotetoids, Phricodothyris sp., Plearopugnoldes sp., Productus sp., rhynchonellids, Rugosochonetes sp., Schizophoria?, Spiriferbisulcatus group, spiriferids, Tornquistia sp., Pseudomonotis?, Pterinopectinella sp., Strebtochondria?, fish material.
• Brigantian: the fauna istypical of the Hurlet (or Main') Limestone.
• Strata between 26.62 and 28.08 m:
• Algal halo?, plant material, trepostomatous bryozoan, Antiquatonia sp., Avonia youngiana, Composita sp., Crurithyris urii, Lingula sp., Pleurapugnoides sp., Productus concinnus?, Productus sp., rhynchonellids, Rugosochonetes sp., Schizophoria sp., Tornquistia cf. polita, T. scotica, Actinopleria persulcata, Aviculopecten?, Pernopecten sowerbii, Pterinapectinella sp., ostracods, crinoid columnals, Archaeoridmis sp., fish material.
• This fauna includes elements of the Macnair fauna (Wilson, 1989, p.104).
• Strata between 29.65 and 30.42 m:
• Plant material, Lingula sp., gastropod.
• Strata at 33.55 m:
• Plant material, productoid spines, rhynchonellids.
• Strata between 33.63 and 34.38 m:
• Plant material, trepostomatous bryozoan, Lingula?, Pleuropugnoides sp., Productus concinnus?, Productus sp., rhynchonellids, gastropod, Myalina?, Polidevtia attenuata, fish material.
• Brigantian: this horizon correlates with the Blackbyre (or 'Under') Limestone.
• Strata between 34.48 and 34.83m:
• Plant material, annelid, bryozoan, Buxtonia, sp., Pleuropugnoides?, Productus cf. concinnus, Productus sp., rhynchonellids, Schizophoria sp., Spirifer bisulcatus group, spiriferid, Modiolus?, ostracods, fish material.
• The cores were examined by I B Paterson and K A T MacPherson, the fossils were identified by M T Dean.

Fossil index

To satisfy the rules and recommendations of the codes of botanical and zoological nomenclature, authors of cited species are included in the index.

• Acanthoplecta mesoloba (Philips, 1836) [brachiopod]
• A. acernaspis sp. [trilobite]
• Actinoconchus? [brachiopod]
• Actinopteria persulcala (McCoy, 1851) [bivalve]
• Alitaria cf. panderi (Muir-Wood & Cooper, 1960) [brachiopod]
• Ambitisporites sp. [acritarch]
• Angiospirifer sp. [brachiopod]
• A. trigonalis (Martin, 1809)
• Anthraconaia sp. [nonmarine bivalve]
• A. cymbula (Wright, 1929)
• A. oblonga (Wright, 1929)
• A. williamsoni (Brown, 1849)
• Anthraconauta aff. Phillipsii (Williamson 1836) [nonmarine bivalve]
• A. aff. tenuis (Davies & Trueman, 1927)
• Anthraconeilo sp. [bivalve]
• A. cf. laevirostrum (Portlock, 1843)
• Anthracosia sp. [nonmarine bivalve]
• A. acutella? (Wright, 1929)
• A. aquilina (J de C Sowerby, 1840)
• A. atra (Trucman, 1929)
• A. beaniana King, 1856
• A. fulva (Davies & Trueman, 1927)
• A. ovum Truman & Weir, 1951
• A. phrygiana (Wright, 1929)
• Antiquatonia sp. [brachiopod]
• A. hindi (Muir-Wood, 1928)
• Ateleaspis tesselata Traquair, 1899 [fish]
• Atrypa reticularis (I.inne, 1767) [brachiopod]
• Aulophyllum sp. [coral]
• A. fungites (Fleming, 1828)
• Aviculopecten sp. [bivalve]
• Avonia youngiana (Davidson, 1860) [brachiopod]
• Beecheria sp. [brachiopod]
• B. .cf. hastata ( J de C Sowerby, 1824)
• Beyrichia sp. [ostracod]
• Beyrichoceratoides sp. [goniatite]
• B. truncatus (Phillips, 1836)
• Birkenia elegans Traquair, 1899 [fish]
• Brachyopterella ritchiei Waterston, 1979 [eurypterid]
• Brachythyris sp. [brachiopod]
• Brochocarina sp. [brachiopod]
• Buxtonia sp. [brachiopod]
• Carbonicola sp. [nonmarine bivalve]
• C. browni Trueman & Weir, 1946
• C. cf. communis Davies & Trueman, 1929
• C. oslancis Wright, 1929
• C. aff. polmontensis (Brown, 1849)
• C. pseudorobusta. (Trueman, 1929)
• C. aff. pyramidata (Brown, 1843)
• C. aff. rhindi (Brown, 1843)
• C. cf. rhomboidalis Hind, 1894
• C. vcf. robusta (J de C Sowerby, 1840)
• Carcinosoma scorpioides Woodward, 1872 [eurypterid]
• Catastroboceras sp. [nautiloid]
• Ceratiocaris sp. [phyllocarid]
• C. papilio Salter in Murchison, 1885
• Clinopistha parvula de Koninck, 1885 [bivalve]
• Colpomya? [bivalve]
• Composita sp. [brachiopod]
• C. ambigua Sowerby, 1822)
• Cornulites sp. [cricoconarid]
• Cravenoceras scotium Currie, 1954[goniatite]
• Crurithyris sp. [brachiopod]
• C. urii (Flemming, 1828)
• Ctenodonta sp. [bivalve]
• Curvirimula sp. [nonmarine bivalve]
• C. cf. candela (Dewar, 1939)
• C. cf. scotica (Etheridge, jun., 1877)
• Cyamocephalus loganensis Currie, 1927 [limulid]
• Cypricardella rectangularis (McCoy, 1844) [bivalve]
• Deceptrix sp. [bivalve]
• Dentalium sp. [scaphopod]
• Dibunophyttum sp. [coral]
• D.muirheadi Nicholson & Thomson, 1876
• Dictyocaris slimoni Salter, 1860 [arthropod]
• Donetzoceras sp. [goniatite] 44
• Dunbarella macgregori (Currie, 1937) [bivalve]
• Echinoconchus sp. [brachiopod]
• E. elegans (McCoy, 1855)
• Edmondia cf. senilis (Phillips, 1836) [bivalve]
• Encrinurus hagshawensis Lamont, 1965 [trilobite]
• Eomarginfera sp. [brachiopod]
• E. lobata (J Sowerby, 1821)
• E. cf. longispina (J Sowerby, 1814)
• Eoschizodus? [bivalve]
• Epirlonuctoceras sp. [nautiloid]
• Erettopterus bilobus (Salter, 1855) [eurypterid]
• Euchondria sp. [bivalve]
• Euestheria [conchostracan]
• Euomphelites sp. [gastropod]
• Euphemites sp. [gastropod]
• E. ardenensis (Weir, 1931)
• E. urii (Fleming, 1828)
• Fenestella sp. [bryozoan]
• Fuchsella amygdalina (J de C Sowerby in Murchison, 1839) [bivalve]
• Geisina arcuata (Bean, 1836) [ostracod]
• Gigantoproductus [brachiopod]
• G. giganteus Sowerby, 1822)
• G. cf. irregularis (Janischewsky, 1954)
• Girtyoceras sp. [goniatite]
• Glabrocingulum sp. [gastropod]
• G. atomarium (Phillips, 1836)
• G. beggi? (E G Thomas, 1940)
• Glassia sp. [brachiopod]
• Glauconome sp. [plant?]
• Goniophora? [bivalve]
• Hemiarges rolfei Lamont, 1965 [trilobite]
• Holopella? [gastropod]
• Homoceratoides jacksoni Bisat, 1930 [goniatite]
• Howellella sp. [brachiopod]
• H. anglica (Lamont & Gilbert, 1945)
• H. elegans (Muir-Wood, 1925)
• Hughmilleria sp. [eurypterid]
• H. lanceolate (Salter, 1856)
• Hyolithus sp. [scaphopod]
• Jamoytius kerwoodi White, 1946 [fish]
• Kansuella sp. [brachiopod]
• Kochiproductus sp. [brachiopod]
• Krotovia sp. [brachiopod]
• K. aculeata Sowerby, 1814)
• K. spinulosa (J Sowerby, 1814)]
• Lachrymula pringlei (Currie, 1937) [brachiopod]
• Lanarkia horrida Traquair, 1899 [fish]
• L. spinulosa Traquair, 1899
• Lanarkopterus dolichoschelus Størmer ex Laurie, M S 1936 [eurypterid]
• Lasanius altus Richie, 1967 [fish]
• L. armatus Traquair, 1899
• L. problematicus Traquair, 1899
• Latiproductus sp. [brachiopod]
• L. latissimus (J Sowerby, 1822)
• Leiopteria sp. [bivalve]
• L. cf. hendersoni (Etheridge, jun., 1878)
• Leptaena cf. rhomboidalis (Wilckens, 1769) [brachiopod]
• Leptagonia smithi Brand, 1972 [brachiopod]
• Limipecten sp. [bivalve]
• L. dissimilis (Fleming, 1828)
• Lingula sp. [brachiopod]
• L. minima J de C Sowerby, 1839
• L. mytilloides J Sowerby, 1812
• L. squamiformis Phillips, 1836
• Linoproductus sp. [brachiopod]
• Liralingua wilsoni Graham, 1970 [brachiopod]
• Logania scotica (Traquair, 1898) [fish]
• L. taiti (Stetson, 1931)
• Lonsdaleia cf. caledonia Smith, 1916 [coral]
• Lotiproductus sp.
• Loxonema sp. [gastropod]
• Martinia sp. nov. [brachiopod]
• Megachonetes sp. [brachiopod]
• Metacoceras sp. [nautiloid]
• Mixopterus sp. [eurypterid]
• Modiolopsis sp. [bivalve]
• Modiolus sp. [bivalve]
• Modiomorpha sp. [bivalve]
• Molinicola sp. I bivalve]
• M. cf. gotlandica Liljedahl, 1984
• Monoclimacis crenulata (Tornquist, 1881) [graptolite]
• M. griestoniensis (Nicol, 1850)
• Monograptus marri Perner, 1897 [graptolite]
• M. priodon (Bronn, 1835)
• M. spiralis (Geinitz, 1842)
• 'Moyeria' cabottii (Cramer, 1970) [nonmarine acritarch]
• Myalina sp. [bivalve]
• M. cf. flemingi (McCoy, 1844)
• M. peralata? de Koninck, 1885
• Mytilarca cf. acutirostra (Hall, 1847) [bivalve]
• Naiadites sp. [NS nonmarinc bivalve]
• N. alatus Weir, 1956
• N. quadratus (J de C Sowerby, 1840)
• Neolimulus falcatus Woodward 1868 [limulid]
• Nodospora? [acritarch]
• Nuculites sp. [bivalve]
• Nuculopsis gibbosa (Fleming, 1828) [bivalve]
• Orbiculoidea craigii (Davidson, 1877) [brachiopod]
• Orthoceras cf. araneosum Barrande, 1868 [cephalopod]
• Pachytheca sp. [plant]
• Palaeolima cf. simplex (Phillips, 1836) [bivalve]
• Paracarbonicola pervetusta Bennison, 1954 [nonmarine bivalve]
• Paracarcinosoma obese H Woodward, 1872 [eurypterid]
• Parka sp. [plant]
• Pentlandella pentlandica (Haswell, 1865) [brachiopod]
• Perrtopecten sp. [bivalve]
• P. carboniferus (Hind, 1903)
• P. fragilis Wilson, 1966 [bivalve]
• P. sowerbii, (McCoy, 1844)
• Phestia sp. [bivalve]
• P. attenuate (Fleming, 1828)
• Phricodothyris cf. lineata Q Sowerby, 1822) [brachiopod]
• Planolites ophthalmoides Jesson, 1949 [trace fossil]
• Platyschisma sp. [gastropod]
• Pleuromphalus? [gastropod]
• P. simulans (Salter, 1861)
• Pleuropugnoides [brachiopod]
• P. cf. greenleightonensis Ferguson, 1966
• P. pleurodon (Phillips, 1836) 41
• Podowrinella straitonensis Lamont, 1965 [trilobite]
• Posidonia sp. [bivalve]
• P. becheri Bron, 1828
• P. corrugata Etheridge, jun., 1873
• P. sulcata (Hind, 1904)
• Praenucula sp. [bivalve]
• Productus sp. [brachiopod]
• P. concinnus J Sowerby, 1821
• Promarginijera trearnensis Sheills, 1966 [brachiopod]
• Protatrypa? [brachiopod]
• Prothyris smtica (Wilson, 1963 [bivalve]
• Pseudarthron [arthropod]
• Pterinopectinella sp. [bivalve]
• Pteronitella sp. [bivalve]
• Pugilis sp. [brachiopod]
• P. pugilis (Phillips, 1836)
• Pugnax cf. pugnus (Martin, 1809) [brachiopod]
• Reticulatia? craigmarkensis (Muir-Wood, 1937) [brachiopod]
• Reticycloceras sp. [cephalopod]
• Retiolites geinitzianus geirritzianus (Barrande, 1850) [graptolite]
• Retispira sp. [gastropod]
• Rhipidomella cf. michelini Leveille, 1835 [brachiopod]
• Rugosochoneles sp. [brachiopod]
• R. caledonicus Brand, 1970
• R. celticus Muir-Wood, 1962
• R. skipseyi (Currie, 1937)
• R. speciosus (Cope, 1938)
• Saccaminopsis fusulinaformis (McCoy, 1849) [alga]
• Sanguinolites sp. [bivalve]
• R. clavatus Etheridge, jun., 1876
• R. costellatus (McCoy, 1851)
• S. variabilis? McCoy, 1851
• Schellwienella sp. [brachiopod]
• Schizodus sp. [bivalve]
• Schizophoria resupinata (Martin, 1809) [brachiopod]
• Sedgwickia sp. [bivalve]
• Semiplanus sp. [brachiopod]
• Serpuloides [annelid]
• S. carbonarius (McCoy, 1844)
• Siphonodendron junceum (Fleming, 1828) [coral]
• S. pauciradiale (McCoy, 1844)
• Skenidioides sp. [brachiopod]
• Slimonia acuminata Salter, 1856 [eurypterid]
• Spathella sp. [bivalve]
• Spirifer sp. [brachiopod]
• S. bisulcatus J de C Sowerby, 1825
• Spiriferellina sp. [brachiopod]
• Straparollus sp. [gastropod]
• S. carbonarius (J de C Sowcrby, 1814)
• Streblochondria sp. [bivalve]
• Streblopteria ornata (Etheridge, jun., 1873) [bivalve]
• Strobeus sp. [gastropod]
• Strophochonetes sp. [brachiopod]
• S. aff. edmundsi H S Williams, 1913
• Stylonurella sinipes Page, 1859 [eurypterid]
• Sudeliceras sp. [goniatite]
• Synsphaeridium? [acritarch]
• Syringopora sp. [coral]
• Taitia catena Crookall, 1930 [plant]
• Tentaculites sp. [NS cricoconarid]
• Tornquistia sp. [brachiopod]
• T. polita (McCoy, 1852)
• T. youngi Wilson, 1966
• Tylonautilus? [nautiloid]
• Vlasta? [bivalve]
• Wilkingia sp. [NS bivalve]

Figure, plates and tables

Figures

(Figure 1) Physiographic and locality map of the Hamilton district.

(Figure 2) Outline geology of the solid rocks in the Hamilton district.

(Figure 3) Silurian and Devonian rocks of the Lesmahagow and Hagshaw Hills inliers. Ornament denotes rocks of continental facies.

(Figure 4) Lateral variation in the Inverclyde and Strathclyde groups in the Hamilton district.

(Figure 5) Main lithostratigraphical units in the Strathclyde and Clackmannan groups in the Central Coalfield.

(Figure 6) Sections illustrating lateral variation in the Limestone Coal Formation in the Hamilton district. Inset map shows location of sections and generalised outcrop of limestone Coal Formation.

(Figure 7) Main lithostratigraphical units in the Coal Measures in the Central Coalfield.

(Figure 8) Main structural elements in the western Midland Valley of Scotland showing position of the Hamilton district.

(Figure 9) Principal faults and folds in the Hamilton district.

(Figure 10) Pre-Dimlington Stadial channel system, Stonehouse–Larkhill–Wishaw area.

(Figure 12) Glacial striae, drumlins and other features in the Hamilton district and surrounding area.

(Figure 13) Chapelton esker–ridge system.

(Figure 14) Diagrams illustrating stages in the deglaciation of west-central Scotland.

(Figure 15) Bouguer gravity anomaly map of the Hamilton district and surrounding area with contours at 1 mGal intervals. Density of 2.70 Mg m ³ used for the Bouguer correction. B = Bathgate; CF = Carmichael Fault; DB = Douglas Basin; Di = Distinkhorn; DSL = Dechmont–Stonehouse Line; G = Gradient; H = High; Ha = Hamilton; HHI = Hagshaw Hills Inlier; KLF = Kerse Loch Fault; L = Low; Li = Linburn; MS = Muirkirk Syncline; NAB = North Ayrshire Block; SAC = South Ayrshire Coalfield.

(Figure 16) Aeromagnetic total field anomaly map of the Hamilton district and surrounding area with contours at 10 nT intervals. CPVF = Clyde Plateau Volcanic Formation; TD = Tertiary dyke. Other abbreviations as (Figure 15).

Plates

(Front cover) Chatelherault, former seat of the Dukes of Hamilton, built of local Upper Coal Measures Chatelherault Sandstone. (D 4998) (Photographer: T S Bain)

(Frontispiece) Trachyandesite volcanic plug of Loudoun Hill, with scree on south face; sand and gravel pit in glacial kame terrace; lavas of the Clyde Plateau Volcanic Formation in distance

(Plate 1) Dippal Burn Formation grey-green bedded siltstone, overlain by massive sandstone, Dippal Burn [NS 693 317] (D 3052).

(Plate 2) Middlefield Conglomerate with well-rounded quartzite boulders, Birkenhead Burn [NS 764 360] (D 2488).

(Plate 3) Greywacke Conglomerate, Pockmuir Burn [NS 777 346] (C 2969).

(Plate 4) Kinnesswood Formation sandstone with carious weathering lying on cornstone, Middlefield Quarries [NS 680 295] (D 3107).

(Plate 5) Middle Coal Measures, 3 m-thick Glasgow Ell Coal, Glencleland Opencast Site [NS 791 571] (C 3821).

(Plate 6) Felsite sill cut by basic dyke, Dunduff Quarry near Kirkmuirhill [[NS 779 410] (C 2961).

(Plate 7) Glacial drainage channel subparallel to contours, near Hawkwood [NS 683 388] (MNS 1867).

(Plate 8) Esker ridge, Kaims of Avon [NS 610 343] (C 2935).

(Plate 9) Foreset bedding in deltaic sand and gravel deposit, dipping to north-east, Ferniegair Gravel Pit [NS 734 493] (C 2959).

(Plate 10) Structures in glacial sand, Torfoot Sand Pit, Drumclog [NS 636 379]. a Ice-wedge cast (D 2305). c Ripple-bedding and planar bedding (D 2304). b Contorted bedding (D 2306). d Contemporaneous faulting (D 2142).

(Plate 11) Subsidence into shallow stoop and-room coal workings, South Bankend, near Coalburn [788 333] (MNS 3334).

(Plate 12) Working sand pit, Snabe [645 387] (MNS 1971).

Tables

(Table 1) Classification of the Carboniferous.

(Table 2) Classification of basic igneous rocks of Carboniferous and Permian age in the Midland Valley of Scotland.

(Table 3) Correlation of the limestone beds in the Dinantian of the Hamilton district.

(Table 4) Quaternary lithostratigraphy and chronostratigraphy of the Hamilton district.

(Table 5) Colliery dewatering rates.