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Geology of the Rothbury district. A brief explanation of the geological map Sheet 9 Rothbury
D J D Lawrence, M T Dean, D C Entwisle, G S Kimbell, and A Butcher
Bibliographic reference: Lawrence, D J D, Dean M T, Entwisle, D C, Kimbell, G S, and Butcher A. 2011. Geology of the Rothbury district — a brief explanation of the geological map. Sheet Explanation of the British Geological Survey. 1:50 000 Sheet 9 Rothbury (England and Wales).
Keyworth, Nottingham: British Geological Survey.
© NERC 2011 All rights reserve. Printed in the UK for the British Geological Survey by B&B Press Ltd, Rotherham.
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(Front cover) Prehistoric cup and ring marks, a form of rock art, cut into the Fell Sandstone at Lordenshaw [NZ 056 991] with Rothbury in the distance. The sandstone exposure into which the symbols are cut is 3.3 m from front to back. (Photographer B McIntyre; P708898).
(Rear cover)
Notes
The word 'district' refers to the area of the geological 1:50 000 Series Sheet 9 Rothbury. Ordnance Survey National Grid references are given in square brackets and prefixed by the 100 km square in which they lie: NT, NU, NY or NZ. Symbols in round brackets after lithostratigraphical names are those used on the geological map. The serial number given with the plate captions is the registration number in the National Archive of Geological Photographs, held at BGS. Boreholes are identified by their BGS Registration Number in the form (NZ29SW/122), where the prefix indicates the 1:10 000 scale National Grid sheet.
Acknowledgments
The authors acknowledge contributions from P J Brand (palaeontology) and thank D Millward for critical review. This Sheet Explanation was edited by D T Aldiss. Figures were produced by C Woodward and page-setting was by A J Hill.
The urban area in the south of the district was surveyed and described with the support of the Department of the Environment. We acknowledge the assistance provided by the Coal Authority, Northumberland County Council, local councils and numerous civil engineering consultants. Landowners, tenants and quarry companies are thanked for permitting access to their lands.
The grid, where it is used on figures, is the National Grid taken from Ordnance Survey mapping.
© Crown copyright. All rights reserved. British Geological Survey 100017897/2011.
Geology of the Rothbury district (summary from rear cover)
An explanation of sheet 9 (England and Wales) 1:50 000 series map
(Rear cover)
The former market town of Rothbury is situated in the valley of the River Coquet, which drains to the sea at Amble. A westerly belt of moorland, covered by heather and coniferous plantation, centred on Rothbury, gives way eastwards to rolling pasture land and this in turn to a low-lying coastal plain. The local economy is dominated by farming, though tourism is important in the west of the district where Rothbury serves as a gateway to the Northumberland National Park. Underground coal mining ceased with the closure of the mine at Whittle in 1987. Surface mining on the coastal plain continues to provide employment on a small scale.
Carboniferous rocks, deposited between about 350 and 310 million years ago, underlie the entire district. Most of the area is covered by a varying thickness of superficial deposits, and only in the northwest does the form of the ground bear any clear relation to the underlying bedrock structure. The Carboniferous succession, nearly 3000 m thick, was deposited in the Northumberland–Solway Basin. The Carboniferous sedimentary rocks represent the gradual infilling of the fault-controlled trough. Deposition in lagoonal coastal-flat environments in the early Carboniferous was followed by fluvial deposition and by the deposition of rhythmic units of limestone, mudstone, sandstone and coal on low-lying deltaic swamps with repeated shallow marine incursions. Extensional movements during early Permian times led to the emplacement of the Whin Dolerite Sill-swarm and of associated dykes of the Northern England Tholeiitic Dyke-swarm. A few basaltic dykes were intruded during the early Palaeogene. Superficial deposits of Quaternary age were deposited during the last, Devensian, glaciation. Postglacial and recent deposits include, alluvium, river terrace deposits and peat inland, with blown sand, tidal river, salt-marsh and marine deposits on the coast.
The revised geological map incorporates information resulting from the exploration for, and extraction of coal. The exploration of northern England for oil and gas in the 1980s, by seismic reflection surveys, provided a means for investigating the subsurface structure. This Sheet Explanation provides summary information for a range of applied geological issues — mineral and water resources, potential geohazards and engineering ground conditions. Lists of information sources and relevant references are also included.
Chapter 1 Introduction
This Sheet Explanation provides a summary of the geology of the district covered by the 1:50 000 scale Geology Series Sheet 9 Rothbury, published in a combined Bedrock and Superficial Deposits edition in 2009. The geology is described using the revised lithostratigraphical classification adopted for Carboniferous rocks in Great Britain (Waters et al., 2007). More comprehensive descriptions of aspects of the geology can be found in individual Technical Reports and Research Reports prepared during the resurvey and in the comprehensive memoir that accompanied the previous revision survey of the district (Fowler, 1936). Many of the detailed observations and descriptions contained in the 1936 memoir still hold, albeit couched in old-fashioned terms. The local geological succession is summarised inside the front cover of this Sheet Explanation, and (Figure 1) provides a comparison between previous and current classifications applied to the rocks.
The Rothbury district affords some of the most delightful scenery in Northumberland; the topography is in no sense spectacular, but it offers considerable variety. Most of the area is covered by a varying thickness of glacial deposits, and only in the north-west does the form of the ground bear any clear relation to the underlying bedrock structure. The distinctive skyline formed by the sandstone escarpment of the Simonside Hills is visible for more than 50 km and can be seen throughout much of the district. Carboniferous rocks, deposited between about 350 and 310 million years (Ma) ago, underlie land that drops eastwards from over 440 m on Tosson Hill [NZ 0045 9825] to the sea. A westerly belt of moorland centred on Rothbury, covered by heather and coniferous plantation, gives way eastwards to rolling pasture land and this in turn to a low-lying coastal plain extending to Druridge Bay and the North Sea.
The former market town of Rothbury is situated in the valley of the River Coquet, which drains into the North Sea at Amble. The local economy is dominated by farming, though tourism is important in the west of the district where Rothbury serves as a gateway to the Northumberland National Park. The coastal plain has been the focus of surface (opencast) coal mining since the 1950s and surface mining continues to provide employment, but on a much smaller scale than at its acme in the 1980s and 1990s. Underground coal mining, active throughout the eastern part of the district at the time of the previous survey, ceased with the closure of the mine at Ashington [NZ 264 882] in 1986. Evidence of the former workings is becoming less obvious. Many buildings have been demolished and the entire sites of some former collieries removed by the surface mining and subsequent restoration. The extent of these workings can be identified by the cross-hatched areas in the east of the map. The few remaining mine spoil heaps have been landscaped, as at Ashington. Areas of subsided ground above collapsed underground coal workings occur locally and ponds and small lakes have formed in the hollows created.
In northern England, a phase of crustal stretching during the Late Devonian and early Carboniferous resulted in the formation of rapidly subsiding basins that were separated by extensional faults from relatively slowly subsiding horst and tilt blocks. These early basin and block structures strongly influenced the later sedimentation. The Rothbury district is located within the Northumberland Trough, between the Alston Block to the south and the Southern Uplands and Cheviot massifs to the north, forming the eastern part of the Northumberland–Solway Basin, which stretches across northern England.
The Carboniferous succession, which underlies the entire district, is almost 3000 m thick and is divided into four groups (inside front cover (Geological succession) and (Figure 1)). Lagoonal coastal-flat and estuarine environments with water of high salinity and subject to periodic desiccation were established in early Carboniferous times, about 350 Ma ago. The lower Carboniferous succession of the Northumberland Trough, deposited along the southern margin of the Southern Uplands, is distinct from that found elsewhere in England, but very similar to that seen in the Midland Valley of Scotland. In recognition of this, recent studies by BGS have extended the use of the Inverclyde Group from Scotland into Northumberland. In late Arundian times, a fluvial system advanced into the trough from the northeast and braided streams deposited the thick Fell Sandstone Formation. Towards the end of the Visean, tectonic extension was replaced by a prolonged period of thermal relaxation and crustal sag, resulting in widespread marine transgression and the gradual submergence of the distinctive block-and-basin structure beneath an increasingly terrigenous sediment cover. Sandstone, siltstone and mudstone accumulated in an extensive low-lying deltaic swamp, but repeated marine incursions spread shallow seas across the district in which limestone was deposited. The Visean to Namurian Yoredale Group is characterised by repeated upward-coarsening cycles on a variety of scales. From the late Namurian, significant marine influence was progressively lost and subsidence and riverborne sedimentation were balanced, so maintaining a stable delta-top environment through to the late Westphalian. Deposition of the Pennine Coal Measures Formation during Westphalian times was characterised by the accumulation of thick deposits of peat, derived from forest swamps that developed widely on the emergent delta surface: these are seen today as coal seams. Further extensional movements during early Permian times, about 296 Ma ago, led to the emplacement of the complex of quartz-dolerite intrusions known collectively as the Whin Dolerite Sill-swarm and the associated dykes of the Northern England Tholeiitic Dyke-swarm. A few basaltic dykes were intruded about 52 Ma ago, during the early Palaeogene. Apart from these, there is no evidence of bedrock younger than the Carboniferous in the district, and a major unconformity that spans a period of about 300 million years exists between the Westphalian deposits and widespread glacial and post-glacial deposits of Quaternary age.
The oldest rocks that crop out in the district are mudstone and limestone of the Ballagan Formation. These are relatively weak and easily eroded and form the lowlying country in the north-west. As the regional dip is to the south-east there is a progressive rise in the sequence in this direction, until Pennine Coal Measures Group strata are encountered along the coastal belt. The regularity of this general sequence is locally interrupted by faults, some of which have large displacements. The Bolton–Swindon Fault in the north-west of the district has a throw to the north of about 200 m. A parallel disturbance to the south-east, the Cragend–Chartners Fault, has its throw of about 400 m down to the south-east; the intervening ground is thus a horst within which most of the high ground of the district occurs, composed for the most part of the resistant, pebbly, coarse-grained sandstone (formerly termed 'grit') of the Fell Sandstone Formation. The Fell Sandstone also makes up the somewhat lower ground north of Rothbury, together with some faulted outliers of the Ballagan Formation.
East of the Cragend–Chartners Fault, away from the hilly Fell Sandstone country, the succession rises in unbroken sequence to the sea. The topography is less strongly marked and there are few significant natural exposures, apart from in places in the gorges of the rivers Coquet and Font, in the Forest Burn, on the coast at Amble and in the prominent sandstone crags at Rothley [NZ 045 880], which have steep scarp faces up to 40 m high. Although the limestones in the Alston Formation are typically 6 to 10 m thick they rarely form scars or crags, although it is possible some did exist before quarrying began. The Great Limestone gives rise to some minor topographical features, as do intrusions of the Great Whin Sill (e.g. Wards Hill, 5 km south-south-east of Rothbury). In the south-west of the district some of the other limestones can be traced by means of swallow holes.
Apart from minor dislocations by faulting, the outcrop of the three main limestones near the top of the Alston Formation can be followed continuously across country from Newton-on-the-Moor [NU 170 053] in the north-east, to Hartington Hall [NZ 024 870] in the extreme south-west, a distance of more than 25 km. In the east of the district, faults with displacements of 50 m are relatively common in the Coal Measures. A few of these faults, generally those with a north-east to south-west orientation, can be traced across the outcrop of the Yoredale Group; perhaps the most notable being that which initially follows the line of the Causey Park Dyke from Druridge Bay [NZ 275 985] continuing to the south-west corner of the district.
Certain minor topographical features such as glacial overflow channels formed during deglaciation. Landslides are a common feature within the glacial deposits along the banks of the River Coquet and its tributaries in the north of the district.
Survey history
The Rothbury district was originally surveyed on a scale of six inches to one mile by W Topley and the maps issued between 1870 and 1880. The Old Series one-inch sheet 109SW was issued in 1895. A revision survey by J Maden, A Fowler, W Anderson, G A Burnett, V A Eyles and R G Carruthers between 1922 and 1926 was published as separate 'solid' and 'drift' one-inch maps for the district in 1934 and an accompanying descriptive memoir by A Fowler published in 1936. Bedrock and superficial editions of the map at 1:50 000 scale were produced in 1977 without geological revision.
Systematic revision of the coalfield area in the south-east of the district was begun in 1987 by I Jackson and D J D Lawrence as part of a programme of surveys commissioned by the Department of the Environment to provide modern geological and thematic maps for the region, with particular attention directed towards aspects of land-use planning (Lawrence and Jackson, 1986, 1990a, b). Resurvey of the remainder of the district was carried out by D J D Lawrence and B Young between 1994 and 2007. In the east of the district, this revision incorporates a large amount of information resulting from the exploration for, and extraction of, coal of Westphalian age. Boreholes and underground information from the mining of the Shilbottle Coal at Whittle Colliery in the 1980s and 90s have enabled improved understanding of the disposition of the Alston Formation in the Longframlington area. The exploration of northern England for oil and gas in the 1980s, primarily by seismic reflection surveys, provided data to investigate the subsurface structure of Northumberland at depths not previously reached and has enabled great advances in the understanding of the evolution of the Carboniferous strata (Chadwick et al., 1995). The Longhorsley Borehole (NZ19SW/6), drilled in 1986, proved the complete thickness of the Tyne Limestone and Fell Sandstone formations within the Rothbury district for the first time.
Chapter 2 Geological description
Concealed geology
Chadwick et al. (1995) investigated the structure of the Northumberland–Solway Basin using seismic reflection data. They found that the top of the Caledonian basement deepens from less than 1 km in the north-west of the Rothbury district to about 4 km in the south-east (Figure 2). The overall shape of the basin closely approaches that of a half-graben. Its southern margin is controlled by the Stublick and Ninety-Fathom faults to the south of the district. The northern boundary of the basin against the Cheviot Block is relatively poorly defined but in this district may be represented by the Cragend–Chartners and Hauxley faults, as seen at the top of the Caledonian basement.
These downthrow to the south-east and are inferred to have been active during the early Carboniferous extensional phase of basin evolution, although the syndepositional displacements on these faults are smaller than those observed on similar structures in the west of the basin (De Paola et al., 2005; Shiells, 1964). The Sweethope and Stakeford faults in the southern part of the district downthrow to the north-west and show evidence of minor Variscan inversion (Chadwick et al., 1995).
The south-eastwards thickening of the Carboniferous sequence across the district is reflected in the Bouguer gravity anomaly map (Figure 3a), which shows decreasing values towards a minimum in the south-east over the Pennine Middle Coal Measures outcrop. The decrease in the gravity field in the north-west is due to the low-density Cheviot Granite Pluton (Kimbell et al., 2006; Lee, 1982), which crops out about 3 km to the north-west of the area shown in (Figure 3a).
The main features revealed by available magnetic survey data are due to intrusive igneous rocks. There is a marked contrast between the resolution of the regional aeromagnetic survey and that achieved in a detailed survey extending into the southwestern corner of the district (Figure 3b). The latter clearly reveals the linear western edge of the late Carboniferous Whin Sill- swarm (M1 in (Figure 3b)) and features associated with offsets and alteration of the sills at geological structures such as the Sweethope Fault (Cornwell and Evans, 1986; Young and Lawrence, 2002). Such detail is not visible in the regional survey but it does detect major east-north-east-trending positive magnetic anomalies associated with the early Permian Causey Park Dyke and High Green Dyke subswarm, shown as M2 and M3 respectively in (Figure 3b). A prominent east-south-east-trending negative magnetic anomaly can be traced from southern Scotland across the northern edge of the district (M4 in (Figure 3b)) and into the North Sea (e.g. Kimbell et al., 2006). This feature is almost certainly caused by a reversely magnetised Palaeogene dyke or dykes. Although it has been associated with the Acklington Dyke (e.g. Kimbell et al., 2006; Kirton and Donato, 1985) its geophysical expression lies about 5 km to the north of the exposed dyke, which is not resolved by the magnetic survey. A further negative magnetic anomaly occurs over the apparent north-westward extension of the Palaeogene dykes observed in the Blyth area south-east of the district (M5 in (Figure 3b)) and several small Palaeogene dykes are resolved by the detailed aeromagnetic survey just to the south-west of the district (Young and Lawrence, 2002). The north-western corner of the area shown in (Figure 3b) extends over a large positive magnetic anomaly associated with the magnetic outer phases of the Cheviot Granite, adjacent metamorphosed Devonian volcanic rocks and the Biddlestone porphyritic rhyolite (Robson and Green, 1980).
Carboniferous
Inverclyde Group
The Inverclyde Group is represented by rocks of the Ballagan Formation (Bgn), which crops out in the north-west of the district. The base of the formation is not seen and areas of outcrop commonly have faulted boundaries. The absence of any continuous section makes it difficult to calculate an accurate total thickness, but it is estimated that at least 530 m of the formation are exposed within the district. These strata, part of the former 'Cementstone Group', comprise a cyclic sequence of interbedded sandstone, siltstone, variegated mudstone, argillaceous ferroan dolostone ('cementstone') and limestone. The beds range in colour from grey and blue-grey, through green and yellow to brown and pinkish red. The cementstones are broadly of two types. Layered cementstones have a sharply-defined top and base and some may show signs of internal stratification and desiccation cracks. Nodular cementstones show a transition into calcareous mudstones above and below and they form a more or less persistent layer or a horizon of nodules. The layered cementstones are thought to be primary in origin and the nodular cementstones to be of secondary origin. The beds are generally less than 0.3 m thick. The interbedded mudstones are calcareous, silty, poorly bedded and range in thickness from a few millimetres to several metres. The highest part of the formation, including three persistent limestone units, is exposed south of the River Coquet. One kilometre south-west of Rothbury at Glebe Quarry [NU 052 005], a section some 6 m thick in limestone is packed with nodular, concentrically laminated oncoids containing the calcareous alga Ortonella furcata Garwood. Micropalaeontological work suggests that the section at this quarry lies at or close to the Chadian–Arundian boundary. There is no direct evidence for the age of the oldest beds of the formation exposed in the district, but comparison with the Ballagan Formation elsewhere shows that they could range down into the Tournaisian.
Border Group
The Border Group of northern England and southern Scotland consists of two formations. The lower of these, the Lyne Formation, does not crop out within the district. The upper, the Fell Sandstone Formation (FeSd) of Arundian–Holkerian age, forms the sandstone crags and scarp faces of the Simonside Hills and the high ground north of Rothbury and on Longframlington Common. At exposures the succession comprises very strong, very thickly bedded, pebbly, poorly to moderately sorted, medium- to coarse-grained sandstone; the original name for the formation was the 'Rothbury and Harbottle Grits' (Lebour, 1876). The sandstones generally range in colour from light buff to pale purple but locally are yellow, deep red or brown. In places sandstone units overlie one another to form multistorey bodies up to 90 m thick. They contain subangular to subrounded fragments of mudstone and a few pebbly conglomerate layers. The well-preserved sedimentary structures include a variety of forms of cross-stratification and examples of steep-sided channels, as well as thick structureless units. Detailed descriptions of the formation in this and adjacent districts can be found in Robson (1956), Hodgson (1978), Turner and Munro (1987) and Turner et al. (1993; 1997). Robson (1956) showed that in the area west of Rothbury and south of the River Coquet, the sandstones were laid down by river channels flowing from a north-easterly direction. This accords with the work of Turner et al. (1997), who described the formation in north-east England as having been deposited by perennial braided river-systems flowing westwards into a shallow, low-energy tideless sea in the vicinity of Bewcastle, about 45 km west-south-west of the district. Lateral variations in thickness and facies were primarily controlled by syndepositional faulting, with the thick sandstone units of the Rothbury district deposited in the active braid-plain of the central graben of the Northumberland Trough.
The junction between the Fell Sandstone and the overlying Yoredale Group is faulted throughout the district, for much of its length along the line of the Cragend– Chartners Fault (page 20). The faulting and intense cross-bedding make it difficult to determine a maximum thickness for the formation exposed within the district, but it has been estimated to be about 350 m (Fowler, 1936). In the Longhorsley Borehole 380 m of strata are attributed to the formation, predominantly comprising sandstone but with some thin beds interpreted from the geophysical logs as claystone and with scattered thin coal seams (Figure 4). Six kilometres to the north of the district on Alnwick Moor [NU 1462 1210], a cored borehole drilled in 1987 through 101 m in the upper part of the formation was composed predominantly (98.5 per cent) of medium- to coarse-grained sandstone with argillaceous material at two levels towards the base of the borehole (Turner et al., 1997). Significantly, the siltstone in the upper argillaceous layer contained an abundant, though restricted, ostracod fauna with rare gastropods, including the first documented occurrence of marine ostracods from the Fell Sandstone. Plant fossils, freshwater ostracods and a large freshwater bivalve Archanodon jukesi (Forbes) had previously been recorded from the Fell Sandstone in Northumberland (Howse, 1878; Johnson, 1980). Palynological evidence from the Longhorsley Borehole indicated a Holkerian age for strata near the top of the formation.
Yoredale Group
The Yoredale Group comprises up to 1480 m of cyclical marine and fluviodeltaic beds of Asbian to late Namurian age. It is divided into three formations, the Tyne Limestone Formation, the Alston Formation and, at the top, the Stainmore Formation, based largely on the relative abundance of the different rock types within the cycles of each division. Limestones generally increase in importance and thickness as the sequence is traced up towards the base of the Namurian (near the top of the Alston Formation) and decline in significance thereafter. The major limestones are described in (Figure 5).
The Tyne Limestone Formation (TyLs) crops out in four widely separated areas, each one fault-bounded on at least one side. The outcrops occur in a very small strip at the very north of the district in Edlingham Woods [NU 125 059]; in a connected series of small faulted outliers near Debdon, north of Rothbury [NU 067 041]; at the western edge of the district south of Hepple [NY 990 995], where only the very lowest beds of the formation are present, and in a narrow strip on the southern slopes of the Simonside Hills between Lordenshaw and Chartners [NY 990 953 to NZ 062 984]. The strata consist of yellow-brown, fine- to medium-grained sandstone, siltstone and seatearth with numerous thin coals developed locally. Coal was formerly worked on a very limited scale in the Debdon area. The faulting, poor exposure and disconnected nature of the outcrops make it extremely difficult to establish a total thickness for the succession at outcrop in the district, although it has been estimated to be about 140 m. However, over 530 m of the Tyne Limestone Formation are interpreted to be present in the Longhorsley Borehole (Figure 4).
The Dun Limestone (DnL) at the top of the formation is exposed in the Blanch Burn [NZ 019 958], where it is repeated by faulting, and the nearby Newbiggin Burn [NZ 010 952]. It was quarried in the Black Burn [NU 080 042] north of Rothbury (Fowler, 1936). Amongst the fossils that it contains, the coral Palaeosmilia murchisoni Milne-Edwards and Haime is a typical Asbian macrofossil, and whilst the coral Lithostrotion maccoyanum Milne-Edwards and Haime is characteristically Brigantian, it seems to have gradually appeared during the Asbian in northern England, indicating a probable late Asbian age for the limestone (Dean, 1999; Frost and Holliday, 1980).
The base of the Alston Formation (An) is formally defined in the neighbouring Bellingham district (Sheet 13) at the base of the Low Tipalt Limestone, equivalent to the Watchlaw Limestone in North Northumberland. However, no limestone can be mapped consistently in the interval between the Dun and the Oxford limestones in the Rothbury district. Consequently, the base of the Alston Formation here is taken at the top of the Dun Limestone. Thus, the majority of the Tyne Limestone Formation in the Rothbury district is equivalent to the 'Scremerston Coal Group' of Northumberland (Figure 1). A number of limestones within the Alston Formation can be traced across the district (Figure 5), although they are generally not well-exposed. The positions of the Three Yard, Four Fathom and Great limestones are locally marked by swallow holes.
Two significant coal seams occur within the formation. The Shilbottle Coal (Shil), up to 1 m, but typically 70 to 80 cm thick, underlies the Three Yard Limestone. The coal was exposed in the excavations for Fontburn reservoir, but appears to be absent farther south. The 1970s saw renewed interest in the coal and it was worked from Whittle Colliery between Guyzance [NU 210 040] and Longframlington [NU 130 010]. Exploratory boreholes from the surface to prove the coal and levels from the underground workings have enabled significant improvements to be made in the mapping of the limestones in the north of the district. The Townhead (Top) Coal (To) lies immediately beneath the Great Limestone in the northern half of the district.
The Great Limestone (GL) is by far the thickest (up to 14 m) and purest limestone within the district. It forms one of the most useful lithostratigraphical marker horizons in northern England. Fairbairn (1978, 1980) has traced its constituent beds from the northern part of the Alston Block across the Ninety Fathom–Stublick Fault System and into the Northumberland Trough. The limestone has been extensively quarried and is excellently exposed at Wards Hill Quarry [NZ 079 965] and Greenleighton Quarry [NZ 034 920]. At Greenleighton Quarry (Plate 1), in its formal type section, the Great Limestone consists of a lower unit (7 m) of limestone with thin mudstone partings (the 'Bench Posts' and 'Main Posts' of Fairbairn, 1980) and a 5 m unit of limestone with prominent mudstone interbeds (the 'Transitional Posts' and Tumbler Beds' of Fairbairn, 1980). The overlying beds (up to 10 m), composed of mudstone with thin and laterally impersistent calcareous siltstone and sandstone beds, are particularly fossiliferous containing a diverse array of bryozoan, brachiopod, bivalve, cephalopod and echinoderm taxa, including the type material of the brachiopod Pleuropugnoides greenleightonensis (Purnell and Cossey, 2004). Correlation of Namurian rocks in Great Britain and beyond is traditionally aided by zonally significant goniatites, but the key species are scarce in northern England. Johnson et al. (1962) recorded the occurrence of the basal Namurian, E1a index goniatite, Cravenoceras leion Bisat from the overburden waste produced during quarrying. This together with evidence from the adjoining Morpeth district (Sheet 14; Dean and Brand, 1998) and the presence of the nonmarine bivalve 'Carbonicola' cf. pervetusta Bennison in the mudstone roof of the Great Limestone at about 342 m depth in the Guyzance Borehole (NU20SW/55) [NU 2200 0269] indicates an early Pendleian (late E1a) age for the limestone. The top of the Alston Formation is taken at the top of the Great Limestone.
Stainmore Formation (ST) rocks underlie the wide area that is predominantly covered by superficial deposits from Cambo in the south-west to Warkworth in the north-east. Exposures of the formation are intermittent and largely limited to quarries, a few rivers and to sporadic areas of sandstone outcrop that are generally slightly higher than the surrounding country. The BGS Stobswood 2 (NZ29SW/122) and Longhorsley (NZ19SW/6) boreholes combine to provide a complete succession through the formation to the south of the Causey Park Dyke (Figure 4). The succession in the area to the north, between Longhorsley and Felton, is very poorly known because of the presence of thick superficial deposits and an absence of borehole information.
The formation comprises a cyclical succession dominated by mudstone and sandstone with a small number of interbedded thin limestones and coals. It increases in thickness from 390 m in the north to about 530 m in the south, which is the maximum development of the formation within the Northumberland Trough (Brand, 2011). The sand bodies are broadly of two types: channel sand bodies and mouth bar/ delta sand bodies. The channel sand bodies are generally much the coarser of the two and are locally pebbly. Many of the limestone beds within the Stainmore Formation (Figure 4) are only intermittently exposed and borehole coverage is insufficient to determine their continuity in the areas covered by superficial deposits. They are typically only a few metres thick; towards the top of the succession they become fewer and much more impure, locally becoming difficult to distinguish from calcareous sandstone or mudstone and almost indistinguishable from the surrounding measures. The limestone units are commonly associated with a coal or coals and in places a horizon may be represented by a coal alone. The Little Limestone (LtLs) is a persistent bed proved in boreholes throughout the district but rarely seen exposed. The Little Limestone Coal (LLCO), underlying the Little Limestone, is the thickest and most consistent coal in the formation and has been worked along much of its outcrop (Fowler, 1936), most recently in opencast excavations. It is known locally as the Chirm Coal. Fowler gave details of mining in the overlying Coatyards, Netherwitton and Stanton seams that ceased long before the current survey. All of these seams were worked opencast at a number of small sites, on a very local scale, in the 1970s and 1980s.
The Rothley Grits Member (RotG) is a multistorey sandstone body interpreted as the deposit of a low sinuosity channel. At Rothley Crags [NZ 042 886] it is a grey-brown, poorly-sorted, pebbly coarse-grained sandstone about 35 m thick. It is cross-bedded throughout with trough cross-bedding dominant and contains some large, simple bar forms; palaeocurrent measurements indicate flows directed towards the south-east (Jones, 1996).
Throughout Northumberland a major macrofaunal change takes place at the Thornbrough Limestone (ThL) with the incoming of a number of marine species new to the area (Brand, 2011). Miospores from mudstone a short distance above this bed in the Morpeth district suggest it is very close to the boundary between the E1 and E2 goniatite zones (Young and Lawrence, 2002).
Borehole records show that the Stainmore Formation becomes much more arenaceous above the Newton Limestone (NwL), comprising sand bodies up to 20 m in thickness separated by thinner mudstone and siltstone units. The sandstones are locally feldspathic, containing beds of coarse-grained sandstone and conglomerate. They correspond to the lower and middle part (First and Second grits) of the 'Millstone Grit' of former classifications and could possibly be assigned to the Millstone Grit Group of Waters et al. (2007). Unlike their equivalents in regions further south, these strata have little or no effect on the topography of the district.
The BGS Stobswood 2 Borehole yielded goniatites that indicated the H to R chronozones (Chokierian to Marsdenian) between 159 and 189 m depth, which accords with palynological evidence for the same interval indicative of Arnsbergian to Alportian age (McNestry, 1993). A goniatite recovered at 93 m depth from the Warkworth Borehole 15, (NU20NW/17) [NU 2401 0553] may represent the Cancelloceras cumbriense Marine Band (Yeadonian).
Pennine Coal Measures Group
The Pennine Coal Measures Group, of Westphalian age, crops out in the eastern part of the district and consists of up to about 580 m of strata comprising cyclothems of mudstone, siltstone, sandstone, seatearth and coal (Plate 2). No goniatites have been found in either Northumberland or Durham to prove the Subcrenatum Marine Band, which defines the base of the Westphalian in western Europe. The Quarterburn Marine Band of Durham is believed to be the most likely correlative in north-east England of the Subcrenatum Marine Band (Mills and Hull, 1968), but it has not been found in the current district. Consequently, in the absence of a definitive faunal horizon, the position of the base of the group has been inferred by comparing sequences of strata from within the district to those containing the Quarterburn Marine Band in areas to the south. Owing to the intermittent and variable nature of exposures, this position could not be identified with certainty across the entire district and may range in level over one or possibly two cyclothems.
The private colliery companies operating at the time of the previous survey (Fowler, 1936) applied a variety of local names to the seams they worked, as have, more recently, individual opencast sites. In the present survey, a common naming system has been applied to the coals across the district; for the most part this corresponds to that adopted by British Coal in the 1960s but with small variations to accord with British Geological Survey practice on adjacent maps. The main named coal seams and local (alternative) names are shown on (Figure 6). Only the Yard and High Main coals maintain a thickness sufficient to have been mined throughout the district, the others show local areas of thinning or absence including 'washouts', which are areas where channel sandstones have cut down through the coal.
The Pennine Lower Coal Measures Formation (PLCM) comprises up to 205 m of strata between the Subcrenatum and Vanderbeckei marine bands. Beneath the Victoria Coal the bulk of the succession is arenaceous. The lowest 30 to 40 m was previously included in the 'Millstone Grit', the top of which was taken at an arbitrary line more or less parallel to the Ganister Clay Coal outcrop. Some impersistent seams beneath the Marshall Green Coal are sufficiently thick in places to have been mined and borehole information available since the previous survey makes it possible to correlate workings in some of these tentatively. The 0.35 m thick 'Victoria' coal worked at Causey Park Drift [NZ 1780 9512], which closed in 1969, would appear to be that immediately above the base of the Pennine Coal Measures. Overlying this coal, pebbly coarse-grained sandstone forms a prominent ridge with a steep scarp facing northwest at Helm [NZ 1880 9642]. The Ganister Clay Coal is thickest in the south of the district and is probably the 'Victoria' coal mined at Eshott [NZ 2088 9760]. The Marshall Green Coal is the lowest coal to have been worked extensively, but is absent north of Acklington. A dark grey mudstone bed with a fauna indicative of the Stobswood Marine Band, named from its occurrence in a borehole at East Stobswood (NZ29SW/58) [NZ 2281 9458], is present between the Marshall Green and Victoria coals throughout much of the district. The succession is more arenaceous than farther south in the coalfield and contains several thick channel sandstones. The Vanderbeckei Marine Band, locally known as the 'Harvey Marine Band' has been identified in boreholes in the south of the district and is exposed intermittently on the foreshore at Amble [NU 2752 0468] (Brand, 1991; Dean, 2001). It consists of up to 1 m of very dark grey to black mudstone containing, predominantly, the brachiopod Lingula mytilloides.
The Pennine Middle Coal Measures Formation (PMCM) is up to 375 m thick. Some major sandstones are present between the base of the formation and the persistent Yard Coal so that the intermediate coals are absent in parts of the district. Even the Northumberland Low Main Coal, locally up to 2.13 m thick and so prominent farther south in the coalfield, is cut out in places.
Towards the southern margin of the district the Metal and High Main coals come together and were worked as a single seam from Ashington Colliery (Lawrence and Jackson, 1990b). The Ashington Coal, up to 1.76 m thick, is the stratigraphically highest to have been mined underground on an extensive scale. Strata above the Maltby Marine Band are present only in two small areas: the first south of Amble, where they dip steeply northwards into the Hauxley Fault, and the second east of Ashington, where the highest strata, of Bolsovian age, dip increasingly steeply southwards towards the Stakeford Fault of the adjacent Morpeth district.
Intrusive rocks
The Carboniferous sedimentary rocks of the Rothbury district are intruded by two age groups of mafic igneous rocks. The older of these, with the larger area of outcrop, belongs to the complex of quartz-dolerite intrusions known collectively as the Whin Dolerite Sill-swarm, which together with the associated dykes of the Northern England Tholeiitic Dyke-swarm has a very early Permian emplacement age at about 296 Ma (Liss et al., 2004). The younger Acklington Dyke is a representative of the tholeiitic basaltic and basaltic andesite dyke-swarm that radiates from the Palaeogene igneous centre on the Isle of Mull.
The Whin Sill-swarm in northern England comprises four separate sills, characterised by regionally consistent relationships with the stratigraphy and palaeomagnetic directions, and with their associated feeder dykes. In the Rothbury district the dolerite sill is the northernmost part of the Great Whin Sill. In the south-west of the district the sill is exposed in a series of apparently isolated occurrences with a lower leaf intruded beneath the Three Yard Limestone at Coldwell [NZ 004 870] and Dyke Head [NZ 024 914], and an upper leaf just beneath the Great Limestone at Gallows Hill [NZ 022 887]. Further north the sill is intruded into the Great Limestone in the disused quarry at Wards Hill [NZ 079 965] and attains its maximum exposed thickness in the district, almost 20 m, in Ewesley Quarry [NZ 060 945]. However, down-dip in the Longhorsley Borehole, a 72 m-thick upper leaf was recorded in the Alston Formation beneath the Eelwell Limestone, and a 26 m-thick lower leaf in the Tyne Limestone Formation beneath the Fourlaws Limestone (Figure 4). Goulty (2005) considered that the 'step-and-stair' transgressions of the bedding by the Great Whin Sill, stepping downwards in the direction of bedding dip, may be explained by the effect of gravity on the fractured roof rocks, which floated on the intruding sill.
The dykes of the Northern England Tholeiitic Dyke-swarm trend east-northeast with locally well-developed en échelon geometries resulting from emplacement during transtensional fault movements. The Causey Park Dyke has been traced at or near the surface over a distance of about 19 km (Poole et al., 1935). The dyke was 19 m wide in mine workings near Bullock's Hall [NZ 241 980] (Fowler, 1936), now within the area of Maiden's Hall Opencast. Several other dykes crop out in the hill country south-west and north-east of Rothbury. The dyke at Wellhope [NU 111 059] is 24 m wide and a 15 m face was exposed during quarrying (Fowler, 1936).
The Palaeogene Acklington Dyke can be traced across much of the north of the district. The quarry at Acklington [NU 231 018], from which the dyke was named, is now under a housing estate, but the dyke has been encountered underground in the Shilbottle Coal workings, in workings at coastal collieries south of the Hauxley Fault, where it was 8 m wide, and during excavations for the Togston opencast site [NU 247 022]. The course of the dyke from the western edge of the district to the Cheviots was traced by Robson (1964) using a ground magnetic survey.
Structure
The Carboniferous rocks exhibit a regional south-easterly dip of generally less than 5°, although the cross-bedded nature of many of the sandstone units, notably within the Fell Sandstone Formation, makes it extremely difficult to establish structural dips in isolated outcrops. Across the district, the rocks are intersected by a series of east-north-east- to north-east-trending major faults, which are named on the map face. Much of the surface fault pattern has been influenced by the basin and block structures established during the early Carboniferous, although not all of the faults identified at depth can be traced at the surface. The Hauxley Fault is prominent on the top Caledonian basement (Figure 2). The fault is exposed on the coast, 1 km east of the district, where it has a throw down to the south of about 290 m, but at the surface it has been traced only as far as the base of the Pennine Coal Measures Group to the west of Acklington [NU 217 025]. No evidence was found during the survey for the continuation of the fault at the surface into the ground covered by thick superficial deposits north of Felton [NU 184 007] and Longframlington [NU 132 010].
In the north-west, the upstanding area of Fell Sandstone rocks that makes up the Simonside Hills and moorland north of Rothbury is a horst bounded by the Bolton–Swindon and Cragend–Chartners faults, believed to have been formed contemporaneously with the Lemington Anticline of the district to the north (Shiells, 1964). Interpretation of seismic sections indicates that the downthrow along the Swindon Fault ranges from 200 to 400 m and that locally it has complex reversed faulting in its hanging-wall block, indicative of transpressive movements (Chadwick et al., 1995). Comparison with the thickness of strata recorded in the Longhorsley Borehole indicates that the Cragend–Chartners Fault throws at least 400 m down to the south.
Within the coalfield the dip of strata increases in the vicinity of major faults and, exceptionally, in the Stobswood opencast site, dips locally exceeding 40° were recorded adjacent to the Stobswood Fault (Plate 3). Faults within the Coal Measures mostly exhibit displacements of 25 m or less, but some faults have throws of up to 70 m. Many faults were seen to terminate against cross faults, whereas others die out gradually as their throw reduces or as they pass into several fractures, commonly with opposing throws. The apparent degree and complexity of faulting within the Coal Measures, compared with the remainder of the district, almost certainly reflects the very much larger volume of data available for the coalfield.
Small-scale folds, typically with amplitudes and wavelengths of only a few metres, are common within the Great Limestone; their length, measured along the fold axes, is usually of the order of a few tens of metres. The orientation of these folds typically varies from north–south to north-east–south-west. Shiells (1964) described a variety of such folds in the Great Limestone and other limestones throughout Northumberland.
Quaternary
With the exception of the Fell Sandstone country and areas of the Ballagan Formation in the north-west of the district, Quaternary deposits mantle almost the entire district and largely conceal the underlying Carboniferous rocks. The unconformity that separates the bedrock and superficial deposits in north-east England represents a period of geological history during which perhaps as much as 2000 m of Mesozoic strata, and some late Carboniferous rocks, were removed by erosion (Holliday, 1993). The long-term evolution of the landscape of northern England prior to the last glaciation is not well known. The dominant control on topography in the district, however, is bedrock lithology; the Fell Sandstone forms several notable escarpments and higher ground in the south and centre of the district is formed by the resistant Great Whin Sill and Great Limestone.
It is likely that the district experienced several periods of glaciation during the Pleistocene, though the deposits seen today date only from the latest (Late Devensian) glaciation. Any earlier superficial deposits have either been removed or recycled by subsequent glaciations. Despite the widespread occurrence of superficial deposits they are typically very poorly exposed. Information on their nature is derived largely from records of boreholes and temporary sections, notably during the recent survey from exposures in opencast coal sites.
The rockhead surface has an appreciably greater relief than the present-day surface (Anson and Sharp, 1960; Lawrence and Jackson, 1986, 1990b). Buried valleys that cannot be delineated easily at the surface occur beneath the coastal lowlands, where ice flowed south-eastwards across pre-existing valleys forming the lower reaches of the rivers Coquet, Wansbeck and Lyne, which before the last glaciation mostly flowed directly towards the North Sea. Some buried channels have been graded to a base level more than 30 m below present sea level, but others have 'humped' longitudinal profiles and clearly formed subglacially. In the Broomhill area [NU 248 000], bores have proved over 45 m of superficial deposits. Exploratory drilling to prove the Shilbottle Coal at Whittle Colliery has shown that a buried channel exists in the area to the north of Longframlington with superficial deposit thickness up to 59 m (NU10SW/40) [NU 1415 0323]. The nature of the deposits in this channel is described in borehole records only as 'boulder clay with sandstone'.
Till covers about 75 per cent of the district. The glacigenic deposits in the west probably represent the North Pennines Glacigenic Subgroup deposited from an ice stream flowing generally eastwards (Stone et al., 2010). Tills laid down by Pennine ice are generally dark brown to grey in colour, with cobbles and boulders dominated by Carboniferous clasts including sandstone, limestone, mudstone, ganister and coal, but with some far-travelled clasts from Scotland and the Lake District (wacke-sandstone and siltstone, granite and granodiorite). Small areas of hummocky glacial deposits have been mapped north-east of Spylaw [NZ 047 979]. The glacial deposits in the east of the district are part of the North Sea Coast Glacigenic Subgroup, deposited by ice sourced mainly in the Scottish Borders and central Scotland. The ice flowed south-eastwards along the eastern coast of England as far as north Norfolk and formed an extensive coastal plain underlain by lodgement till deposited beneath a wet-based ice-sheet. This subgroup includes a suite of brownish grey to reddish brown deposits that contain clasts derived predominantly from the Carboniferous rocks of north-east England (yellow, grey and white sandstones, mudstone, limestone, ganister, coal, dolerite), and Lower Palaeozoic and Devonian rocks of southern Scotland (wacke-sandstone and mudstone, granite, andesite, red sandstone). Where unweathered, this till is typically a stiff, grey to greyish brown, silty, sandy or stony clay. It commonly exhibits a reddened topmost layer up to 8 m thick that is traditionally interpreted as a separate stratigraphical unit ('Upper Boulder Clay'), although it has also been described as a post-depositional weathering profile (Eyles and Sladen, 1981). The tills contain intraformational sands and gravels, and beds of laminated glaciolacustrine silt and clay up to 8 m thick. Glaciofluvial deposits, mainly composed of sand and gravel, were formed in subglacial meltwater channels. These sediments have commonly been partially eroded and incorporated within overlying till, giving rise in places to an apparent tripartite stratigraphy of an upper red till, middle sands and gravels and a lower grey till (Hughes et al., 1998).
Although the location of many buried valleys is known from borehole records, it is rarely possible to examine in detail the complex sequences that fill them. However, in 1999, excavation of the Maiden's Hall opencast site [NZ 235 980] revealed an approximately 30 m-deep, west–east orientated linear depression cut into bedrock beneath till of the North Sea Coast Glacigenic Subgroup (Stone et al., 2010). A basal unit of weathered, shelly till of possible pre-Devensian age is confined to pockets at the base of the incision, and is overlain by iron-stained gravel deposited in a braided river environment. The overlying succession includes seven or more distinct cyclic sequences composed of units of laminated silt and clay, sand, gravel and diamicton, many of the last containing well-dispersed clasts typical of deformation till (Plate 4). The uppermost till unit oversteps the entire succession within the linear depression to rest on bedrock at its margin. The cyclic sequences were probably formed entirely subglacially as a result of a corresponding number of surge events of the North Sea ice stream across a 'tunnel valley', although readvances of ice across an ice-marginal lake confined within the channel cannot be ruled out (Stone et al., 2010).
There are several glacial drainage channels ('dry valleys') within the district. The best examples are Selby's Cove [NZ 021 976] on the southern face of the Simonside Hills and the flat-bottomed steep-sided channel west of Rothley Crags [NZ 042 887]. It is likely that these channels were formed in the vicinity of the ice margin at transitory positions of actively retreating ice. A well-defined esker west of Blueburn [NZ 050 952] and kame-like deposits between Spylaw Burn and Nunnykirk represent other ice-marginal features preserved within the district.
Fowler (1936) recognised that fluting and grooving are common on weathered faces within the Fell Sandstone Formation and that widening of horizontal and vertical joints has produced small caves, as on the southern flank of the Simonside Hills, 500 m south of the summit [NZ 023 981]. Self and Mullan (2005) have compared the weathering in appearance to karst: possible pan forms (kamenitza) are recorded from Little Church Rock, Simonside [NZ 026 993] together with a cave remnant [NZ 026 998]. Solution along grain boundaries is believed to have weakened the fabric of the rock, allowing individual grains to fall out or be washed away, a process that has taken place since the Devensian glaciation.
River terrace deposits, composed of sand and gravel, occur along the rivers Font and Coquet. Terrace surfaces lie between 3 m and 10 m above present river level. In places, several individual terraces are identifiable but no lateral correlations have been established.
Alluvium occurs as narrow, discontinuous tracts flanking rivers and small streams throughout the district. The alluvial deposits usually do not exceed 3 m in thickness and generally consist of laterally variable clay, silt and fine-grained sand. Lenses of peat or peaty clay may be present and gravel is common at the base. Alluvial fan deposits occur locally, where tributaries meet main streams. These small deltas of alluvial material are generally similar in composition to the nearby river alluvium. The largest peat bogs in the district are in the Harwood Forest south of the Simonside Hills.
The coastal zone of Northumberland is an important area for late Quaternary studies, in particular research on sea level and other coastal changes. It occupies a pivotal position between areas of postglacial crustal uplift in the north and areas of net subsidence to the south, and is thus a key area for the establishment of sea-level index points. Relative sea-level change shows a mid Holocene sea-level maximum about 2.5 m above present in north Northumberland and about 0.5 m above present at Warkworth in the north of the Rothbury district (Shennan et al., 2000). Sandy beach and dune environments are characteristic of the coast. Blown sand forms a narrow fringe of dunes all along the coast with maximum height of 19 m above sea level at Hemscott Hill [NZ 2760 9622]. The dunes are draped over rock and till and in places peat layers of mid to late Holocene age are preserved beneath the dunes. Peat layers are exposed in the intertidal zone at Amble and Druridge Bay. An organic shelly layer in the dune sequence at Low Hauxley [NU 286 033], immediately east of the district, has been dated at 980 ± 50 BP (Bridgland et al., 1999). Marine beach deposits, composed largely of sand and gravel, occur in the intertidal zone. Small areas of grey clay and silt assigned to raised marine deposits, tidal river or creek deposits and salt-marsh deposits are present in the Coquet valley north of Amble. Substantial areas of artificially modified ground have been mapped within the district. Worked ground indicates areas of man-made excavation, created either for engineering purposes or in the pursuit of mineral resources. Only the largest areas are shown and these relate to sites of bedrock extraction, mainly disused limestone and sandstone quarries within the Yoredale Group outcrop, but including the Stobswood North Extension opencast coal site [NZ 230 947] that was active at the completion of survey in 2007. Made ground is where the natural surface has been artificially raised. In the western half of the district made ground typically consists of small patches of waste rock and earth from quarry workings. In the coalfield it is largely of two types: waste material excavated from opencast sites as at Hebron [NZ 200 895], consisting largely of mudstone, siltstone and sandstone, or spoil from former underground coal mining, much of which has subsequently been landscaped as at Ashington [NZ 270 890]. Made ground also includes areas raised for engineering purposes, such as road embankments. Infilled ground indicates areas where excavations have been filled, predominantly opencast sites backfilled using waste rock previously excavated from the workings as overburden.
Chapter 3 Applied geology
Earth science factors are important in landuse planning and have been assessed in detail in the most urbanised part of the district, in the south-east (Lawrence and Jackson, 1986, 1990a). Their early consideration can help to ensure that developments are compatible with ground conditions and that mineral resources are not sterilised. The principal land use in this predominantly rural district is farming. The western part of the district lies within the Northumberland National Park and tourism plays a large part in the local economy, and consideration of geodiversity and the conservation of geological and landscape features is important in maintaining the attraction of the area (Lawrence et al., 2007).
There is a long history of mining and quarrying in the district, which in the east includes the northernmost part of the Northumberland and Durham Coalfield, here referred to as the 'Northumberland Coalfield'. Deep mining of coal has now ended, though limited production continues from opencast mines. Centuries of coal mining have had a profound influence on the social and economic history of the district and have affected the environmental and engineering characteristics of the land. Other local mineral products include limestone, iron ore, building stone, sand and gravel, roadstone, and brick and refractory clay. Lead ore has been mined on a very small scale.
Energy resources
Coal mining in this part of Northumberland extends back over many centuries. By far the greatest coal production from the district was obtained from the Westphalian Coal Measures of the Northumberland Coalfield. In the Pennine Coal Measures Group at least 26 named seams are known to have been worked, many of them extensively (Figure 6). However, with the closure in 2005 of Ellington Colliery, to the south of the present district, all underground coal mining in the Northumberland Coalfield ceased, although resources of coal remain at depth. Lawrence and Jackson (1990b) summarise the important aspects of the Westphalian coals in the south of the district, together with comments on the extent of extraction of individual seams. Extensive opencast extraction of the Coal Measures has taken place during the past half century and continues on a limited scale; exploration for workable reserves continues. During the 1980s the Butterwell site [NZ 213 895] in the south of the district was the largest opencast coal site operating in Europe with an output in excess of one million tonnes per annum.
Coal has also been worked at stratigraphical levels beneath the Pennine Coal Measures Group. The oldest coals worked were those towards the top of the Tyne Limestone Formation in the Debdon area, north of Rothbury, in strata equivalent to the 'Scremerston Coal Group' of the area to the north-east. The most important coal in the Alston Formation is the Shilbottle Coal, a very high quality bituminous coal. This coal was up to 0.8 m thick in Whittle, where it was worked until 1987; resources probably still remain. The Top (or Townhead) Coal was worked locally for calcining of the overlying Great Limestone, notably along its outcrop north of the Font valley. Several coals in the Stainmore Formation were locally sufficiently thick to have been mined, both underground and at the surface in the south-west of the district. The Little Limestone Coal was worked extensively underground along the line of its outcrop between Brinkburn [NZ 117 983] and Rothley Shield [NZ 041 907], where it was also surface-mined in the 1980s. Higher in the formation, the Coatyards coals were worked underground at a colliery west of Longwitton [NZ 062 897] and surface-mined at Rothley Cross Roads [NZ 050 898]. The Netherwitton coals were mined underground and later in an opencast site at Folly House [NZ 093 930] and the Stanton Coal was mined near Stanton House [NZ 135 910].
Throughout the Northumberland Coalfield coal rank typically increases southwards in response to the higher geothermal gradients associated with the basement strata of the Alston Block, including the North Pennine Batholith. Within the Rothbury district coal rank is relatively low (84–86 per cent carbon) with the exception of the coals immediately north of the Causey Park Dyke near Red Row, which, very locally, have higher rank owing to the heating effect of the igneous intrusion. Coals from this part of the Northumberland Coalfield were formerly important as household coals and for gas making. More recently, they have supplied the power station market both locally and further afield and have, in addition, been blended with other coals for metallurgical and other uses.
The coalbed methane potential of the Northumberland and Durham Coalfield is likely to be low due to low adsorbed methane, the extent of former mining and the lack of a thick impervious cover sequence (Glover et al., 1993).
Other mineral resources
A variety of industrial and bulk minerals within the district have been exploited, though none were being worked at the time of survey. Limestone has been worked on a very modest scale producing small amounts of lime mainly for nearby agricultural use. Several limestones (the 'cementstones') in the higher beds of the Ballagan Formation were also worked for building lime and the quarry at Glebe [NU 0515 0055] was used for roadstone. The Great Limestone has been the most extensively worked and was also quarried for use as roadstone, most recently at Greenleighton Quarry [NZ 034 920]. Dolerite of the Great Whin Sill provides a good roadstone, used widely in northern England. It was formerly worked at Wards Hill Quarry [NZ 079 965] and, on a larger scale, at Ewesley [NZ 061 942] (Plate 5). A resumption of dolerite quarrying within the district is unlikely. Several of the dykes have also been worked on a small scale. Few of the numerous sandstone units within the Carboniferous succession of the district are of sufficient quality for use as building stone, although a number have been quarried for this purpose on a small scale for local use. One of the thicker and more durable sandstones near the base of the Fell Sandstone Formation was worked at Pondicherry Quarry [NU 045 018] (Plate 6) to provide stone for Rothbury and beds at about the same stratigraphical level were extracted nearby at Cragside [NU 074 022] for most of the buildings on the estate. Mudstone and siltstone from the Pennine Coal Measures Group have been worked for brick materials, commonly as a by-product of coal mining. Fireclays were taken from beneath the High Main and Five-Quarter seams at Ashington Colliery. The mudstone roof of the Low Main Coal was also used extensively, as at Pegswood Colliery [NZ 2295 8738]. More recently fireclays have been extracted in conjunction with opencast coal mining. Pegswood Moor opencast site [NZ 207 877] was providing clay for the manufacture of pottery at Bardon Mill, in the Tyne valley to the south of the district, during the recent survey. Some Quaternary deposits have been worked for brick and tile making, but this industry is now almost extinct.
Sand and gravel has been worked, mainly on a small scale, from several small pits in the larger areas of superficial deposits, but is not of any great importance within the district.
There are bands and concretions of ironstone in the form of siderite mudstone ('clay ironstone') within mudstone at many levels in the Carboniferous rocks of the district, but these are generally too small to be worth working. One exception is in the mudstone overlying the Three Yard Limestone in the Coquet valley where ironstone nodules were quarried near Pauperhaugh [NZ 104 995] and smelted at the adjacent Brinkburn Ironworks.
Mining for lead ore has been tried on a small scale within the district. Smith (1923) reported several small, isolated lead veins in the area including those on the northern slopes of the Simonside Hills near Whittondean [NZ 055 999], at Redpath mine on the Fallowlees Burn [NZ 010 930], and at Hartington Farm [NZ 023 883]. The northwestern part of the district was included in a reconnaissance geochemical drainage survey across the lower Carboniferous rocks of the Northumberland Trough that identified not only the known mining areas but also a number of other areas with anomalously high metal values (Bateson et al., 1983). High barium values in concentrates were obtained from an area to the south of Rothbury at Ewesley Farm [NZ 061 922]. Subsequent soil sampling also revealed high levels of barium, and it is considered that unexposed barium mineralisation exists in the area, probably associated with a fracture cross-cutting dolerite of the Great Whin Sill.
Water resources
Public water supply is the dominant use of abstracted water in the area, comprising approximately 80 per cent of the total licensed quantity for the area. A smaller proportion is abstracted for industrial and commercial use, spray irrigation and domestic and agricultural purposes (Environment Agency, 2003). Public water supply is derived mainly from surface water sources, on the River Coquet at Warkworth Water Treatment Works (WTW) [NU 237 059], yielding about 45 000 m3/d, and on the River Font at the Fontburn Reservoir WTW [NU 049 938] yielding about 15 000 m3/d. Some groundwater (about 3000 m3/d) abstracted in the Rothbury area is put into supply at Tosson Springs WTW [NU 032 002].
Several farm boreholes tap aquifers for local supplies. Intensive farming is rarely practised in the area due to the poor farming land and so the aquifers have been little affected by agricultural contamination. The Fell Sandstone Formation is the major aquifer within the district and has provided water to the public supply for more than a century. Studies of the Fell Sandstone in the Berwick area, to the north, show it to be hydrogeologically complex and multilayered, with several discrete sandstone aquifers separated by laterally persistent layers of very low permeability mudstone (Turner et al., 1993). Such mudstone layers are not present in the Rothbury district, but water flow is affected by bedding discontinuities and although there is some water flow in thin, coarse-grained horizons, most water flow is through fissures (Bell, 1978; Hodgson and Gardiner, 1971; Turner et al., 1993). Thick sandstone and limestone units within the Inverclyde, Yoredale and Coal Measures groups act as individual aquifers confined by low permeability mudstone. However, although intergranular permeability occurs in some coarse-grained sandstones, groundwater storage and movement is again predominantly within fissures. Major fissures occur along fault planes and minor fissures are present as joints and along bedding planes. Most close with depth so that permeability decreases downwards within a single aquifer and deeper aquifers are generally less permeable than shallower ones. Borehole yields depend on the number and size of fractures encountered in a productive horizon and many boreholes penetrate more than one productive horizon.
Large springs may be associated with dykes intruded into fault zones. Near Rothbury, Cartington Spring [NU 0417 0438] and Tosson Springs [NU 030 005] jointly yield in excess of 9000 m3/d. Other springs include the White Park Well [NZ 039 993] and Swan Well on the slopes of the Simonside Hills, the latter yielding about 500 m3/d.
In the past, mine dewatering operations at Linton Colliery [NZ 262 914] yielded sufficient supply to serve Lynemouth just to the east of the district. Mine workings in the Northumberland Coalfield were extensively interconnected and pumping maintained a water table at about 150 m below the ground surface. With the end of underground mining activity, mine dewatering in the coalfield has declined and groundwater levels have risen (with the associated potential for flooding by acidic ferruginous mine waters), although some pumping accompanies activities at opencast sites. Chemical analyses of mine drainage water reflect the presence of infiltrated sea-water, but suggest that normal groundwater is probably a sodium sulphate type with subordinate amounts of chloride.
The main significance of Quaternary and Holocene deposits in the context of regional hydrogeology is that they form a confining bed over the main aquifers and control recharge and influence water chemistry. Aquifers also occur within sand and gravel deposits and are confined by interbedded silts, clays and till. Supplies from these sources are likely to fluctuate rapidly in response to variations in precipitation. The water is generally hard, due to bicarbonate or sulphate concentration, and may be ferruginous. Brackish water occurs in the marine deposits along the coast.
Ground conditions
The bedrock of the district generally provides adequate bearing capacity for domestic and light industrial structures using normal foundations. Sandstone, limestone and the igneous rocks in the district are typically strong to very strong whereas siltstone and mudstone are weak to moderately strong. However, the strength of the rock mass is significantly reduced by the presence of discontinuities such as joints and faults, and of the rock material by the effects of weathering and groundwater. Some thicker limestones, notably in the Alston Formation, contain swallow holes and, unusually, the Fell Sandstone Formation locally contains karst in the form of short caves, small elliptical tubes and protocaves.
The alignment of large excavations, such as those for opencast coal, should take into consideration the location of faults. Faces aligned near-parallel to faults tend to fail, whereas faces aligned near perpendicular to faults are more stable (Hughes and Clarke, 2002).
The superficial deposits have more variable foundation conditions. Peat is highly compressible and unsuitable for foundations. Buildings founded on alluvium containing peat or organic-rich layers are likely to suffer from differential settlement. The coarse-grained superficial deposits (much of the alluvium, river terrace deposits and glaciofluvial sand and gravel) provide adequate foundation media for most domestic or light industrial purposes. However, blown sand is not suitable for foundations, unless stable. All of these deposits can be excavated relatively easily, but trench supports may be necessary and dewatering will be needed in excavations below the water table. Cuttings will require drainage measures to remove water from perched water tables and to relieve relatively high water pressures in confined aquifers, such as sands and gravels overlain by less permeable clays, in order to avoid slope failure or heave in excavations.
Most of the glacial deposits provide good foundation conditions. Till, when fresh, is a firm to stiff or very stiff gravelly sandy clay with cobbles and sometimes boulders, which occur most commonly at or near the base. Its weathered upper layer (commonly 3 to 4 m but locally up to 8 m thick) is generally weaker and fissured. The top 0.5 to 2 m will soften after wet weather and may need to be removed prior to construction. Within the till are beds of loose to very dense, sometimes thickly interbedded, glaciofluvial sand or gravelly sand up to 3 m thick, and beds of sometimes soft but generally firm to stiff, laminated glaciolacustrine silt or clay up to 8 m thick. The sand and gravel and laminated clay beds within the till play an important role in determining ground behaviour, especially in excavations. In slopes, the sand intercalations can be sources of water, forming seepages that soften the till, sometimes resulting in the formation of cavities or in the failure of the slope. Excavated slopes containing laminated clay may become unstable, as the clays have a relatively low shear strength and a tendency to soften rapidly when unloaded, particularly where associated with beds of water-bearing sand. Slope failures are more likely at or near the base of excavations. Spoil heaps and embankments may fail if they are constructed on laminated clay and silt. Fine-grained deposits are typically of low to intermediate plasticity and so changes in water content are unlikely to cause volume-change (shrink/ swell) problems. The superficial deposits generally have low sulphate content and are slightly acidic to alkaline (pH 6.7 to 8.1), although the pH of peat can be as low as 4. Differential settlement is possible on till where foundations bridge the typical till and boulders or rock rafts within it (Eyles and Sladen, 1981).
The artificial deposits display a wide range of geotechnical properties, as they contain a variety of natural and man-made materials and have been deposited using different methods. Leachate plumes, solid and liquid contaminants, potentially explosive and noxious gases and uneven ground settlement, which can be associated with artificial deposits, have implications for health and construction. Site history and the implication for contamination should be assessed as appropriate during desk study and site investigation.
Potential geological hazards and environmental geology
Small landslides occur within over-steepened, generally water-bearing, glacial deposits in the banks of the River Coquet and its tributaries in the north of the district. Coal extraction and undermining has taken place across most of the outcrop of the Pennine Coal Measures Group, locally within the Stainmore and Alston formations, notably in the Shilbottle Coal, and in the Tyne Limestone Formation. It is essential that, prior to any development, the type, extent and depth of any abandoned mining and the existence of abandoned mine shafts and adits are determined accurately. However, this can be difficult to ascertain for the oldest workings as old mines and bell pits were commonly documented poorly or not at all. Although a large number of shafts and adits have been recorded in the district, it is likely that many more are present for which no records exist. Many old shafts may be inadequately filled or capped and may present significant hazards. The collapse of these openings is possible. Information regarding disused shafts and adits and advice on treatment is available from the Coal Authority.
Gases, notably methane but also including hydrogen sulphide, carbon dioxide and carbon monoxide, may be generated or trapped in old workings or in permeable rock beneath a low permeability cover such as till. Discharge into the atmosphere can occur through natural or man-made fissures, shafts or adits, through collapsed workings or through permeable rock such as sandstone. Gas emissions are more likely during periods of low atmospheric pressure. Rising water levels will also displace accumulations of gas generated and trapped in old workings. Where there is no continuous, impermeable cover discharge tends to take place safely over a wide area, with rapid dilution by the atmosphere. However, potentially dangerous emissions may occur where an impermeable seal that confines significant volumes of gas is breached. The construction of boreholes, pits and trenches, and other deep excavations can form pathways for the gases and areas in which they may collect.
Low-lying areas adjacent to rivers are potentially at risk from flooding. An indication of those areas that are most susceptible is given by the extent of alluvium on the map and by Environment Agency flood-risk maps. Low river terraces, especially where eroded, can also be inundated during exceptional weather events. Flooding may also follow the cessation of pumping in the deep mines; the regional recovery of the water table can result in the discharge to surface of large volumes of acid water. Water may reach the surface through abandoned mine openings or boreholes, or through faulted and fissured ground. Such mine water can contaminate rivers by precipitation of heavy metal compounds or ochreous hydrated iron hydroxide deposits, or both.
Little information is available on radon concentrations in the district. However, in a national assessment Appleton and Ball (1995) considered the area of the Yoredale Group outcrop as one of moderate radon potential; the Pennine Coal Measures Group outcrop was classified as an area of low to moderate radon potential though with local areas susceptible to moderate or high levels of radon emissions. However, cover of clayey superficial deposits will generally provide a barrier against radon transmission to the surface.
Conservation and geological heritage
The geological heritage of the district forms a resource for tourism, education and scientific research and is also an issue in planning and development. Greenleighton [NZ 034 920] and Glebe [NU 052 005] quarries are designated as geological Sites of Special Scientific Interest (SSSI). Greenleighton is of importance for the rich marine shell faunas contained in the Great Limestone and the overlying mudstone and the Ballagan Formation in Glebe Quarry contains one of the richest and thickest deposits of continuous algal (oncolite) limestone to be found at any site representing the British lower Carboniferous. At the time of writing, a number of Sites of Nature Conservation Importance (SNCI) and other sites with significant educational value are under consideration for designation as Regionally Important Geological and Geomorphological Sites (RIGS). Details of these may be obtained from the Royal Society for Nature Conservation, The Kiln, Mather Road, Newark NG12 1WT.
Information sources
Sources of further geological information held by the British Geological Survey relevant to the Rothbury district and adjacent areas are listed here. Enquiries concerning geological data for the district should be addressed to the National Geoscience Information Service, BGS, Edinburgh. The current BGS Catalogue of Geological Maps and Books is available on request and at the BGS website (www.bgs.ac.uk). BGS maps, memoirs, books, and reports relevant to the district may be consulted at BGS and some other libraries. They may be purchased from the BGS Sales Desk, or via the bookshop on the BGS website. This website also provides details of BGS activities and services, and information on a wide range of environmental, resource and hazard issues. Searches of indexes to some of the materials and documentary records collections can be made on the BGS website.
Geological enquiries, including requests for geological reports on specific sites, should be addressed to the BGS Enquiry Service at Edinburgh.
Maps
- Geological maps
- 1:10 000 and 1:10 560
- The maps at 1:10 000 scale covering all or part of 1:50 000 Sheet 9 Rothbury are listed here, with the surveyors' initials and dates of survey. The surveyors were: I Jackson, D J D Lawrence and B Young. The maps are not published, but are available for consultation in the BGS libraries. Coloured print-on-demand copies are available for purchase from BGS sales desks. The earlier maps are available for public reference at the BGS libraries in Edinburgh and Keyworth.
- Digital geological map data
- The 1:10 000 and 1:50 000 scale BGS maps of the Rothbury district are available in digital form, which allows the geological information to be used in GIS applications.
| Map number | Surveyor | Date of Survey |
| NT90NE (Part) | BY | 1997 |
| NT90SE (Part) | BY | 1997 |
| NU00NE (Part) | BY | 1997 |
| NU00SW | BY | 1997 |
| NU00SE | BY | 1997 |
| NU10NW (Part) | BY | 1997 |
| NU10NE (Part) | BY, DJDL | 1996, 2006 |
| NU10SW | BY | 1997–98 |
| NU10SE | BY, DJDL | 1998–99, 2008 |
| NU20NW (Part) | BY, DJDL | 1999, 2008 |
| NU20NE (Part) | BY, DJDL | 1999, 2008 |
| NU20SW | BY, DJDL | 1999 |
| NU20SE | DJDL | 1991–92, 2005–06 |
| NY98NE (Part) | BY | 1999 |
| NY99NE (Part) | BY | 1997 |
| NY99SE (Part) | BY | 1998–99 |
| NZ08NW (Part) | IJ, DJDL | 1993 & 1998 |
| NZ08NE (Part) | DJDL | 1998 |
| NZ09NW | BY | 1997 |
| NZ09NE | DJDL | 1997–98 |
| NZ09SW | DJDL | 1994–1997 |
| NZ09SE | DJDL | 1995 |
| NZ18NW (Part) | DJDL | 1996 |
| NZ18NE (Part) | DJDL | 1986 |
| NZ19NW | DJDL, BY | 1998–99 |
| NZ19NE | BY, DJDL | 1998, 2002 |
| NZ19SW | DJDL | 1995–2000 |
| NZ19SE | DJDL, BY | 1995–2002 |
| NZ28NW | DJDL | 1988–89 |
| NZ28NE | IJ | 1988 |
| NZ29NW | DJDL | 1992–2001, 2007–08 |
| NZ29NE | DJDL | 1992–2001 |
| NZ29SW | DJDL | 1992–1999, 2007 |
| NZ29SE | IJ, DJDL | 1991–2002 |
| These data must be licensed for use. Details are available from the Intellectual Property Rights Manager at BGS, Keyworth. Details are given on the BGS website. |
Geochemical atlas
Southern Scotland and part of northern England (1993)
The Geochemical Baseline Survey of the Environment (G-Base) is based on the collection of stream sediment and stream water samples. The geochemical atlas is also available in digital form (on CD-ROM) under licensing agreement. BGS offers a client-based service for interactive interrogation of G-Base data.
Geophysical maps
1:625 000
Gravity anomaly map of the UK: North sheet (2007)
Magnetic anomaly map of the UK: North sheet (2007)
Geophysical interpretations
Regional Geophysics of Southern Scotland and Northern England. Version 1.0 on CD-ROM (Kimbell, et al., 2006)
Hydrogeological maps
1:100 000
Groundwater Vulnerability of West Northumberland (Sheet 1) (1990) Groundwater Vulnerability of Coastal Northumberland (Sheet 2) (1990)
Books
British regional geology guides
Northern England, Fifth edition (2010)
Memoirs
The geology of the country around Rothbury, Amble and Ashington (Fowler, 1936)
Popular geology
The Geodiversity Audit and Action Plan for the Northumberland National Park and surrounding area (Lawrence et al., 2007) describes the geology of the north-western part of the district and its links with other aspects of the natural and cultural heritage.
Documentary records collections
Collections of records of borehole and site investigations relevant to the district are available for consultation at the BGS, Edinburgh, where copies of most records can be purchased. BGS also maintains a mining and quarrying database.
Hydrogeological data
Records of water wells, springs, and aquifer properties held at BGS Wallingford can be consulted through the BGS Hydrogeology Enquiry Service.
Geophysical data
These data are held digitally in the National Gravity Databank and the National AeroMain magnetic Databank at BGS Keyworth.
BGS lexicon of named rock units Definitions of the stratigraphical units shown on BGS maps, including those named on Sheet 9 (Rothbury), are held in the BGS Lexicon of named rock units database, which can be consulted on the BGS website.
BGS photographs
The photographs used in this Sheet Explanation are part of the National Archive of Geological Photographs, held at BGS in Keyworth and Edinburgh.
Materials collections
Information on the collections of rock samples, thin sections, borehole samples (including core) and fossil material can be obtained from the Chief Curator, BGS Keyworth. Indexes can be consulted on the BGS website.
References
Most of the references listed here are held in the libraries of the British Geological Survey at
Keyworth (Nottingham) and Edinburgh. Copies of BGS publications can be obtained from the sources described in the previous section. The BGS Library may be able to provide copies of other material, subject to copyright legislation. Links to the BGS Library catalogue and other details are provided on the BGS website.
Anson, W W, and Sharp, J J. 1960. Surface and rock-head relief features in the northern part of the Northumberland coalfield. Kings College, University of Durham, Department of Geography Research Series, No. 2 (Newcastle upon Tyne).
Appleton, J D, and Ball, T K. 1995. Radon and background radioactivity from natural sources: characteristics, extent and the relevance to planning and development in Great Britain. British Geological Survey Technical Report, WP/95/2.
Bateson, J H, Johnson, C C, and Evans, A D. 1983. Mineral reconnaissance in the Northumberland Trough. Institute of Geological Sciences Mineral Reconnaissance Programme Report, No. 62.
Bell, F G. 1978. Petrographical factors relating to porosity and permeability in the Fell Sandstone. Quarterly Journal of Engineering Geology and Hydrogeology, Vol. 11, 113–126.
Brand, P J. 1991. Report on faunas from some boreholes drilled on 1:10 000 sheet NZ 29. British Geological Survey Technical Report, PD/91/227.
Brand, P J. 2011. Serpukhovian and Bashkirian (Carboniferous, Namurian and basal Westphalian) faunas of Northern England. Proceedings of the Yorkshire Geological Society, Vol. 58, 143–165.
Bridgland, D R, Horton, B P, and Innes, J B (editors). 1999. The Quaternary of North-east England: Field guide. (London: Quaternary Research Association.) ISBN 0907780687
Chadwick, R A, Holliday, D W, Holloway, S, and HulBert, A G. 1995. The structure and evolution of the Northumberland–Solway Basin and adjacent areas. Subsurface Memoir of the British Geological Survey.
Cornwell, J D, and Evans, A D. 1986. Magnetic surveys and structures in the Whin Sill, northern England. 65–74 in Geology in the real world — the Kingsley Dunham volume. Nesbitt, R W, and Nichol, I (editors). (London: The Institution of Mining and Metallurgy.)
De Paola, N, Holdsworth, R E, McCaffrey, K J W, and Barchi, M R. 2005. Partitioned transtension: an alternative to basin inversion models. Journal of Structural Geology, Vol. 27, 607–625.
Dean, M T. 1999. Fauna of the Great Limestone and associated beds exposed at Greenleighton Quarry (disused), Northumberland. British Geological Survey Technical Report, WH/99/42R.
Dean, M T. 2001. England Sheet 9 (Rothbury): a palaeontological and biostratigraphical summary. British Geological Survey Internal Report, IR/01/10.
Dean, M T, and Brand, P J. 1998. English Sheet 14 (Morpeth): a palaeontological and biostratigraphical summary. British Geological Survey Technical Report, WH/98/77R.
Environment Agency. 2003. The Northumberland Rivers Catchment Abstraction Strategy. (Leeds: Environment Agency.)
Eyles, N, and Sladen, J A. 1981. Stratigraphy and geotechnical properties of weathered lodgement till in Northumberland, England. Quarterly Journal of Engineering Geology and Hydrogeology, Vol. 14, 129–141.
Fairbairn, R A. 1978. Lateral persistence of beds within the Great Limestone (Namurian, E1) of Weardale. Proceedings of the Yorkshire Geological Society, Vol. 41, 533–544.
Fairbairn, R A. 1980. The Great Limestone (Namurian) of south Northumberland. Proceedings of the Yorkshire Geological Society, Vol. 43, 159–167.
Fowler, A. 1936. The geology of the country around Rothbury, Amble and Ashington. Memoir of the Geological Survey of Great Britain, Sheets 9 and 10 (England and Wales). ISBN X780009141
Frost, D V, and Holliday, D W. 1980. Geology of the country around Bellingham. Memoir of the Geological Survey of Great Britain, Sheet 13 (England and Wales). ISBN 0 11 884137
Glover, B W, Holloway, S, and Young, S R. 1993. An evaluation of coalbed methane potential in Great Britain. British Geological Survey Technical Report, WA/93/24.
Goulty, N R. 2005. Emplacement mechanism of the Great Whin and Midland Valley dolerite sills. Journal of the Geological Society of London, Vol. 162, 1047–1056.
Hodgson, A V. 1978. Braided river bedforms and related sedimentary structures in the Fell Sandstone Group (Lower Carboniferous) of North Northumberland. Proceedings of the Yorkshire Geological Society, Vol. 41, 509–532.
Hodgson, A V, and Gardiner, M D. 1971. An investigation of the aquifer potential of the Fell Sandstone of Northumberland. Quarterly Journal of Engineering Geology and Hydrogeology, Vol. 4, 91–109.
Holliday, D W. 1993. Mesozoic cover over northern England: interpretation of apatite fission track data. Journal of the Geological Society of London, Vol. 150, 657–660.
Howse, R. 1878. Notice of discovery of Archanadon jukesi in the Lower Carboniferous rocks of Northumberland with plate. Transactions of the Natural History Society of Northumberland and Durham, Vol. 8, 173–175.
Hughes, D B, and Clarke, B G. 2002. Faulting and slope failures in surface coal mining — some examples from North East England. Geotechnical and Geological Engineering, Vol. 20, 291–332.
Hughes, D B, Clarke, B G, and Money, M S. 1998. The glacial succession in lowland Northern England. Quarterly Journal of Engineering Geology and Hydrogeology, Vol. 31, 211–234.
Johnson, G A L. 1980. Robson's geology of North East England. Special Publication of the Natural History Society of Northumbria.
Johnson, G A L, Hodge, B L, and Fairbairn, R A. 1962. The base of the Namurian and of the Millstone Grit in north-eastern England. Proceedings of the Yorkshire Geological Society, Vol. 33, 341–362.
Jones, N S. 1996. Report on the sedimentology of Namurian strata examined at selected localities on the Morpeth and Rothbury sheets. British Geological Survey Technical Report, WH/96/87R.
Kimbell, G S, Carruthers, R M, Walker, A S D, Williamson, J P, Busby, J P, McDonald, A J W, Marsh, S H, and Stone, P. 2006. Regional geophysics of southern Scotland and Northern England. [CD-ROM]. Version 1.0. Keyworth, Nottingham: British Geological Survey.
Kirton, S R, and Donato, J A. 1985. Some buried Tertiary dykes of Britain and surrounding waters deduced by magnetic modelling and seismic reflection methods. Journal of the Geological Society of London, Vol. 142, 1047–1057.
Lawrence, D J D, and Jackson, I. 1986. Geology of the Ponteland–Morpeth district. Research Report of the British Geological Survey. ISBN X780130951
Lawrence, D J D, and Jackson, I. 1990a. Geology and land-use planning: Morpeth–Bedlington–Ashington. Part 1 Land-use planning. British Geological Survey Technical Report, WA/90/14.
Lawrence, D J D, and Jackson, I. 1990b. Geology and land-use planning: Morpeth–Bedlington–Ashington. Part 2 Geology. British Geological Survey Technical Report, WA/90/19.
Lawrence, D J D, Arkley, S L B, Everest, J D, Clarke, S M, Millward, D, Hyslop, E K, Thompson, G L, and Young, B. 2007. Northumberland National Park — Geodiversity audit and action plan. British Geological Survey Commissioned Report, CR/07/037N. ISBN 978 0 85272 599 3
Lebour, G A. 1876. On the larger divisions of the Carboniferous system in Northumberland. Transactions of the North of England Institute of Mining and Mechanical Engineers, Vol. 25, 225–237.
Lee, M K. 1982. Regional geophysics of the Cheviot area. Report of the Environmental Protection Unit, Institute of Geological Sciences, ENPU82-2.
Liss, D, Owens, W H, and Hutton, D H W. 2004. New palaeomagnetic results from the Whin Sill complex: evidence for a multiple intrusion event and revised virtual geomagnetic poles for the late Carboniferous for the British Isles. Journal of the Geological Society of London, Vol. 161, 927–938.
McNestry, A. 1993. Carboniferous palynology of 43 samples from the Stobswood No. 2 Borehole. British Geological Survey Technical Report, WH/93/34R.
Mills, D A C, and Hull, J H. 1968. The Geological Survey borehole at Woodland, Co. Durham (1962). Bulletin of the Geological Survey of Great Britain, Vol. 28, 1–37.
Poole, G, Whetton, J T, and Taylor, A. 1935. Magnetic observations on concealed dykes and other intrusions in the Northumberland coalfield. Transactions of the North of England Institute of Mining Engineers, Vol. 89, 34–47.
Purnell, M A, and Cossey, P J. 2004. Northumberland Trough. 151–153 in British Lower Carboniferous stratigraphy. Cossey, P J, Adams, A E, Purnell, M A, Whiteley, M J, Whyte, M A, and Wright, V (editors). Geological Conservation Review Series, No. 29.
Robson, D A. 1956. A sedimentary study of the Fell Sandstones of the Coquet Valley, Northumberland. Quarterly Journal of the Geological Society, Vol. 112, 241–262.
Robson, D A. 1964. The Acklington Dyke — a proton magnetometer survey. Proceedings of the Yorkshire Geological Society, Vol. 34, 293–308.
Robson, D A, and Green, A G. 1980. A magnetic survey of the aureole around the Cheviot granite. Scottish Journal of Geology, Vol. 16, 11–27.
Self, C A, and Mullan, G J. 2005. Rapid karst development in an English quarzitic sandstone. Acta Carsologica, Vol. 34, 415–424.
Shennan, I, Horton, B, Innes, J, Gehrels, R, Lloyd, J, McArthur, J, and Rutherford, M. 2000. Late Quaternary sea-level changes, crustal movements and coastal evolution in Northumberland, UK. Journal of Quaternary Science, Vol. 15, 215–237.
Shiells, K A G. 1964. The geological structure of north-east Northumberland. Transactions of the Royal Society of Edinburgh, Vol. 65, 449–484.
Smith, S. 1923. Lead and zinc ores of Northumberland and Alston Moor. Special Report on the Mineral Resources of Great Britain, Memoir of the Geological Survey of Great Britain, Vol. 25. ISBN X780015540
Stone, P, Millward, D, Young, B, Merritt, J W, Clarke, S M, McCormac, M, Lawrence, D J D, Barnes, R P, Butcher, A S, and Entwisle, D C. 2010. British regional geology: Northern England (5th edition). (Nottingham: British Geological Survey.) ISBN 9780852726525
Turner, B R, and Monro, M. 1987. Channel formation and migration by mass-flow processes in the Lower Carboniferous fluviatile Fell Sandstone Group, north-east England. Sedimentology, Vol. 34, 1107–1122.
Turner, B R, Younger, P L, and Fordham, C E. 1993. Fell Sandstone lithostratigraphy southwest of Berwick-upon-Tweed: implications for the regional development of the Fell Sandstone. Proceedings of the Yorkshire Geological Society, Vol. 49, 269–281.
Turner, B R, Dewey, C, and Fordham, C E. 1997. Marine ostracods in the Lower Carboniferous fluviatile Fell Sandstone Group: evidence for base level change and marine flooding of the central graben, Northumberland Basin. Proceedings of the Yorkshire Geological Society, Vol. 51, 297–306.
Waters, C N, Browne, M A E, Dean, M T, and Powell, J H. 2007. Lithostratigraphical framework for Carboniferous successions of Great Britain (Onshore). British Geological Survey Research Report, RR/07/001.
Young, B, and Lawrence, D J D. 2002. Geology of the Morpeth district. Sheet Explanation of the British Geological Survey, Sheet 14 Morpeth (England and Wales). ISBN 0852724276
Index to the 1:50 000 Series maps of the British Geological Survey
(Index map)
The map below shows the sheet boundaries and numbers of the 1:50 000 Series geological maps. The maps are numbered in three sequences, covering England and Wales, Northern Ireland, and Scotland. The west and east halves of most Scottish 1:50 000 maps are published separately. Almost all BGS maps are available flat or folded and cased.
The area described in this sheet explanation is indicated by a solid block.
British geological maps can be obtained from sales desks in the Survey's principal offices, through the BGS London Office at the Natural History Museum, and from BGS-approved stockists and agents.
Northern Ireland maps can be obtained from the Geological Survey of Northern Ireland.
Figures and plates
Figures
(Figure 1) Nomenclature and classification of Carboniferous rocks.
(Figure 2) Depth to top Caledonian basement (after Chadwick et al., 1995). Contours are at 200 m intervals relative to Ordnance Datum. The extent of the Rothbury district is outlined with a black rectangle. Main faults: CCF –Cragend–Chartners Fault; HF – Hauxley Fault; SF – Swindon Fault; SWF – Sweethope Fault; STF – Stakeford Fault.
(Figure 3a) Bouguer gravity anomaly map contoured at 1 mGal intervals. Anomalies calculated against the Geodetic Reference System 1967 and referred to the National Gravity Reference Net 1973. There are approximately 440 gravity stations in the area shown. A variable reduction density was used that increases from about 2.5 Mg/m3 in the south-east to about 2.65 Mg/m3 in the north-west.
(Figure 3b) Colour shaded-relief image of the total magnetic field with illumination from the north. Most of the area was flown at an elevation of 305 m along north–south aeromagnetic flight lines at 2 km spacings; the detailed survey in the southwest corner was flown at an elevation of 60 m along north-west–south-east flight lines at 250 m spacings. The extent of the Rothbury district is outlined with a black rectangle on both maps.
(Figure 4) Longhorsley and Stobswood boreholes.
(Figure 5) Major limestones in the Yoredale Group.
(Figure 6) Named coal seams of the district
Plates
(Plate 1) General view of the Great Limestone and overlying mudstone at Greenleighton Quarry [NZ 034 920]. Refer to text for scale (p.11). The two limestone beds, with a thin mudstone parting, at the top of the limestone sequence and the underlying mudstone (just above the tree) are believed to be equivalent to the 'Tumbler Beds' of the Alston Block (Photographer D J D Lawrence; (P741911)).
(Plate 2) Section between the top and bottom leaves of the Top Busty seam at West Chevington East Extension opencast site [NZ 277 965] (1990) (Photographer T S Bain; P220608).
(Plate 3) Steeply dipping strata on the downthrown southern side of the Stobswood Fault are seen in the foreground, forming a face 4 m high. Stobswood North Extension opencast site [NZ 230 947], looking south-west (2007) (Photographer D J D Lawrence; P741910).
(Plate 4) Superficial deposits at Maiden's Hall opencast site [NZ 235 980]. Laminated silty clay passing up into a melange of ripped-up silt and clay, capped by deformation till. A total thickness of approximately 2.5 m of sediments is shown in the photograph (Photographer J W Merritt; P543555).
(Plate 5) Ewesley Whin Quarries working in 1928 [NZ 061 946]. The quarry face was up to 15 m high. Looking north-north-west (Photographer J Rhodes; P204463).
(Plate 6) Cross-bedded sandstone (Fell Sandstone Formation) in the former working face at Cove Quarry, Pondicherry [NU 045 018], which supplied stone for the construction of the town of Rothbury. The face is approximately 6 m high. Diagonal tooling marks made by the quarrymen show that the stone was worked by hand (Photographer E K Hyslop; P741909).
(Geological succession) Summary of the geological succession in the Rothbury district.one at Lordenshaw [NZ 056 991] with Rothbury in the distance. The sandstone exposure into which the symbols are cut is 3.3 m from front to back. (Photographer B McIntyre; P708898).
(Rear cover)
(Front cover) Prehistoric cup and ring marks, a form of rock art, cut into the Fell Sandstone at Lordenshaw [NZ 056 991] with Rothbury in the distance. The sandstone exposure into which the symbols are cut is 3.3 m from front to back. (Photographer B McIntyre; P708898).
(Index map) Index to the 1:50 000 Series maps of the British Geological Survey
Figures
(Figure 5) Major limestones in the Yoredale Group
| Formation | Regional name (and local alternatives) | Map code | Lithology | Key localities | Regional substage |
| STAINMORE | Newton Limestone (Pigdon) | NwL | Limestone, grey, bioclastic, fine-grained. It is commonly argillaceous and contains thick mudstone beds. Its fauna includes coral colonies and large productid brachiopods | Poorly exposed, but proved in numerous boreholes. Thickest occurrence from 83 to 91 m depth in the Morpeth Water Bore (NZ18NE/4) [NZ1873 8799] | Arnsbergian |
| Thornbrough Limestone (Stanton, Brainshaugh) | ThL | Throughout much of the district as two beds of limestone separated by mudstone. The thinner upper bed characteristically contains abundant crinoid ossicles | Quarry north-east of Stanton House [NZ1385 9118] | Arnsbergian | |
| Pike Hill Limestone | PHL | Limestone, grey, bioclastic, fine-grained. Locally has an ochreous appearance, as in the Mere Burn | Mere Burn [NU1969 0295] | Arnsbergian? | |
| Corbridge Limestone (Todbridge, Felton-St-Mary's, Sugar Sands, Acton Park) | CorL | Limestone, grey, bioclastic, fine-grained. It has numerous persistent argillaceous partings and thin mudstone partings thickening towards the top. Fauna dominated by brachiopods (especially productids including gigantoproductids) and molluscs | Old quarry north of Todburn [NZ1226 9642] | Pendleian? | |
| Coatyards Limestone | CyL | Limestone, grey, bioclastic | Coat Yards Farm [NZ0775 9440] | Pendleian | |
| Belsay Dene Limestone (Shothaugh) | BsDL | Limestone, grey, bioclastic, rather impure, yellow-weathering | River Coquet at Shothaugh [NZ1670 9990] | Pendleian | |
| Little Limestone | LtLs | Limestone, medium-grey, bioclastic. It contains a relatively good fauna dominated by inarticulate and articulate brachiopods (especially productids) and molluscs (especially bivalves, including pectenids) | Although proved in numerous boreholes the limestone is not seen at outcrop within the district | Pendleian | |
| ALSTON | Great Limestone (Dryburn, Catsbit) | GL | Limestone, medium-grey, slightly bituminous, crinoidal. Complete or fragmentary shells of brachiopods and some bivalves are locally conspicuous and in places both solitary and colonial corals are common | Greenleighton Quarry [NZ034 920], Wards Hill Quarry [NZ079 965] | Lower Pendleian |
| Four Fathom Limestone (Eight Yard) | FFL | Limestone, packstone, fine-grained, medium and dark grey, thick-bedded and wavy-bedded, with few mudstone partings; somewhat argillaceous, particularly at the top. Typically contains nodular layers and lenses of black chert | Swarland Burn [NU1441 0345] | Brigantian | |
| Three Yard Limestone (Six Yard, Acre) | TYL | Limestone, packstone, fine-grained, medium and dark grey, thickly bedded. Compact and crinoidal with a sub- glassy fracture. The foraminiferan Saccamminopsis fusulinaformis (McCoy) is common | Swarland Burn [NU1431 0372], Forest Burn [NZ0950 9877] | Brigantian | |
| Eelwell Limestone | EL | Limestone, massive, grey, bioclastic, generally devoid of mudstone partings; typically rich in algal remains with a prominent fauna of brachiopods and corals; appreciably dolomitised in part, giving a brown-weathered and vuggy appearance | Forest Burn [NZ0939 9875] | Brigantian | |
| Colwell Limestone | CwL | Limestone, dark grey, evenly bedded | Exposed only in the south-western corner of the district [NY9885 8715] | Brigantian | |
| Oxford Limestone | OxL | Limestone, thick-bedded at base, becoming thin-bedded, nodular and rather flinty near the top; grey to dark grey and particularly rich in corals and brachiopods with numerous red-weathered Osagia (an alga) haloes | Fallowlees Burn [NZ0126 9318], Forest Burn [NZ0524 9643] | Brigantian | |
| TYNE LIMESTONE | Dun Limestone | DnL | Dark grey, argillaceous, crinoidal, shelly limestone with algal nodules, weathering to a rusty ochreous colour, up to 3.5 m thick | Blanch Burn [NZ0167 9595] and
[NZ 0188 9588] |
Asbian |
| Ages are based on faunal identifications contained in Dean (1999, 2001) Local limestone names are given in parentheses |
(Figure 6) Named coal seams of the district
| Formation | Coal seam | Alternative names | County index letter | Map code | Maximum thickness (metres) | Details of coal working |
| PENNINE MIDDLE COAL MEASURES | Usworth | Uswo | 0.76 | Not worked | ||
| Burradon | Bur | 0.62 | Not worked | |||
| Rowlington | Rowl | 1.27 | Surface mined | |||
| Top Ryhope Five-Quarter | Gosforth | C1 | TRFQ | 0.61 | Surface mined | |
| Bottom Ryhope Five-Quarter | Bottom Hauxley | C | BRFQ | 0.45 | Surface mined | |
| Ryhope Little | Seventy Fathom | Ryl | 0.53 | Surface mined | ||
| Moorland | Blackclose | D | Moor | 1.08 | Surface and deep-mined | |
| Ashington | High Main | D/E | Ash | 1.76 | Surface and deep-mined | |
| High Main | Top Main, Diamond, Bensham | E | HM | 1.70 | Steam coal; surface and deep-mined | |
| Metal | Middle Main, Main | F1 | Me | 1.47 | Surface and deep-mined | |
| Five-Quarter | Bottom Main, Grey, Main | F2 | FQ | 1.60 | Surface and deep-mined | |
| Bentinck | Prince, Top Yard, First Below Grey | G1 | Bent | 0.92 | Surface and deep-mined | |
| Yard | Albert, Bottom Main | G or G2 | Yard | 1.86 | Excellent house coal; surface and deep-mined | |
| Top Bensham | Stone, Top Maudlin | H1 | TBen | 1.20 | Steam coal; surface and deep-mined | |
| Bensham | Queen, Maudlin, Quarry | H or H2 | Ben | 1.32 | Steam coal; surface and deep-mined | |
| Durham Low Main | Band,
Little Wonder, Five-Quarter |
J | DLM | 1.96 | Steam coal; surface and deep-mined | |
| Northumberland Low Main | Pegswood Yard, Brass Thill, Princess, Top of Broomhill, Radcliffe | K | NLM | 2.13 | First class steam coal; surface and deep-mined | |
| Broomhill Main | Duke, Main | K/L | BrM | 1.70 | Steam coal; surface and deep-mined | |
| Plessey | Hutton, Bottom | L | Ply | 1.83 | Splinty coal; surface and deep-mined | |
| Bottom Plessey | Cheeveley | M | BPly | 1.24 | Surface and deep-mined | |
| PENNINE LOWER COAL MEASURES | Beaumont | Harvey | N | Bm | 1.83 | Surface and deep-mined |
| Bottom Beaumont | N2 | BBm | Surface and deep-mined | |||
| Tilley | Widdrington Yard, Givens | P | Tily | 1.53 | Surface and deep-mined | |
| Top Busty | Pegswood Harvey, Widdrington Five-Quarter | Q1 | TBu | 1.07 | Surface and deep-mined | |
| Bottom Busty | Old Man, Widdrington Main | Q2 | BBu | 1.22 | Surface and deep-mined | |
| Three-Quarter | Top Busty, Little, Bottom Widdrington Main | R | TQ | 1.50 | Surface and deep-mined | |
| Stobswood New | R/S | StNw | 0.67 | Surface and deep-mined | ||
| Brockwell | Bandy | S | Broc | 1.22 | Locomotive coal; surface and deep-mined | |
| Victoria | Brockwell, Bessie Gray | T | Vict | 1.18 | Good household coal; surface and deep-mined | |
| Marshall Green | Choppington Victoria | U | MGn | 1.63 | Household, steam and gas coal; surface and deep-mined | |
| Ganister Clay | V | GC | Not worked | |||
| STAINMORE | Hazon Lee | HL | 0.61 | Deep-mined | ||
| Stanton | St | 0.34 | Deep-mined | |||
| Top Netherwitton | TNw | 1.10 | Surface and deep-mined | |||
| Bottom Netherwitton | BNw | 0.70 | Surface and deep-mined | |||
| Top Coatyards | TCy | 1.10 | Surface and deep-mined | |||
| Bottom Coatyards | BCy | 0.64 | Surface and deep-mined | |||
| Little Limestone | Chirm | LLCO | 0.90 | Surface and deep-mined | ||
| ALSTON | Townhead | Top | To | 0.60 | Mined, mainly for calcining limestone | |
| Shilbottle | Shil | 1.00 | Good bituminous household coal; Deep mined |
