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Easter field meeting in North-West Dartmoor
Easter Field Meeting in North-West Dartmoor 26-30 March 1959 Report by the Directors: W. R. DEARMAN and N. E. BUTCHER Received 11 May 1959 THE OBJECT of the meeting was to demonstrate recent work (Dearman & Butcher, 1959) on the stratigraphy, structure and petrology of the Upper Devonian and Carboniferous sediments and associated igneous rocks along the north-west margin of the Dartmoor granite. The party of eighteen members assembled in Okehampton on the evening of 26 March, when the Directors gave a brief outline of the geology of the area and the programme to be followed during the next four days. The Association had previously visited part of the area in 1922 (Sherlock, 1923).
Friday, 27 March
Director: W. R. DEARMAN Okehampton and Meldon. The day was devoted to the examination of the Upper Culm Measures in the neighbourhood of Okehampton and the inlier of the Lower Culm Measures at Meldon. The party drove first to Knowle Quarry (594962)1 where the symmetrical folds in the unaltered Upper Culm Measures were contrasted with the overfolds in the neighbouring Okehampton Brickworks Quarry (594961). Returning to Okehampton the roadside exposures in the Upper Culm along New Road (586948 to 583943) were examined. The party rejoined the bus and drove to the British Railways Quarries (567926) at Meldon. Here in the 860 North Bay the typical faulted zig-zag overfolds of the Lower Culm Measures inlier were well displayed. Particular attention was paid to the metasomatic development of coarse-grained calc-silicate hornfels from the black radiolarian cherts which occur near the top of the Lower Culm succession. The party walked southwards to the disused face opposite the crushing plant to see the faulted inlier of the Slate-with-Lenticles Group. The full sequence of the Lower Culm, with two basic dykes, was exposed in the adjoining 860 South Bay. After lunch the party walked up the incline road to the 950 Waste Tip, and from this vantage point the Director pointed out the relationship between the geology and the topography as far south as the granite of Yes Tor. A short walk along the 950 South Cutting brought the 1
All National Grid reference numbers OCcurwithin the 100 km, square 20 (sX).
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party to an exposure of deeply weathered limestones and cherts. The Meldon Aplite Quarries (567920) were next visited, and the traverse of the inlier was completed by walking up the valley of the Red-a-ven Brook to the disused Red-a-ven Mine (570917). Saturday, 28 March
Director: W. R. DEARMAN Be/stone and Stick/epath. The day was spent in a further examination of the Lower and Upper Culm Measures along the northern border of Dartmoor. The party left Okehampton by the footpath leading to Ball Hill (598947) and Fatherford Viaduct (603597), examining small exposures of the Upper Culm sandstones and shales on the way. The East Okement River was crossed by the footbridge just upstream from the viaduct, and by following the footpath on the west bank of the river to Chapel Ford (608934) a traverse was made from the northern normal limb of the Lower Culm anticlinal inlier, across the core of the Slate-with-Lenticles Group, into the steep beds of the southern limb of the anticline. Some members of the party continued upstream on the eastern bank to complete the traverse to the Upper Culm, and then climbed Watchet Hill (615930) to reach Belstone (619935) for lunch. The rest of the party followed the footpath northwards to the hilltop above West Cleave (609941) for the view north across the Permian of the Crediton Valley to Exmoor, then joined the road and walked to Belstone. After lunch the party walked south-westwards across the heather-covered outcrop of the Upper Culm Measures of Watchet Hill to the granite tors of Belstone Common. Below in the valley of the Taw River lay the great waste of Taw Marsh (620910); recent borings for water have revealed that beneath the marsh there is over one hundred feet of granite detritus resting on solid granite. Returning to Belstone the party took the path along the disused leat leading to Skai~ (631939) on the northern slopes of Belstone Cleave, passing crags of tuff and agglomerate of the Volcanic Group of the Lower Culm Measures. The disused workings of Ivy Tor Mine (627934) could be seen on the southern bank of the Taw River, and the Director pointed out that this marked the southern margin of the Lower Culm inlier which had also been mined farther east at Ford (643935). The dumps of the disused Belstone Consols (Mid-Devon) Mine (632945), at the northern margin of the Lower Culm inlier, were visited next. The party then walked from the mine to the end of the lane (637942) high on the hill above StickIepath. There was a clear view from here across the StickIepath Fault Zone along which the Belstone Consols lodes have been displaced over one mile south-east to the disused Ramsley Mine (650930). The party followed the steep path down into StickIepath and walked along the main road to Ramsley. Brecciated
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W. R . DEARMAN AND N . E. BUTCHER
granite in a sand-pit (649928) near Fulford, directly opposite the mine dumps, again clearly indicated the magnitude of the offset along the Sticklepath Fault. The bus was waiting to take the party to the disused South Tawton Limestone Quarries, where the day's work ended with a fruitless search for fossils in the easily accessible exposures in the most northerly of the quarries (658951). Sunday, 29 March
Director : N. E. BUTCHER Brentor and Lydford. The day was mainly devoted to an examination of the Brentor and Lydford inliers of the Transition Series within the Culm Measures outcrop. The party drove first to Brent Tor (471804) from the summit of which low cloud unfortunately allowed only a restricted view of the north-western slopes of Dartmoor. The crags of pyroclastic rocks were examined on descending the tor. In the nearby South Brentor quarries (480806) the Upper Devonian Clymenia Zone beds were seen, lamellibranchs and ostracods being collected. The party then walked on to a quarry (480809) near North Brentor in green slaty beds of ? Wocklumeria Zone age. The coach was rejoined in North Brentor and the party drove to the Manor Hotel near Lydford Station where a nearby stream bank locality (502832) in Platyclymenia Zone slates was visited. After lunch Lydford Gorge was entered at its lower end at the elbow of river capture (Dewey, 1916), the geomorphological features exciting much discussion. The River Lyd Slate-with-Lenticles Group was examined in passing through the gorge, at the upper end of which most members visited the Devil's Cauldron. Continuous rain and a walk along a swinging plank which was awash due to the high-water level made the visit to this large pot-hole in the depths of the gorge a memorable experience. Leaving the gorge the party visited a quarry (515854) at Lydford in contact-altered calcareous siltstones of the Transition Series and then brieflycompared and contrasted them with calc-flintas of the Lower Culm Watervale Calcareous Group in stream exposures by the main road (518839) near Watervale. The day's work ended with a walk up the valley of the River Lyd from the main road towards the granite, examining the lithology and deformation structures of the River Lyd Group within the metamorphic aureole of the granite . Finally some members of the party visited a quarry (533864) in the Upper Culm Measures close to the granite near Nodden Gate . Monday, 30 March
Director : N . E. BUTCHER Marytavy and Tavistock, The main object of the day's programme was to examine the Lower and Upper Culm Measures, comparing and con-
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trasting the rocks with those seen in the northern part of the area on the first two days. The party travelled from Okehampton along the main road towards Tavistock, features of geological interest being pointed out on the way. The first stop was at Wheal Betsy (510810) where the mine dumps were briefly searched for galena before the party proceeded to examine the nearby stream section in the Upper Culm Measures. Cast structures on the bottoms of the inverted sandstones of this Black Down Group particularly attracted attention. Wheal Friendship at Marytavy was next visited where the Director gave an account of the operations of this large disused coppermine. The party then examined the nearby Famennian inlier (505792), of ? Clymenia Zone age. The coach was rejoined and a brief stop made outside Pitts Cleave Quarry (501761), the Director giving an account ofthe igneous rock types-oligoclase-dolerites, hornblende-dolerites and trachytes -to be found as sills in the quarry. The party proceeded to Wilminstone Quarry (490755) to examine the hinge of a recumbent fold in which occurs a 200-foot thick sill of dolerite. After lunch in Tavistock the top of the inverted Lower Culm Measures succession was studied in Lower Deerpark Quarry (483743) and adjacent exposures. The Whitchurch Down Greywacke Group, the southernmost outcrop of the Upper Culm Measures in the area, was examined in the River Tavy exposures immediately north of the footbridge (479741). The Director demonstrated the facies differences between these rocks and the Upper Culm Measures to the north. The party then drove over Whitchurch Down, visiting a quarry (512739) in the Upper Devonian Slate Series which crops out to the south of the Culm Measures. After Dr. G. W. Himus had proposed a vote of thanks to the Directors, the party returned to Okehampton by way of Tavistock. REFERENCES DEARMAN, W. R. & N. E. BUTCHER. 1959. The Geology of the Devonian and Carboniferous Rocks of the North-West Border of the Dartmoor Granite, Devonshire. Proc. Geol. Ass., Lond., 70, 51-92. DEWEY, H. 1916. On the Origin of some River-Gorges in Cornwall and Devon. Quart. J. geol. Soc. Lond., 72, 63-76. SHERLOCK, R. L. 1923. Long Excursion to Lydford and Brent Tor, Devon. Proc. Geol. Ass., Lond., 34, 32-8.
Friday, 27 March
Director: W. R. DEARMAN Okehampton and Meldon. The day was devoted to the examination of the Upper Culm Measures in the neighbourhood of Okehampton and the inlier of the Lower Culm Measures at Meldon. The party drove first to Knowle Quarry (594962)1 where the symmetrical folds in the unaltered Upper Culm Measures were contrasted with the overfolds in the neighbouring Okehampton Brickworks Quarry (594961). Returning to Okehampton the roadside exposures in the Upper Culm along New Road (586948 to 583943) were examined. The party rejoined the bus and drove to the British Railways Quarries (567926) at Meldon. Here in the 860 North Bay the typical faulted zig-zag overfolds of the Lower Culm Measures inlier were well displayed. Particular attention was paid to the metasomatic development of coarse-grained calc-silicate hornfels from the black radiolarian cherts which occur near the top of the Lower Culm succession. The party walked southwards to the disused face opposite the crushing plant to see the faulted inlier of the Slate-with-Lenticles Group. The full sequence of the Lower Culm, with two basic dykes, was exposed in the adjoining 860 South Bay. After lunch the party walked up the incline road to the 950 Waste Tip, and from this vantage point the Director pointed out the relationship between the geology and the topography as far south as the granite of Yes Tor. A short walk along the 950 South Cutting brought the 1
All National Grid reference numbers OCcurwithin the 100 km, square 20 (sX).
338
FIELD MEETING IN NORTH-WEST DARTMOOR
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party to an exposure of deeply weathered limestones and cherts. The Meldon Aplite Quarries (567920) were next visited, and the traverse of the inlier was completed by walking up the valley of the Red-a-ven Brook to the disused Red-a-ven Mine (570917). Saturday, 28 March
Director: W. R. DEARMAN Be/stone and Stick/epath. The day was spent in a further examination of the Lower and Upper Culm Measures along the northern border of Dartmoor. The party left Okehampton by the footpath leading to Ball Hill (598947) and Fatherford Viaduct (603597), examining small exposures of the Upper Culm sandstones and shales on the way. The East Okement River was crossed by the footbridge just upstream from the viaduct, and by following the footpath on the west bank of the river to Chapel Ford (608934) a traverse was made from the northern normal limb of the Lower Culm anticlinal inlier, across the core of the Slate-with-Lenticles Group, into the steep beds of the southern limb of the anticline. Some members of the party continued upstream on the eastern bank to complete the traverse to the Upper Culm, and then climbed Watchet Hill (615930) to reach Belstone (619935) for lunch. The rest of the party followed the footpath northwards to the hilltop above West Cleave (609941) for the view north across the Permian of the Crediton Valley to Exmoor, then joined the road and walked to Belstone. After lunch the party walked south-westwards across the heather-covered outcrop of the Upper Culm Measures of Watchet Hill to the granite tors of Belstone Common. Below in the valley of the Taw River lay the great waste of Taw Marsh (620910); recent borings for water have revealed that beneath the marsh there is over one hundred feet of granite detritus resting on solid granite. Returning to Belstone the party took the path along the disused leat leading to Skai~ (631939) on the northern slopes of Belstone Cleave, passing crags of tuff and agglomerate of the Volcanic Group of the Lower Culm Measures. The disused workings of Ivy Tor Mine (627934) could be seen on the southern bank of the Taw River, and the Director pointed out that this marked the southern margin of the Lower Culm inlier which had also been mined farther east at Ford (643935). The dumps of the disused Belstone Consols (Mid-Devon) Mine (632945), at the northern margin of the Lower Culm inlier, were visited next. The party then walked from the mine to the end of the lane (637942) high on the hill above StickIepath. There was a clear view from here across the StickIepath Fault Zone along which the Belstone Consols lodes have been displaced over one mile south-east to the disused Ramsley Mine (650930). The party followed the steep path down into StickIepath and walked along the main road to Ramsley. Brecciated
340
W. R . DEARMAN AND N . E. BUTCHER
granite in a sand-pit (649928) near Fulford, directly opposite the mine dumps, again clearly indicated the magnitude of the offset along the Sticklepath Fault. The bus was waiting to take the party to the disused South Tawton Limestone Quarries, where the day's work ended with a fruitless search for fossils in the easily accessible exposures in the most northerly of the quarries (658951). Sunday, 29 March
Director : N. E. BUTCHER Brentor and Lydford. The day was mainly devoted to an examination of the Brentor and Lydford inliers of the Transition Series within the Culm Measures outcrop. The party drove first to Brent Tor (471804) from the summit of which low cloud unfortunately allowed only a restricted view of the north-western slopes of Dartmoor. The crags of pyroclastic rocks were examined on descending the tor. In the nearby South Brentor quarries (480806) the Upper Devonian Clymenia Zone beds were seen, lamellibranchs and ostracods being collected. The party then walked on to a quarry (480809) near North Brentor in green slaty beds of ? Wocklumeria Zone age. The coach was rejoined in North Brentor and the party drove to the Manor Hotel near Lydford Station where a nearby stream bank locality (502832) in Platyclymenia Zone slates was visited. After lunch Lydford Gorge was entered at its lower end at the elbow of river capture (Dewey, 1916), the geomorphological features exciting much discussion. The River Lyd Slate-with-Lenticles Group was examined in passing through the gorge, at the upper end of which most members visited the Devil's Cauldron. Continuous rain and a walk along a swinging plank which was awash due to the high-water level made the visit to this large pot-hole in the depths of the gorge a memorable experience. Leaving the gorge the party visited a quarry (515854) at Lydford in contact-altered calcareous siltstones of the Transition Series and then brieflycompared and contrasted them with calc-flintas of the Lower Culm Watervale Calcareous Group in stream exposures by the main road (518839) near Watervale. The day's work ended with a walk up the valley of the River Lyd from the main road towards the granite, examining the lithology and deformation structures of the River Lyd Group within the metamorphic aureole of the granite . Finally some members of the party visited a quarry (533864) in the Upper Culm Measures close to the granite near Nodden Gate . Monday, 30 March
Director : N . E. BUTCHER Marytavy and Tavistock, The main object of the day's programme was to examine the Lower and Upper Culm Measures, comparing and con-
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trasting the rocks with those seen in the northern part of the area on the first two days. The party travelled from Okehampton along the main road towards Tavistock, features of geological interest being pointed out on the way. The first stop was at Wheal Betsy (510810) where the mine dumps were briefly searched for galena before the party proceeded to examine the nearby stream section in the Upper Culm Measures. Cast structures on the bottoms of the inverted sandstones of this Black Down Group particularly attracted attention. Wheal Friendship at Marytavy was next visited where the Director gave an account of the operations of this large disused coppermine. The party then examined the nearby Famennian inlier (505792), of ? Clymenia Zone age. The coach was rejoined and a brief stop made outside Pitts Cleave Quarry (501761), the Director giving an account ofthe igneous rock types-oligoclase-dolerites, hornblende-dolerites and trachytes -to be found as sills in the quarry. The party proceeded to Wilminstone Quarry (490755) to examine the hinge of a recumbent fold in which occurs a 200-foot thick sill of dolerite. After lunch in Tavistock the top of the inverted Lower Culm Measures succession was studied in Lower Deerpark Quarry (483743) and adjacent exposures. The Whitchurch Down Greywacke Group, the southernmost outcrop of the Upper Culm Measures in the area, was examined in the River Tavy exposures immediately north of the footbridge (479741). The Director demonstrated the facies differences between these rocks and the Upper Culm Measures to the north. The party then drove over Whitchurch Down, visiting a quarry (512739) in the Upper Devonian Slate Series which crops out to the south of the Culm Measures. After Dr. G. W. Himus had proposed a vote of thanks to the Directors, the party returned to Okehampton by way of Tavistock. REFERENCES DEARMAN, W. R. & N. E. BUTCHER. 1959. The Geology of the Devonian and Carboniferous Rocks of the North-West Border of the Dartmoor Granite, Devonshire. Proc. Geol. Ass., Lond., 70, 51-92. DEWEY, H. 1916. On the Origin of some River-Gorges in Cornwall and Devon. Quart. J. geol. Soc. Lond., 72, 63-76. SHERLOCK, R. L. 1923. Long Excursion to Lydford and Brent Tor, Devon. Proc. Geol. Ass., Lond., 34, 32-8.