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Field meeting in the Forest of Dean
Field Meeting in the Forest of Dean 10-12 June 1966 Report by the Director: J. G. CAPEWELL Received 18 November 1966
Friday, 10 June of the Association assembled at Littledean House, Littledean, Gloucestershire, for the meeting. After dinner, the Director gave a short talk on the geology of the Forest of Dean and adjacent areas and outlined the programme for the week-end. The object of the excursion was to present as broad a view as possible, within the available time, of the succession, structure and geomorphology of the area, and this would necessitate going outside the strict limits of the Forest of Dean.
TWENTY-SEVEN MEMBERS
Saturday,l1 June On a misty morning the party travelled by coach via Cinderford and Nailbridge to Plump Hill, the geological features being pointed out en route. From a suitable vantage point (SO 663173) above The Point, the Director demonstrated as far as was possible the general features of the eastern flank of the Dean syncline. Closer at hand, the Director was able to show the position of the mappable boundary between Devonian and the Carboniferous, where the yellowish sandstones at the top of the Tintern Sandstone Group are succeeded abruptly by the first limestones of the Lower Limestone Shales. The latter formation was then studied in the easternmost of the series of quarries along the northern side of the main road at Plump Hill. The overturning of the dip of one of the limestone bands was noted and its origin discussed. The party then moved into the large quarry in Lower Dolomite, the unbedded nature of most of which provoked discussion. The Director suggested that this rock may have originally been a reef limestone, since dolomitisation did not normally obliterate the bedding. The quarry exposes virtually the full thickness of the Lower Dolomite; its steep dip becomes apparent in the uppermost beds. The Crease Limestone, a coarse crystalline dolomite and the principal haematitised horizon, was then seen in a series of old excavations running northward along the strike from a point on the main road immediately west of the chapel (SO 661171). From here the party moved across to the quarry, now virtually infilled, 200 yards north-north-west of the chapel, in the Whitehead Limestone. Enough rock is exposed in situ, however, to demonstrate the algal nature of this limestone. The party then walked through the plantation northward for about half 207
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J. G. CAPEWELL
a mile, noting small exposures of the Drybrook Sandstone, and then turned north-westward across the Wigpool syncline to Puddlebrook. Some time was spent here in the quarry (SO 656183), occasionally worked for building- and brick-facing sand, in Drybrook Sandstone. Its unexpected, deltaic, facies and its palaeogeographical significance provoked discussion. From the conspicuous shale band in the quarry, plant-remains, including sporophylls of Lepidostrobophyllum (Allen, 1961),were collected. After lunch at a nearby inn, the Director demonstrated the positions of the outcrops on the western flank of the Wigpool syncline, the party then moving on into the road-cutting north-east of Euroclydon (SO 643187). This displays a fine section in the Tintern Sandstone Group, consisting largely of greenishhighly micaceouscurrent-bedded sandstones (for detailed succession see Welch & Trotter, 1961,51). At the corner at the northern end of the cutting, the topographical features of the Hope Mansel hemidome, the complementary structure to the Wigpool syncline, were demonstrated. At Bailey Gate, about 300 yards down the hill, road-side crags of the Quartz Conglomerate were examined. At this point the Director made reference to unpublished work on the sedimentology of the Upper Old Red Sandstone by Mr. A. J. Halliwell, who, from a study of the petrography, had been led to infer a northerly or north-westerly provenance for the sediments, from a region of Pre-Cambrian rocks. The party then went by coach via Lea to Longhope railway station, thence walking up the lane to the well-known Hobbs Quarry in the Wenlock Limestone (SO 695190), thence across the fields towards the railway bridge. On the way the oolitic and partly dolomitised development of the upper part ofthe Wenlock Limestone wasseen in an old quarry (SO 694189). In the sunken lane farther down, the fossiliferous, calcareous siltstones of the Ludlow Series were seen; reference was made to the detailed work of Lawson (1955) on these beds and his conclusions summarised. From Longhope station the party returned to headquarters by way of the Wilderness Quarry (SO 672185), which exposes a fine section showing typical rocks of the Brownstones division of the Lower Old Red Sandstone. Sunday, 12 June The party went by coach to the Strand at Westbury-on-Severn and walked along the foreshore to the section of Keuper and Rhaetic beds at Garden Cliff (SO 718128).The detailed succession was studied with the aid of the diagram compiled by L. Richardson and published on the occasion of the Gloucester meeting in 1934 (Gardiner and others, 1934, 134). The palaeontological and sedimentological features of the Rhaetic aroused great interest. Having returned to the coach, the party then travelled via Newnham and Blakeney to Severn Bridge station. In a heavy downpour, the party
FIELD MEETING IN THE FOREST OF DEAN
209
made its way through Purton and under the railway to the Severn foreshore, where the deposition at high water of estuarine silts was noted and the result experienced. In the cliff and shore section, the Raglan Marls of the Old Red Sandstone were compared and contrasted with the Keuper Marls, seen at the previous stop. The greater tectonic deformation of the Raglan Marls was the most obvious feature, and at one point (SO 671044), a steeply dipping reverse fault, with marked drag on the upthrow side, is well exposed. The presence of strong bands of current-bedded sandstones, and of nodular cornstones, the origin of which caused some discussion, were other features noted. The party moved on along the shore, pausing to note the use of local Pennant Sandstone in the construction of the landward piers of the now disused Severn Railway Bridge, before returning to the coach by field-path. A rapid traverse by coach across the Forest of Dean was then made by way of Blakeney, Parkend, Cannop and Berry Hill to the Rock Inn, near Symond's Vat, features of interest being pointed out en route. After lunch the party went on as far as the new coach-park at Symond's Vat. The earthworks shown on the 2t-in. Ordnance Survey map as Offa's Dyke were briefly inspected and the party moved on to the wooden foot-bridge, and examined an excellent exposure of the Crease Limestone, here a coarse crystalline dolomite. A hundred yards down the hill a problematic feature in the Lower Dolomite, probably an infilled collapse structure, was studied. A return was then made to the summit of Symond's Vat Rock (SO 564160), also composed of Lower Dolomite; here the view and its geomorphological implications were much admired. On the return journey to headquarters, a final halt was made at some road-side quarries near Miry Stock (SO 617148), where typical Pennant Sandstone is well exposed. A search for plant fossils in a shale lens was unsuccessful. At this point, appreciative votes of thanks to the Director and to the Secretary (Mr. F. E. Jarvis) were proposed by the Rev. F. M. Hodgess Roper. The party reached Littledean at 4 p.m. and dispersed. REFERENCES ALLEN, K. c. 1961. Lepidostrobophyllum fimbriatum (Kidston, 1883) fromthe Drybrook Sandstone (Lower Carboniferous). Geol. Mag., 98, 225-9. GARDINER, C. I.. G. A. KELLAWAY, S. H. REYNOLDS, STANLEY SMITH & A. E. TRUEMAN. 1934. The Geology of the Gloucester District. Proc. Geol. Ass., 45, 109-44. LAWSON, J. D. 1955. The Geology of the May Hill Inlier. Q. JI geol. Soc. Lond., 111, 85-116.
WELCH, F. B. A. & F. M. TROTTER. 1961. Geology of the Countryaround Monmouth and Chepstow. Mem, geol, Surv, U.K. Department of Civil Engineering The City University 51. John Street, London E.C.I PROC. GEOL. ASS., VOL. 79. PART 2,1968
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Friday, 10 June of the Association assembled at Littledean House, Littledean, Gloucestershire, for the meeting. After dinner, the Director gave a short talk on the geology of the Forest of Dean and adjacent areas and outlined the programme for the week-end. The object of the excursion was to present as broad a view as possible, within the available time, of the succession, structure and geomorphology of the area, and this would necessitate going outside the strict limits of the Forest of Dean.
TWENTY-SEVEN MEMBERS
Saturday,l1 June On a misty morning the party travelled by coach via Cinderford and Nailbridge to Plump Hill, the geological features being pointed out en route. From a suitable vantage point (SO 663173) above The Point, the Director demonstrated as far as was possible the general features of the eastern flank of the Dean syncline. Closer at hand, the Director was able to show the position of the mappable boundary between Devonian and the Carboniferous, where the yellowish sandstones at the top of the Tintern Sandstone Group are succeeded abruptly by the first limestones of the Lower Limestone Shales. The latter formation was then studied in the easternmost of the series of quarries along the northern side of the main road at Plump Hill. The overturning of the dip of one of the limestone bands was noted and its origin discussed. The party then moved into the large quarry in Lower Dolomite, the unbedded nature of most of which provoked discussion. The Director suggested that this rock may have originally been a reef limestone, since dolomitisation did not normally obliterate the bedding. The quarry exposes virtually the full thickness of the Lower Dolomite; its steep dip becomes apparent in the uppermost beds. The Crease Limestone, a coarse crystalline dolomite and the principal haematitised horizon, was then seen in a series of old excavations running northward along the strike from a point on the main road immediately west of the chapel (SO 661171). From here the party moved across to the quarry, now virtually infilled, 200 yards north-north-west of the chapel, in the Whitehead Limestone. Enough rock is exposed in situ, however, to demonstrate the algal nature of this limestone. The party then walked through the plantation northward for about half 207
208
J. G. CAPEWELL
a mile, noting small exposures of the Drybrook Sandstone, and then turned north-westward across the Wigpool syncline to Puddlebrook. Some time was spent here in the quarry (SO 656183), occasionally worked for building- and brick-facing sand, in Drybrook Sandstone. Its unexpected, deltaic, facies and its palaeogeographical significance provoked discussion. From the conspicuous shale band in the quarry, plant-remains, including sporophylls of Lepidostrobophyllum (Allen, 1961),were collected. After lunch at a nearby inn, the Director demonstrated the positions of the outcrops on the western flank of the Wigpool syncline, the party then moving on into the road-cutting north-east of Euroclydon (SO 643187). This displays a fine section in the Tintern Sandstone Group, consisting largely of greenishhighly micaceouscurrent-bedded sandstones (for detailed succession see Welch & Trotter, 1961,51). At the corner at the northern end of the cutting, the topographical features of the Hope Mansel hemidome, the complementary structure to the Wigpool syncline, were demonstrated. At Bailey Gate, about 300 yards down the hill, road-side crags of the Quartz Conglomerate were examined. At this point the Director made reference to unpublished work on the sedimentology of the Upper Old Red Sandstone by Mr. A. J. Halliwell, who, from a study of the petrography, had been led to infer a northerly or north-westerly provenance for the sediments, from a region of Pre-Cambrian rocks. The party then went by coach via Lea to Longhope railway station, thence walking up the lane to the well-known Hobbs Quarry in the Wenlock Limestone (SO 695190), thence across the fields towards the railway bridge. On the way the oolitic and partly dolomitised development of the upper part ofthe Wenlock Limestone wasseen in an old quarry (SO 694189). In the sunken lane farther down, the fossiliferous, calcareous siltstones of the Ludlow Series were seen; reference was made to the detailed work of Lawson (1955) on these beds and his conclusions summarised. From Longhope station the party returned to headquarters by way of the Wilderness Quarry (SO 672185), which exposes a fine section showing typical rocks of the Brownstones division of the Lower Old Red Sandstone. Sunday, 12 June The party went by coach to the Strand at Westbury-on-Severn and walked along the foreshore to the section of Keuper and Rhaetic beds at Garden Cliff (SO 718128).The detailed succession was studied with the aid of the diagram compiled by L. Richardson and published on the occasion of the Gloucester meeting in 1934 (Gardiner and others, 1934, 134). The palaeontological and sedimentological features of the Rhaetic aroused great interest. Having returned to the coach, the party then travelled via Newnham and Blakeney to Severn Bridge station. In a heavy downpour, the party
FIELD MEETING IN THE FOREST OF DEAN
209
made its way through Purton and under the railway to the Severn foreshore, where the deposition at high water of estuarine silts was noted and the result experienced. In the cliff and shore section, the Raglan Marls of the Old Red Sandstone were compared and contrasted with the Keuper Marls, seen at the previous stop. The greater tectonic deformation of the Raglan Marls was the most obvious feature, and at one point (SO 671044), a steeply dipping reverse fault, with marked drag on the upthrow side, is well exposed. The presence of strong bands of current-bedded sandstones, and of nodular cornstones, the origin of which caused some discussion, were other features noted. The party moved on along the shore, pausing to note the use of local Pennant Sandstone in the construction of the landward piers of the now disused Severn Railway Bridge, before returning to the coach by field-path. A rapid traverse by coach across the Forest of Dean was then made by way of Blakeney, Parkend, Cannop and Berry Hill to the Rock Inn, near Symond's Vat, features of interest being pointed out en route. After lunch the party went on as far as the new coach-park at Symond's Vat. The earthworks shown on the 2t-in. Ordnance Survey map as Offa's Dyke were briefly inspected and the party moved on to the wooden foot-bridge, and examined an excellent exposure of the Crease Limestone, here a coarse crystalline dolomite. A hundred yards down the hill a problematic feature in the Lower Dolomite, probably an infilled collapse structure, was studied. A return was then made to the summit of Symond's Vat Rock (SO 564160), also composed of Lower Dolomite; here the view and its geomorphological implications were much admired. On the return journey to headquarters, a final halt was made at some road-side quarries near Miry Stock (SO 617148), where typical Pennant Sandstone is well exposed. A search for plant fossils in a shale lens was unsuccessful. At this point, appreciative votes of thanks to the Director and to the Secretary (Mr. F. E. Jarvis) were proposed by the Rev. F. M. Hodgess Roper. The party reached Littledean at 4 p.m. and dispersed. REFERENCES ALLEN, K. c. 1961. Lepidostrobophyllum fimbriatum (Kidston, 1883) fromthe Drybrook Sandstone (Lower Carboniferous). Geol. Mag., 98, 225-9. GARDINER, C. I.. G. A. KELLAWAY, S. H. REYNOLDS, STANLEY SMITH & A. E. TRUEMAN. 1934. The Geology of the Gloucester District. Proc. Geol. Ass., 45, 109-44. LAWSON, J. D. 1955. The Geology of the May Hill Inlier. Q. JI geol. Soc. Lond., 111, 85-116.
WELCH, F. B. A. & F. M. TROTTER. 1961. Geology of the Countryaround Monmouth and Chepstow. Mem, geol, Surv, U.K. Department of Civil Engineering The City University 51. John Street, London E.C.I PROC. GEOL. ASS., VOL. 79. PART 2,1968
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