Difference between revisions of "Geology of Eyemouth and Burnmouth - an excursion"

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The upper 50 m of Old Red Sandstone, exposed on the foreshore west of Burnmouth Harbour ([[:File:YGS_NORTROCK_FIG_02_3.jpg|Figure 2.3]]), contains up to five relatively poorly exposed fining-upward sequences, each comprising an erosively based, coarse-grained, trough cross-bedded fluvial channel sandstone overlain by fine-grained fluvio-lacustrine sandstones, siltstones and silty mudstones containing calcretes similar to those at Pease Bay (Excursion 1). The Old Red Sandstone on the foreshore is faulted and intruded by a northeast–southwest trending quartz dolerite dyke. Further west in the hillside above the harbour Old Red Sandstone is faulted against the Silurian greywackes.
 
The upper 50 m of Old Red Sandstone, exposed on the foreshore west of Burnmouth Harbour ([[:File:YGS_NORTROCK_FIG_02_3.jpg|Figure 2.3]]), contains up to five relatively poorly exposed fining-upward sequences, each comprising an erosively based, coarse-grained, trough cross-bedded fluvial channel sandstone overlain by fine-grained fluvio-lacustrine sandstones, siltstones and silty mudstones containing calcretes similar to those at Pease Bay (Excursion 1). The Old Red Sandstone on the foreshore is faulted and intruded by a northeast–southwest trending quartz dolerite dyke. Further west in the hillside above the harbour Old Red Sandstone is faulted against the Silurian greywackes.
  
The Old Red Sandstone is conformably overlain by the Lower Carboniferous Cementstone Group which is completely exposed on the foreshore to a total thickness of 450 m. The best exposures, to the east and south of the harbour, consist of micaceous, fluvial channel sandstones regularly interbedded with fluvio-lacustrine overbank siltstones, mudstones, sandstones and subordinate cementstones composed of ferroan dolomite. The succession is characterized by a crudely cyclic pattern of deposition of the type cementstone-mudstone-sandstone-mudstone-cementstone. The cementstones are extremely hard and splinter when hit. They consist of a number of different but gradational types, including sandy and banded cement-stones, which contain a variety of sedimentary structures. A typical sequence comprises massive structureless cementstone at the base, overlain by flat laminations and convolute laminations at the top. Cementstone beds are <45 cm thick and in parts of the succession they have a nodular appearance, and locally contain vugs lined with calcite pseudomorphs of original evaporite minerals such as gypsum. Lateral accretion surfaces, indicative of point bar deposition and meandering river channels, have been identified in some sandbodies seen on aerial photographs but they are extremely difficult to recognize in the field because of the steep dip of the beds. Burrows, rootlets, bivalves, fish fragments and mudcracks have been found in the overbank fines, but vary in abundance through the succession. The lower part is predominantly argillaceous and red to purple in colour. It contains thin (<30 cm) cementstones yielding rare fish fragments and bivalves. Joints in one cement-stone bed near the east wall of the harbour are coated with galena. Plants are common throughout this part of the succession which contains three major sandbodies up to 7.7 m thick. Two of them are relatively simple, single storey, fining-upward sandbodies containing trough cross-bedding overlain by ripple cross-lamination. Channel sandstone It ([[:File:YGS_NORTROCK_FIG_02_3.jpg|Figure 2.3]]) contains a well developed local lenticular basal channel lag conglomerate whilst the other one, next to the east wall of the harbour, is more complex in its internal organisation (Figure 2.3, sandstone 12).
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The Old Red Sandstone is conformably overlain by the Lower Carboniferous Cementstone Group which is completely exposed on the foreshore to a total thickness of 450 m. The best exposures, to the east and south of the harbour, consist of micaceous, fluvial channel sandstones regularly interbedded with fluvio-lacustrine overbank siltstones, mudstones, sandstones and subordinate cementstones composed of ferroan dolomite. The succession is characterized by a crudely cyclic pattern of deposition of the type cementstone-mudstone-sandstone-mudstone-cementstone. The cementstones are extremely hard and splinter when hit. They consist of a number of different but gradational types, including sandy and banded cement-stones, which contain a variety of sedimentary structures. A typical sequence comprises massive structureless cementstone at the base, overlain by flat laminations and convolute laminations at the top. Cementstone beds are <45 cm thick and in parts of the succession they have a nodular appearance, and locally contain vugs lined with calcite pseudomorphs of original evaporite minerals such as gypsum. Lateral accretion surfaces, indicative of point bar deposition and meandering river channels, have been identified in some sandbodies seen on aerial photographs but they are extremely difficult to recognize in the field because of the steep dip of the beds. Burrows, rootlets, bivalves, fish fragments and mudcracks have been found in the overbank fines, but vary in abundance through the succession. The lower part is predominantly argillaceous and red to purple in colour. It contains thin (<30 cm) cementstones yielding rare fish fragments and bivalves. Joints in one cement-stone bed near the east wall of the harbour are coated with galena. Plants are common throughout this part of the succession which contains three major sandbodies up to 7.7 m thick. Two of them are relatively simple, single storey, fining-upward sandbodies containing trough cross-bedding overlain by ripple cross-lamination. Channel sandstone It ([[:File:YGS_NORTROCK_FIG_02_3.jpg|Figure 2.3]]) contains a well developed local lenticular basal channel lag conglomerate whilst the other one, next to the east wall of the harbour, is more complex in its internal organisation ([[:File:YGS_NORTROCK_FIG_02_3.jpg|Figure 2.3]], sandstone 12).
  
 
In the middle part of the succession the mudrocks are mostly grey and sandy, and the associated cementstones thicker and more abundant. Cementstones reach a maximum stage of development here and beds containing bivalves and serpulids become more common. Only two major fining-upward channel sandstones occur in that part of the succession, both cross-bedded in the lower part and ripple cross-laminated in the upper part, with local hard calcareous lenses. Towards the top of the succession the beds are typically calcareous. They contain a greater abundance of rootlets but bivalves and fish fragments are rare. The finer grained beds are generally red, purple and green, often variegated and mottled, but the associated cementstones are much less common and thinner. Major channel sandstones increase in abundance and thickness up-section. They differ from those in the lower part in the following ways: they have a well developed sheet-like geometry with scoured bases locally overlain by intraclast conglomerates; they commonly have a more complex, internally scoured, multistoried form with less evidence of lateral accretion surfaces; and they show little sequential ordering of stratification types which are dominated by low angle trough cross-bedding and ripple cross-lamination. The change in character of the sandbodies through the succession suggests that they were deposited by channels of variable sinuosity and size, but with a general trend towards larger, less sinuous channels towards the top of the succession. These changes relate to base level changes in response to tectonic and/or climatic factors heralding deposition of the Fell Sandstone Group. The presence of cementstone intraclasts in channel sands, and the inverse relationship between the abundance of sandstones and cementstones, indicates an early diagenetic origin for the cementstones during significant periods of non-deposition. They were precipitated below the sediment-water interface in interdistributary lakes and lagoons on a low relief, semi-arid coastal alluvial plain.
 
In the middle part of the succession the mudrocks are mostly grey and sandy, and the associated cementstones thicker and more abundant. Cementstones reach a maximum stage of development here and beds containing bivalves and serpulids become more common. Only two major fining-upward channel sandstones occur in that part of the succession, both cross-bedded in the lower part and ripple cross-laminated in the upper part, with local hard calcareous lenses. Towards the top of the succession the beds are typically calcareous. They contain a greater abundance of rootlets but bivalves and fish fragments are rare. The finer grained beds are generally red, purple and green, often variegated and mottled, but the associated cementstones are much less common and thinner. Major channel sandstones increase in abundance and thickness up-section. They differ from those in the lower part in the following ways: they have a well developed sheet-like geometry with scoured bases locally overlain by intraclast conglomerates; they commonly have a more complex, internally scoured, multistoried form with less evidence of lateral accretion surfaces; and they show little sequential ordering of stratification types which are dominated by low angle trough cross-bedding and ripple cross-lamination. The change in character of the sandbodies through the succession suggests that they were deposited by channels of variable sinuosity and size, but with a general trend towards larger, less sinuous channels towards the top of the succession. These changes relate to base level changes in response to tectonic and/or climatic factors heralding deposition of the Fell Sandstone Group. The presence of cementstone intraclasts in channel sands, and the inverse relationship between the abundance of sandstones and cementstones, indicates an early diagenetic origin for the cementstones during significant periods of non-deposition. They were precipitated below the sediment-water interface in interdistributary lakes and lagoons on a low relief, semi-arid coastal alluvial plain.
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=== Locality 3, Bucket Rocks syncline [NU 008 532] ===
 
=== Locality 3, Bucket Rocks syncline [NU 008 532] ===
  
At this locality the lower leaf of the Upper Bath-House Wood Limestone thins and passes into calcareous shale. A 21 cm thick coal seam and root-penetrated fireclay or seatearth, stained yellow by jarosite, occurs beneath the calcareous shale. The fireclay passes down into a cross-bedded and ripple cross-laminated sandstone spotted with reddish iron oxide burrow infills. The sandstone, which was deposited by palaeocurrents flowing to the southeast, is underlain by a calcareous sandstone with ripples in the coarser, more sandy parts. Two small faults, down-throwing to the north, drop the Upper Bath-House Wood Limestone to the base of the cliff on the north side of the point below the coastguard lookout. Here, it is a rubbly-looking fossiliferous limestone, the upper part of which has been selectively dolomitized; the presence of cavities suggest local de-dolomitization. Fossils occur scattered throughout the limestone or concentrated into distinct bands, whilst bedding surfaces are covered with superb ''Zoophycos ''feeding traces. Another feature of the limestone is the presence of numerous stylolite seams.
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At this locality the lower leaf of the Upper Bath-House Wood Limestone thins and passes into calcareous shale. A 21 cm thick coal seam and root-penetrated fireclay or seatearth, stained yellow by jarosite, occurs beneath the calcareous shale. The fireclay passes down into a cross-bedded and ripple cross-laminated sandstone spotted with reddish iron oxide burrow infills. The sandstone, which was deposited by palaeocurrents flowing to the southeast, is underlain by a calcareous sandstone with ripples in the coarser, more sandy parts. Two small faults, down-throwing to the north, drop the Upper Bath-House Wood Limestone to the base of the cliff on the north side of the point below the coastguard lookout. Here, it is a rubbly-looking fossiliferous limestone, the upper part of which has been selectively dolomitized; the presence of cavities suggest local de-dolomitization. Fossils occur scattered throughout the limestone or concentrated into distinct bands, whilst bedding surfaces are covered with superb ''Zoophycos'' feeding traces. Another feature of the limestone is the presence of numerous stylolite seams.
  
 
=== Locality 4, Ladies Skerrs dome [NU 008 536] ===
 
=== Locality 4, Ladies Skerrs dome [NU 008 536] ===
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Some 30 m of shales, sandstones, seatearths and thin coals of the Scremerston Coal Group are exposed on the foreshore north of Huds Head. The first limestone, the 1.5 m, crinoidal, Dun Limestone, marks the base of the Lower Limestone Group ([[:File:YGS_NORTROCK_FIG_03_3.jpg|Figure 3.3]]). It forms a foreshore feature dipping 30° offshore, but is best seen where it rises in the cliff on the north side of Huds Head, where a seatearth and 36 cm coal is visible immediately beneath it. A band rich in the coral ''Siphonodendron junceum ''occurs near the base, together with large productid brachiopods and scattered other fossils. Brachiopods also occur in the shales above. The Dun Limestone cycle is the first of a series of coarsening-upward cycles consisting of marine limestone, shales, usually with ironstone bands and nodules, silts, sands, often first as starved ripples, lenticular and/or tabular units of cross-bedded sandstone, then usually a seatearth and coal. The environmental change at the base of the Limestone Groups is a subtle increase in marine influence and decrease in periods of emergence. Scattered thin coals occur throughout the succession and the limestones, at least below the level of the Oxford Limestone, are relatively thin and well spaced with thick intervening shales and sandstones, some showing strong channel form.
 
Some 30 m of shales, sandstones, seatearths and thin coals of the Scremerston Coal Group are exposed on the foreshore north of Huds Head. The first limestone, the 1.5 m, crinoidal, Dun Limestone, marks the base of the Lower Limestone Group ([[:File:YGS_NORTROCK_FIG_03_3.jpg|Figure 3.3]]). It forms a foreshore feature dipping 30° offshore, but is best seen where it rises in the cliff on the north side of Huds Head, where a seatearth and 36 cm coal is visible immediately beneath it. A band rich in the coral ''Siphonodendron junceum ''occurs near the base, together with large productid brachiopods and scattered other fossils. Brachiopods also occur in the shales above. The Dun Limestone cycle is the first of a series of coarsening-upward cycles consisting of marine limestone, shales, usually with ironstone bands and nodules, silts, sands, often first as starved ripples, lenticular and/or tabular units of cross-bedded sandstone, then usually a seatearth and coal. The environmental change at the base of the Limestone Groups is a subtle increase in marine influence and decrease in periods of emergence. Scattered thin coals occur throughout the succession and the limestones, at least below the level of the Oxford Limestone, are relatively thin and well spaced with thick intervening shales and sandstones, some showing strong channel form.
  
This lower part of the sequence can be traced in relatively undisturbed north-northwest striking beds across the foreshore south to the far side of Redshin Cove. The thick sandstones above the Dun Limestone forming Huds Head contain intersecting lenticular fine-grained cross-stratified units near the base and medium to coarse grained, multistoried, mainly planar cross-bedded sets deposited by migrating channel bars towards the top. In the cliff above, a major distributory channel filled with small-medium scale, mainly trough cross-bedded sandstones cuts down to the south through the shales overlying the prograding deltaic sequence. Opposite a prominent embayment where the cliff is replaced by a grassy slope, a prominent calcrete is overlain by the Woodend Limestone, which forms a feature on the foreshore, striking into the cliff to the south. This limestone is notable for its excellent fauna of fasciculate corals, mainly ''Siphonodendron junceum ''but also ''S. martini ''(with corallite diameters 7–8 mm). Many of the ''S.junceum ''colonies are virtually uncrushed and in position of growth. Towards the top of the limestone, and particularly on the top bedding surface, scattered colonies of ''Lithostrotion maccoyanum ''(massive with polygonal corallites 3–4 mm diameter) occur. The solitary corals ''Dibunophyllum ''(with a spider's web axial structure) and ''Caninia ''(no axial structure) are present together with the calcareous sponge ''Chaetetes septosus.''
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This lower part of the sequence can be traced in relatively undisturbed north-northwest striking beds across the foreshore south to the far side of Redshin Cove. The thick sandstones above the Dun Limestone forming Huds Head contain intersecting lenticular fine-grained cross-stratified units near the base and medium to coarse grained, multistoried, mainly planar cross-bedded sets deposited by migrating channel bars towards the top. In the cliff above, a major distributory channel filled with small-medium scale, mainly trough cross-bedded sandstones cuts down to the south through the shales overlying the prograding deltaic sequence. Opposite a prominent embayment where the cliff is replaced by a grassy slope, a prominent calcrete is overlain by the Woodend Limestone, which forms a feature on the foreshore, striking into the cliff to the south. This limestone is notable for its excellent fauna of fasciculate corals, mainly ''Siphonodendron junceum ''but also ''S. martini ''(with corallite diameters 7–8 mm). Many of the ''S.junceum ''colonies are virtually uncrushed and in position of growth. Towards the top of the limestone, and particularly on the top bedding surface, scattered colonies of ''Lithostrotion maccoyanum ''(massive with polygonal corallites 3–4 mm diameter) occur. The solitary corals ''Dibunophyllum ''(with a spider's web axial structure) and ''Caninia ''(no axial structure) are present together with the calcareous sponge ''Chaetetes septosus''.
  
 
The Woodend Limestone is overlain by another classic coarsening-upward sequence ending with the Woodend Coals, associated with slumping in the grassy and degraded cliff and thus poorly exposed. About 15 m higher in the succession is a 1.2 m cementstone band and some 2 m higher a very distinctive 50 cm Algal Band, consisting of subspherical algal oncolites, up to 9 cm diameter, which become more densely packed towards the top of the bed. A 4.5 m black oil shale, forming a broad slack on the foreshore, succeeds the Algal Band. In the sequence above, the Watchlaw Limestone can be identified just before a prominent step on the foreshore onto the thick sequence of cross-bedded sandstones of the Maidenkirk Brae Sandstone. Further progress south is difficult and only possible at low tide.
 
The Woodend Limestone is overlain by another classic coarsening-upward sequence ending with the Woodend Coals, associated with slumping in the grassy and degraded cliff and thus poorly exposed. About 15 m higher in the succession is a 1.2 m cementstone band and some 2 m higher a very distinctive 50 cm Algal Band, consisting of subspherical algal oncolites, up to 9 cm diameter, which become more densely packed towards the top of the bed. A 4.5 m black oil shale, forming a broad slack on the foreshore, succeeds the Algal Band. In the sequence above, the Watchlaw Limestone can be identified just before a prominent step on the foreshore onto the thick sequence of cross-bedded sandstones of the Maidenkirk Brae Sandstone. Further progress south is difficult and only possible at low tide.

Revision as of 16:38, 6 August 2019

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Scrutton, Colin, Editor Northumbrian rocks and landscape: a field guide. 2004. 2nd edition. Yorkshire Geological Society.
Figure 2.1 Geological map of the Eyemouth–Burnmouth area indicating the localities described.
Figure 2.2 Downward facing Silurian greywackes of the Hawick Group at Agate Point, Eyemouth (Locality 3). Photo: C. T. Scrutton.
Figure 2.3 Generalized geological map and sections of the foreshore exposures of the Cementstone and Fell Sandstone Groups at Burnmouth.
Figure 2.4 View north of Maidenstone Stack, c.15 m high, composed of near vertical Fell Sandstone. The erosive contact with the underlying Cementstone Group is at the stack foot on the left. Photo: B. R. Turner.

2 The geology of Eyemouth and Burnmouth

By Colin Scrutton and Brian Turner University of Durham

Purpose

This excursion examines folded Silurian greywackes of the Hawick Group, early Devonian autobrecciated lavas unconformably succeeded by late Devonian (Old Red Sandstone) breccias, a transitional sequence across the Devonian–Carboniferous boundary, and an abandoned Pleistocene drainage channel.

Logisistics

This section was compiled in 2006 when the printed guidebook was published. Before visiting this site please ensure you have up-to-date contact and access information.

Most are coastal outcrops and Locality 5 (Burnmouth Foreshore) requires low tide. Localities 1, 2 and 3 can be visited at half tide and Locality 4 is not tide-dependent. The itinerary described here should be adjusted to take account of tide times. Eyemouth has good parking with toilet and refreshment facilities. Parking at Burnmouth is possible along the side of the minor road. Parking at Burnmouth Harbour at the foot of the cliffs is limited.

The foreshore rocks may be slippery. Hard hats and wellingtons are essential. Localities 4 and 5 are not recommended for large parties.

Maps

O.S. 1:50 000 Sheet 67 Duns, Dunbar & Eyemouth; B.G.S. (Scotland) 1:50 000 Sheet 34 Eyemouth.

Geological background

By the early Silurian, the microcontinent of Eastern Avalonia, including the area of England, Wales and southeast Ireland, had moved northwards to lie in close proximity to the margin of Laurentia (Figure 3a). The intervening Iapetus Ocean had been reduced to the width of a narrow seaway. On the Laurentian side; thick deposits of poorly sorted sand grade sediments, derived from rising land to the north, began to cover the thin sequences of black, graptolitic shales that had accumulated on deep shelf and, in some places, oceanic crust. These coarser sediments were introduced by turbidity flows, each resulting in a few cms to 1 m+ thick greywacke, usually separated by thin bands of background mud-grade sediment, sometimes graptolitic. In southern Scotland, these beds extend from the Llandovery up into the Wenlock when sedimentation ceased as the seaway narrowed, filled up and was eventually uplifted as a result of the final oblique compression between Avalonia, Baltica and Laurentia at the end of the Caledonian Orogenic Cycle (Figure 3a). In the Southern Uplands, the Lower Palaeozoic rocks are arranged in a series of northeast–southwest trending fault-defined slices, variously interpreted as reflecting a syn-depositional accretionary prism, or the post-depositional thrust slicing of the thick sediment wedge on the Laurentian margin by the late Silurian compressional event. In the Eyemouth area, individual greywacke beds are seldom more than 0.5 m thick and are strongly folded; the sequence is unfossiliferous but is compared to the Hawick Group elsewhere and is thus most likely to be of Llandovery age.

Magmas generated at depth by the Caledonian continental collision resulted in extensive late Silurian/early Devonian intrusions and volcanic activity, represented by dykes and extensive lavas, agglomerates and tuffs in the Eyemouth area and, to the southwest, by the Cheviot Volcano (Excursion 4). Locally, contemporary sediments were largely derived from the newly formed volcanic centres. More earth movements in the mid Devonian produced gentle folds in the early Devonian rocks. In the late Devonian, this hilly landscape of Lower Palaeozoic greywackes and younger volcanic rocks was deeply weathered in a semi-arid climate and the debris stripped off by flash floods to form fans of breccias and channel sands on the lower ground. By the early Carboniferous, the relief was somewhat reduced and the Cementstone Group was deposited on a broad floodplain crossed by meandering to low sinuosity channels with the formation of early diagenetic cementstones during periods of non-deposition. Subsequently, rivers flowing from the east and northeast brought thick sequences of fluviodeltaic cross-bedded medium- to coarse-grained sandstones into the area, building out to the southwest. The Fell Sandstone Group succeeds the Cementstones on the coast south of Burnmouth.

A full Carboniferous sequence is developed further south in Northumberland. Locally, however, the next events of geological interest were the intrusion oflate Carboniferous/?Permian basic dykes and the effects of the Variscan Orogeny. The latter resulted in local folding and faulting, but the most spectacular structure is the basement controlled, east facing Berwick Monocline, striking just west of north along the coast and responsible for the vertical Cementstone Group sequence at Burnmouth, where the steep limb is cut by a high-angle reverse fault.

Locally, the effects of the Pleistocene glaciation include a variable covering of till, thick in the valleys but thin and patchy on the high ground extending to the coast at Burnmouth. An infilled preDevensian river valley occurs in Eyemouth bay. The present form of Burnmouth Glen and the Eye Water gorge below Ayton are the result of post-glacial deepening of older valleys.

Further details will be found in Greig (1988; in McAdam et al. 1992).

Excursion details

From the A1 about 11 km north of Berwick, take the At 107 to Eyemouth. Cross the Eye Water and turn right. At the church keep straight on to the northern end of the town, then turn sharply back to the right to the car park with toilets and cafe on the esplanade [NT 944 645].

Locality 1, Eyemouth Beach [NT 944 646]

From the car park, turn left and walk to a point where access to a corner of the beach can be obtained via a ramp (Figure 2.1). On the lower foreshore and in the cliffs where they rise towards the north are extensive exposures of rocks of agglomeratic texture which have been interpreted by Greig (1988) as autobrecciated lavas of early Devonian age. Around mid-low tide level, exposures are heavily coated with seaweed, but at the edge of the beach are a number of good, sand polished surfaces in which the fragments are relatively small, angular to rounded and variable in shape, set in an igneous matrix. These dark coloured, reddish weathering rocks with scattered green reduction spots are pyroxene andesites, with well formed phenocrysts of plagioclase feldspar. Some outcrops have the appearance of debris flows of volcaniclastic material. Further north round the bay and in the cliffs, however, the rocks are paler, purplish grey dacites, more acid in composition. Clast size increases, some in excess of 1 m across, and most are tabular with little intervening matrix. They show good flow-banding, with aligned feldspar, hornblende and biotite phenocrysts.

In the back of the bay, the volcanic rocks in the cliff are cut by a channel reaching to beach level. The softer fill, of strikingly reddened layers of unconsolidated breccio-conglomerates and sands capped by several metres of till, is weathered back to form a gully. The mixture of angular and rounded clasts in the coarse unit accessible at beach level is dominated by fragments, <30 cm, of Silurian greywackes, fine–medium grained red sandstones and volcanic rocks, with a vague imbrication indicating water flow towards the sea. The channel is interpreted as that of a pre-Devensian stream.

Locality 2, Eyemouth Fort [NT 944 649]

Proceed with care across the foreshore to a small bay on the point beneath the fort, where the unconformity between the autobrecciated volcanic rocks, here with individual blocks several metres in length, and the overlying red breccias and sandstones of the Upper Old Red Sandstone can be seen. The deposits represent alluvial fans of a semi-arid environment, fed by flash floods stripping material from the surrounding hills. The texture of the Devonian rocks can be studied in the many fallen blocks. Towards the far end of the small bay, a large joint face shows excellent imbrication in the breccias indicating flow from the east. The clasts are similar to those in the Pleistocene channel but more dominantly greywackes, with red sandstones and volcanic rocks. At some levels, where the fines have been washed out of the deposit, they are cemented by calcite.

The small offshore islands are of Silurian greywackes just on the east side of the westerly downthrowing, north-northeast trending Eyemouth Fault which is responsible for juxtaposing the Devonian rocks against the Silurian greywackes seen on the east side of Eyemouth harbour.

Return to the car park in Eyemouth and drive out of the village to the Ai 107. Turn left, cross the Eye Water and then immediately left again down The Avenue. Follow the road to a parking point [NT 947 643] on the east bank of Eyemouth harbour just before the old Customs House.

Locality 3, Nestends [NT 950 647]

Walk in front of the Customs House straight ahead to Nestends, then follow the cliff-edge path southeast round the golf course. Superb exposures of folded and faulted Silurian greywackes of the Hawick Group can be seen from the cliff top and along the promontories, with access to foreshore level possible in several bays. The rocks are fine sand-grade turbidites of varying thickness with reddish mudstone interbeds; no diagnostic fossils have been found. Graded bedding is apparent in places and some units have poorly developed sole structures at the base and small scale ripples and channels towards the top, all indicating way up. The folds strike east-northeast–west-southwest and have a complex form, due to a significant sinistral shear element during compression. They are tight, almost isoclinal in places, with highly variable 1–15 m wavelengths as folds develop or die out along strike. Fold axes vary considerably in their plunge and in some cases, the cleavage, which is well developed in the mud grade beds, is folded. Just beyond the end of the golf course, in a north–south zone about 150 m wide west from Agate Point [NT 955 642], the shearing associated with the compressional event has resulted in the juxtaposition along strike of southwest plunging folds which are upward-facing with downward-facing folds plunging to the northeast (Figure 2.2). These are probably sheath folds and all these features are the result of a single phase of deformation.

From Agate Point, follow the wall along the southern edge of the golf course into the housing estate, then turn right to reach the car park. Return to the A 1107, turn left and after t km turn left again onto the Burnmouth road. Park on the roadside where the houses of Burnmouth start on the left [NT 953 612].

Locality 4, road cutting below Burnmouth Hill [NT 957 611]

Immediately north of the railway bridge, take the road to the east that descends steeply to the shore (Figure 2.1). At the top of the hill, fork left and follow the narrow road downhill with outcrops of Silurian Hawick Group greywackes dipping predominantly about 65° northwest on the left. After some 180 m, approach a roadside outcrop on a left-hand bend, where the first beds are near vertical and younging towards the sea. A 0.7 m greywacke sandstone bed has excellent flute marks, suggesting turbidity flow from the northeast; these sole structures can be traced around the folds here and are well seen just off the road lower down the hill. Viewed along strike, the cleavage in the soft, shaly interbeds can be seen refracting and dying out in the more competent greywacke sandstones. Immediately downhill is a synclinal axis, cut on its seaward side by a 4.25 m wide Siluro-Devonian porphyrite dyke, in which the chilled margin and the coarser centre with plagioclase feldspar phenocrysts can be compared. Beyond the dyke is the nose of a small anticline followed immediately by a sharp syncline with a faulted axis. It is in the seaward limb of this syncline that the sole structures can be well seen.

If time and tide allow, descend the hill to where similar rocks can be studied on the foreshore north of Partanhall [NT 958 612]. Here the cross-cutting relationships of the porphyrite intrusions, roughly parallel to the north–south strike of the Silurian greywackes, can be seen, together with a later generation (late Carboniferous-?Permian) of east–west quartz dolerite dykes. On the foreshore at low tide, the northward extension of the faulted Berwick Monocline can be seen with the Carboniferous Cementstone Group on the seaward side.

From the foot of the hill at Partanhall, cross to Burnmouth Harbour [NT 959 609]. Alternatively, return to the vehicle and take the road down to Burnmouth Harbour, or to Cowdrait at the end of the narrow tarred road along the seafront, where there is limited parking for one or two cars or a minibus.

Locality 5, Burnmouth Foreshore [NT 958 611]

At low tide the foreshore section provides continuous exposure over a distance of some 2 km, and involves rough walking and scrambling over vertically dipping and differentially eroded, wet, slippery rocks.

The sediments exposed include the Upper Old Red Sandstone and Lower Carboniferous Cementstone Group, overlain by the Fell Sandstone Group (Figure 2.3). The sediments young seawards on the steep northerly limb of the east-facing Berwick Monocline, with the beds vertical to slightly overturned and dipping between 55 and 80° to the west-northwest and west-southwest. The structure is most tightly compressed in the north where it is also more extensively faulted.

The upper 50 m of Old Red Sandstone, exposed on the foreshore west of Burnmouth Harbour (Figure 2.3), contains up to five relatively poorly exposed fining-upward sequences, each comprising an erosively based, coarse-grained, trough cross-bedded fluvial channel sandstone overlain by fine-grained fluvio-lacustrine sandstones, siltstones and silty mudstones containing calcretes similar to those at Pease Bay (Excursion 1). The Old Red Sandstone on the foreshore is faulted and intruded by a northeast–southwest trending quartz dolerite dyke. Further west in the hillside above the harbour Old Red Sandstone is faulted against the Silurian greywackes.

The Old Red Sandstone is conformably overlain by the Lower Carboniferous Cementstone Group which is completely exposed on the foreshore to a total thickness of 450 m. The best exposures, to the east and south of the harbour, consist of micaceous, fluvial channel sandstones regularly interbedded with fluvio-lacustrine overbank siltstones, mudstones, sandstones and subordinate cementstones composed of ferroan dolomite. The succession is characterized by a crudely cyclic pattern of deposition of the type cementstone-mudstone-sandstone-mudstone-cementstone. The cementstones are extremely hard and splinter when hit. They consist of a number of different but gradational types, including sandy and banded cement-stones, which contain a variety of sedimentary structures. A typical sequence comprises massive structureless cementstone at the base, overlain by flat laminations and convolute laminations at the top. Cementstone beds are <45 cm thick and in parts of the succession they have a nodular appearance, and locally contain vugs lined with calcite pseudomorphs of original evaporite minerals such as gypsum. Lateral accretion surfaces, indicative of point bar deposition and meandering river channels, have been identified in some sandbodies seen on aerial photographs but they are extremely difficult to recognize in the field because of the steep dip of the beds. Burrows, rootlets, bivalves, fish fragments and mudcracks have been found in the overbank fines, but vary in abundance through the succession. The lower part is predominantly argillaceous and red to purple in colour. It contains thin (<30 cm) cementstones yielding rare fish fragments and bivalves. Joints in one cement-stone bed near the east wall of the harbour are coated with galena. Plants are common throughout this part of the succession which contains three major sandbodies up to 7.7 m thick. Two of them are relatively simple, single storey, fining-upward sandbodies containing trough cross-bedding overlain by ripple cross-lamination. Channel sandstone It (Figure 2.3) contains a well developed local lenticular basal channel lag conglomerate whilst the other one, next to the east wall of the harbour, is more complex in its internal organisation (Figure 2.3, sandstone 12).

In the middle part of the succession the mudrocks are mostly grey and sandy, and the associated cementstones thicker and more abundant. Cementstones reach a maximum stage of development here and beds containing bivalves and serpulids become more common. Only two major fining-upward channel sandstones occur in that part of the succession, both cross-bedded in the lower part and ripple cross-laminated in the upper part, with local hard calcareous lenses. Towards the top of the succession the beds are typically calcareous. They contain a greater abundance of rootlets but bivalves and fish fragments are rare. The finer grained beds are generally red, purple and green, often variegated and mottled, but the associated cementstones are much less common and thinner. Major channel sandstones increase in abundance and thickness up-section. They differ from those in the lower part in the following ways: they have a well developed sheet-like geometry with scoured bases locally overlain by intraclast conglomerates; they commonly have a more complex, internally scoured, multistoried form with less evidence of lateral accretion surfaces; and they show little sequential ordering of stratification types which are dominated by low angle trough cross-bedding and ripple cross-lamination. The change in character of the sandbodies through the succession suggests that they were deposited by channels of variable sinuosity and size, but with a general trend towards larger, less sinuous channels towards the top of the succession. These changes relate to base level changes in response to tectonic and/or climatic factors heralding deposition of the Fell Sandstone Group. The presence of cementstone intraclasts in channel sands, and the inverse relationship between the abundance of sandstones and cementstones, indicates an early diagenetic origin for the cementstones during significant periods of non-deposition. They were precipitated below the sediment-water interface in interdistributary lakes and lagoons on a low relief, semi-arid coastal alluvial plain.

The Fell Sandstone Group conformably overlies the Cement-stones, with a sharp but locally erosive contact. Some of the best exposures occur on the foreshore by Maidenstone Stack (Figure 2.4), where the base of the Fell Sandstone cuts down 2–3 m into the underlying Cementstones. This is the most northerly exposure of Fell Sandstone in the Tweed Basin and one of the few localities where the contact between the Fell Sandstone and the Cementstones can be clearly seen. The Fell Sandstone here comprises two, erosively based fining-upward sequences, but only the lower sequence and the base of the overlying one is completely exposed at low tide. The lower sequence, about to m thick, consists of planar and trough cross-bedded, medium to locally coarse-grained pebbly sandstone containing intraclasts of cementstone. The grain size and scale of the cross-beds decreases towards the top of the sandbody which is overlain by up to 50 cm of rippled and horizontally laminated fine sandstone, siltstone and silty shale, with reddish mottling at the top. Above is the erosive, intraclast-strewn base of the cross-bedded sandstone at the bottom of the overlying sequence. Some cross-beds show evidence of liquefaction and deformation, contemporaneous with deposition. The Fell Sandstone was deposited within a tectonically active environment by perennial braided streams flowing to the southwest, in a similar direction to the Cementstone channels (Figure 2.3). The sandstones, less micaceous and more feldspathic than those in the Cementstone Group, sparkle due to the reflection of light from secondary crystal faces deposited in optical continuity on detrital quartz grains.

When returning to the cliff top, note that the road joins the At at the Flemington Inn 300 m to the south.

3 The Carboniferous rocks around Berwick-upon-Tweed

Brian Turner and Colin Scrutton University of Durham

Purpose

This excursion examines the sedimentology, palaeontology and structure of the Lower Carboniferous succession in the Tweed Basin.

Logisistics

This section was compiled in 2006 when the printed guidebook was published. Before visiting this site please ensure you have up-to-date contact and access information.

Localities 1–5 represent a good half-day and Localities 6–7 a full-day excursion. Berwick on the north side of the River Tweed estuary and Tweedmouth–Spittal on the south both have easy parking, and full facilities are available at all but Cocklawburn Reach (Locality 7). All the localities are on the coast and require low to Mid tide. The rocky foreshore can be slippery; wellingtons and hard hats are recommended.

Maps

O.S. 1:50 000 Sheet 75 Berwick-upon-Tweed; B.G.S. 1:50 000 Sheet 1/2 Berwick-upon-Tweed and Norham (Solid and Drift).

Geological background

In late Devonian and early Carboniferous times the structure of northern England consisted of a number of basins and blocks. The largest of these basins, the Northumberland Basin, was separated from the smaller Tweed Basin to the north by the Cheviot axis. This remained a positive area until late Asbian times, when regional subsidence occurred and the two basins merged into one. The Northumberland Basin owed its origin to Carboniferous extensional re-activation of a northerly dipping crustal scale shear zone. A similar origin is favoured for the Tweed Basin, which is interpreted to have formed in response to reactivation of a thrust slice in the Southern Uplands.

Up to 2200 m of Lower Carboniferous sediments were deposited in the Northumberland Basin and 1300 m in the Tweed Basin. Lower Carboniferous Dinantian stratigraphy in both basins is the same and consists of the Cementstone Group, the Fell Sandstone Group, the Scremerston Coal Group and the Lower and Middle Limestone Groups. The sequence seen on this excursion, from the top of the Scremerston Coal Group to the base of the Namurian Upper Limestone Group, records a series of marine transgressions and regressions across a lower delta plain environment, resulting in vertically stacked Yoredale cyclothems comprising a shallow marine bioclastic limestone, overlain by mudstone, siltstone and sandstone capped by a seatearth and coal. The marine influence progressively increases up-section.

Berwick-upon-Tweed lies on the north bank of the River Tweed and is one of Britain's most historic towns. It was constantly fought over during the Anglo-Scottish wars of the Middle Ages, changing hands some 13 times until 1482 when it was taken by the English; since then it has remained part of England. One of the most impressive features of the town is the Elizabethan walls, built 1558–1570; other features of interest include the Georgian Town Hall, the Barracks (the first to be purpose-built in Britain), the Royal Border viaduct railway bridge built by Robert Stephenson and the 300-year-old low-level stone bridge across the Tweed built of Lower Carboniferous sandstone.

Excursion details

Locality 1, Berwick cliff top, overlooking Green's Haven [NU 004 537] and Fisherman's Haven [NU 004 536].

From the A1, turn off to Berwick on the A6105 (Figure 3.3). Immediately after crossing over the railway bridge turn left towards Berwick Holiday Centre. Turn right past the centre across the municipal golf course to the cliff top where limited parking is available. Alternative parking is available in The Parade between the Barracks and Holy Trinity Church: access to the cliff top is through Walkergate and across the golf course.

The Lower Carboniferous Middle Limestone Group succession exposed on the foreshore at Berwick ranges from the sandstone below the Oxford Limestone to the Acre Limestone (Figure 3.1). From the cliff top at low tide, the spectacularly folded and faulted upper Middle Limestone Group strata can be seen on the foreshore at Green's Haven. A series of domes and basins is defined by the outcrop pattern of the more resistant limestone beds (Figure 3.2). The most prominent of these is the Eelwell Limestone which is exposed at intervals along the coast south of the Tweed (for example Locality 7), and further south at Beadnell Bay.

Locality 2, Meadow Haven Bay [NU 007 527]

Walk south along the cliff top path from Locality 1 to the Meadow Haven fault zone (Figure 3.1) where the steep cliffs change to slope features, and a footpath allows easy access to the beach. Beware of golfers and golf balls when walking along the cliff top path! The first outcrops associated with the Meadow Haven Fault zone occur at the base of the cliff and consist of reddish and white, rippled and cross-bedded sandstones and siltstones containing locally developed, deep-red iron oxide concretions and burrow structures, which become particularly common in the upper part. Ganister can be seen adjacent to part of the Meadow Haven fault zone which comprises sheared and faulted blocks showing both oblique slip and dextral movement. On the north side the beds dip into the fault plane, cut by a number of small tension faults. Shales, containing thin rippled silty and sandy interbeds and lenses, and a few brown ironstone nodules and lenses, are exposed in the unstable cliff face. These are underlain by the Upper Bath-House Wood Limestone which rises in the cliff c.30–40 m to the north. It is a hard, argillaceous biomicrite with a soft shaley top containing crinoid ossicles, brachiopods, gastropods, corals and bivalves. Bedding surfaces are covered with the swirling traces of Zoophycos caudagalli (locally called cock's tails), ubiquitous to all the limestones in the succession. They are the feeding burrow systems of an unknown worm-like organism.

Locality 3, Bucket Rocks syncline [NU 008 532]

At this locality the lower leaf of the Upper Bath-House Wood Limestone thins and passes into calcareous shale. A 21 cm thick coal seam and root-penetrated fireclay or seatearth, stained yellow by jarosite, occurs beneath the calcareous shale. The fireclay passes down into a cross-bedded and ripple cross-laminated sandstone spotted with reddish iron oxide burrow infills. The sandstone, which was deposited by palaeocurrents flowing to the southeast, is underlain by a calcareous sandstone with ripples in the coarser, more sandy parts. Two small faults, down-throwing to the north, drop the Upper Bath-House Wood Limestone to the base of the cliff on the north side of the point below the coastguard lookout. Here, it is a rubbly-looking fossiliferous limestone, the upper part of which has been selectively dolomitized; the presence of cavities suggest local de-dolomitization. Fossils occur scattered throughout the limestone or concentrated into distinct bands, whilst bedding surfaces are covered with superb Zoophycos feeding traces. Another feature of the limestone is the presence of numerous stylolite seams.

Locality 4, Ladies Skerrs dome [NU 008 536]

South of the southerlydownthrowing normal fault, the following sequence can be seen in the cliff; burrowed shales, the Lower Bath-House Wood Limestone, c.6 m fossiliferous shales, then the Upper Bath-House Wood Limestone. The latter consists of two limestone units. A 1 m lower leaf is underlain by a 1–3 cm coal. It is separated by 1.5 m of shales from the upper leaf which consists of three beds separated by shaley partings. Bedding surfaces show abundant horizontal burrow traces.

Locality 5, Green's Haven Bay [NU 004 538]

North of the fault, the Upper Bath-House Wood Limestone forms the prominent, well jointed bedding plane surface, dipping 21° northwest. The overlying shales are marine with rare brachiopods at the base, but thin ironstone bands a little higher up suggest a decreasing marine influence. Further north, tiny rippled silty lenses (starved ripples) occur in the thick shales and thicker siltstone, and fine sandstone interbeds with plane laminations, ripples and convolute laminations appear; some of the thicker interbeds have burrowed and bioturbated tops. Towards the top of this sequence, where the shales become more sandy, a thick unit, containing bulbous concretions, is extensively burrowed and bioturbated obliterating most of the primary sedimentary structures. This is overlain by a thick, 2 m seatearth with black rootlet traces, and a thin 2–20 cm coal. Black, locally burrowed shales beneath the Shotto Wood Limestone succeeded by more shale are exposed approaching the steps (Figure 3.1). The Eelwell Limestone is brown, vuggy and dolomitized in much of its outcrop, but on the northwest side of the paddling pool, it is un-dolomitized and contains abundant Gigantoproductus brachiopods, together with scattered corals including Siphonodendron junceum (a distinctive branching coral with close-spaced corallites about 3 mm in diameter). At the northern end of Green's Haven Bay shales occur adjacant to the southwest side of the northwest–southeast trending Green's Haven Fault; on the northeast side is a softish, feldspathic, medium to coarse and locally pebbly, trough and rarely, planar cross-bedded sandstone deposited by currents flowing from the west-northwest (Figure 3.1). The fault plane dips 65° southwest, and shows a dominant oblique slip movement with some dextral translation. The fault can be traced southeast via small reefs of rock into the dome at Ladies Skerrs.

From the parking area return to the A1167 (old A1) through Berwick, cross the Tweed Bridge and continue south (Figure 3.3). At the roundabout [NT 995 5166] turn left for Spittal and continue to the end of Main Street where there is parking [NU 009 510].

Locality 6, Spittal

South from here for 4 km, there is almost complete foreshore exposure from the top of the Scremerston Coal Group, through the Lower and Middle Limestone Groups, and into the basal Upper Limestone Group. If possible, a second vehicle parked at Cocklawburn Beach (Locality 7) will enable a single continuous traverse to be made.

Some 30 m of shales, sandstones, seatearths and thin coals of the Scremerston Coal Group are exposed on the foreshore north of Huds Head. The first limestone, the 1.5 m, crinoidal, Dun Limestone, marks the base of the Lower Limestone Group (Figure 3.3). It forms a foreshore feature dipping 30° offshore, but is best seen where it rises in the cliff on the north side of Huds Head, where a seatearth and 36 cm coal is visible immediately beneath it. A band rich in the coral Siphonodendron junceum occurs near the base, together with large productid brachiopods and scattered other fossils. Brachiopods also occur in the shales above. The Dun Limestone cycle is the first of a series of coarsening-upward cycles consisting of marine limestone, shales, usually with ironstone bands and nodules, silts, sands, often first as starved ripples, lenticular and/or tabular units of cross-bedded sandstone, then usually a seatearth and coal. The environmental change at the base of the Limestone Groups is a subtle increase in marine influence and decrease in periods of emergence. Scattered thin coals occur throughout the succession and the limestones, at least below the level of the Oxford Limestone, are relatively thin and well spaced with thick intervening shales and sandstones, some showing strong channel form.

This lower part of the sequence can be traced in relatively undisturbed north-northwest striking beds across the foreshore south to the far side of Redshin Cove. The thick sandstones above the Dun Limestone forming Huds Head contain intersecting lenticular fine-grained cross-stratified units near the base and medium to coarse grained, multistoried, mainly planar cross-bedded sets deposited by migrating channel bars towards the top. In the cliff above, a major distributory channel filled with small-medium scale, mainly trough cross-bedded sandstones cuts down to the south through the shales overlying the prograding deltaic sequence. Opposite a prominent embayment where the cliff is replaced by a grassy slope, a prominent calcrete is overlain by the Woodend Limestone, which forms a feature on the foreshore, striking into the cliff to the south. This limestone is notable for its excellent fauna of fasciculate corals, mainly Siphonodendron junceum but also S. martini (with corallite diameters 7–8 mm). Many of the S.junceum colonies are virtually uncrushed and in position of growth. Towards the top of the limestone, and particularly on the top bedding surface, scattered colonies of Lithostrotion maccoyanum (massive with polygonal corallites 3–4 mm diameter) occur. The solitary corals Dibunophyllum (with a spider's web axial structure) and Caninia (no axial structure) are present together with the calcareous sponge Chaetetes septosus.

The Woodend Limestone is overlain by another classic coarsening-upward sequence ending with the Woodend Coals, associated with slumping in the grassy and degraded cliff and thus poorly exposed. About 15 m higher in the succession is a 1.2 m cementstone band and some 2 m higher a very distinctive 50 cm Algal Band, consisting of subspherical algal oncolites, up to 9 cm diameter, which become more densely packed towards the top of the bed. A 4.5 m black oil shale, forming a broad slack on the foreshore, succeeds the Algal Band. In the sequence above, the Watchlaw Limestone can be identified just before a prominent step on the foreshore onto the thick sequence of cross-bedded sandstones of the Maidenkirk Brae Sandstone. Further progress south is difficult and only possible at low tide.

In the corner of the cove just beyond the step, climb a grassy dip slope to the cliff top path. From here, the lenticular nature of beds below the Watchlaw Limestone can be clearly seen on the foreshore to the north.

For those with a single vehicle, return to it and rejoin the Ai 167 at the roundabout and turn south for Scremerston. After 2 km turn left at a signpost for Cocklawburn Beach and rejoin the coast at Seahouse. Turn left, cross a cattle grid and proceed c.150 m into a field from where there is a cliff path down to Cargie's Kiln, or right to where vehicles can be parked at various points on the roadside opposite Saltpan Rocks and the Skerrs.

Locality 7, Cargie's Plantation [NU 019 498] to Cocklawburn Beach [NU 036 478]

Descend to the foreshore at Cargie's Plantation to continue the section southwards (Figure 3.3). The sequence here beneath the Oxford Limestone is confused by a series of small east–west to northwest–southeast faults. At Cargie's Kiln, the 5 m thick, poorly fossiliferous but crinoidal Oxford Limestone marks the base of the late Dinantian Middle Limestone Group. The limestone is split into posts by shaley partings, one of which near the base of the limestone contains particularly prominent rolled algal nodules. In the limestone, red algal haloes around bioclasts stand out on wave polished surfaces. The Middle Limestone Group consists of cycles in which the limestones are thicker and more laterally persistent on a county-wide scale than in the Lower Limestone Group and the sandstones thinner, although thin coals are still developed at the tops of some cycles.

Several thin limestones dipping between 30–60° east-northeast intervene between the Oxford and the next thick limestone, the Eelwell. Each forms the base for a coarsening-upward cycle, often with well developed ripples, small-scale lenticular cross-bedded sets and hummocky cross-stratification (Reynolds 1992) developed in the sandstones, a seatearth and sometimes a thin coal. Following each transgression, the fluvio-deltaic processes carrying progressively coarser elastic material into the basin now show more evidence of shoreline, particularly storm dominated reworking, and less evidence of distributary channel, interdistributary bay and floodplain sediments than in lower parts of the succession (Reynolds 1992).

The Eelwell Limestone, 8 m thick and easily distinguished by a prominent fauna of large Gigantoproductus brachiopods and corals including Siphonodendron junceum, develops small folds and minor associated thrusts throughout its foreshore outcrop. The limestone is locally dolomitized, brown weathering and vuggy. Just south of Seahouse, it is involved in a sharp overfold facing west (Figure 3.4), which further south is thrust through its short limb. On the top surfaces of the gentle, whaleback folds to the east and south, polished sections through the rich fauna of spiriferid brachiopods at the very top of the limestone can be seen. Out on the foreshore here the 30 cm Acre Coal occurs beneath sandstones showing medium–large scale lenticular cross-bedded sets. Where the coal crops out in the cliffat the back of the foreshore, it is involved in a small thrust with 1 m vertical displacement. Just above are excellent examples of climbing ripples and a rootlet bed. The 4.5 m thick Acre Limestone, also developing minor folds on the foreshore, has a very thin, impersistent sulphurous coal beneath it. The limestone, crinoidal, with scattered fossils and small algal nodules, is brown weathering, dolomitized and vuggy in the cliff.

The sequence of outcrops on the beach to the south, the Skerrs, are all formed of the 8.5 m thick Sandbanks Limestone, a series of thin limestone beds with shale partings. Near Skerr to Middle Skerr is a broad, shallow syncline and from Middle Skerr to Far Skerr a complementary anticline. Polished surfaces at Middle Skerr show excellent sections of Zoophycos. There are layers rich in brachiopods, and prominent solitary corals, mainly Aulophyllum fungites (with a dense axial structure) and rarer Palaeosmilia murchisoni (with many septa and no axial structure). In the sandstones below trace fossils are well seen; first the beaded burrow Eione moniliforme and then at lower levels, 1 o cm long, dumbell-like depressions which are the bedding plane traces of the U-shaped burrow Diplocraterion. A careful search ofjoint surfaces may reveal vertical sections through the latter. Dark brown chert nodules occur towards the top of the limestone at Far Skerr; chert is characteristic of this limestone (=Four Fathom Limestone) across Northern England.

The next reef on the beach to the south is a thin, brown-weathering un-named limestone. Cheswick Black Rocks beyond, the last outcrop on the beach to the south, are formed of coarse, cross-bedded sandstones immediately beneath the Great Limestone. This limestone, known locally as the Dryburn, marks the base of the Upper Limestone Group and the Namurian (Upper Carboniferous). It was formerly quarried inland at Scremerston but it cannot now be seen on the shore.


Glossary

Bibliography

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