Severnside FIELD TRIPS 2015 - 2016
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Severnside FIELD TRIPS 2015 - 2016 Page 3 Forest of Dean 7 Garth Hill 10 Black Mountain 15 Pembrokeshire 18 Wye Valley 20 Cardiff Building Stones 25 Weston-Super-Mare 29 Wigpool Syncline 31 Fforest Fawr Geopark 34 Malvern Hills 38 Hereford Building Stones 42 Brecon Beacons This little book contains extracts from ―Sabrina Times‖, the newsletter of the Severnside Branch of the Open University Geological Society. The field trips included were all undertaken during 2015 and 2016 and the write-ups have been provided by members of Severnside Branch. Thanks are due to those members, to the Branch and Events Organisers and especially to the various leaders on the trips. 2 Introductory Day 15th March 2015 Soudley Valley - Forest of Dean Leader: Dave Green FGS For this years Introductory Day we returned to the Soudley Valley Geology Trail in the Forest of Dean. During the course of the day we visited sites covering nearly 100 million years of the Earth‘s history. We began at Soudley Railway Cutting revealing the unconformity between the Lower Old Red Sandstone (Brownstone) and Upper Old Red Sandstone (Quartz Conglomerate) and then gradually worked our way through the geological succession ending at an exposure of the Coal Measures/Pennant Sandstone. Geological Structure of the Forest of Dean West East Soudley Valley Coal Measures Carboniferous Limestone Tintern Sandstone Group Quartz Conglomerate Under the excellent guidance of Dave Green we tried our hand at mapping the exposures. Strike and dip were measured at each site and found to be consistent: Strike = 192° Dip = 64° West. At each site these details, together with the lithology, were transferred to the locality map provided by Dave and throughout the day we built up a picture of the geology in the area. It soon became very evident how much work is involved in creating a comprehensive geological map of an area. Lines on dial aligned with North — South lines on map Dial set to 192° Strike marked along edge of compass Transferring a measured strike of 192° to a map 3 Introductory Day 15th March 2015 Soudley Valley - Forest of Dean 4 Introductory Day 15th March 2015 Soudley Valley - Forest of Dean 5 Introductory Day 15th March 2015 Soudley Valley - Forest of Dean 6 Severnside went up a hill and came down a mountain 12th April 2015 Leader: Stephen Howe A small group gathered on the edge of Taffs Well Green on a cool, cloudy-bright, April morning to explore the southern rim of the South Wales Coalfield. As we set off across the road and through Garth Wood, Stephen set the tone of the day by pointing out the clues we were to follow, the early spring growth barely covering the thin soil over shallow exposures. The weather had been dry and we were surprised by a wet patch over the path as we entered the wood. Stephen pointed to the source of water, a small spring erupting from the junction of beds and a patch of flow stone, deposited lime, all surrounded by lime-loving hart‘s tongue fern and wild garlic (ramsons). The path took us through a wood dominated by beech, whose shallow, spreading roots can thrive in the thin lime soils. Stephen led us to the remains of the Barry Railway Viaduct which had carried a branch line in the early 20th century to carry coal to Barry Dock avoiding the monopoly of Cardiff Docks. The path took us passed an exposure of Friars Point Limestone, the upper member of the Black Rock Limestone Group, part of the Pembrokeshire Limestone succession (formerly the Carboniferous Limestone). See figure 1 Brigantian Not Present Asbian Holkerian Hunts Bay Oolite 180 m. Cefnyrhendy Oolite 20 m. Arundian High Tor Limestone 65 m. Caswell Bay Mudstone 5 m. Gully Oolite 25 m. Chadian Friars Point Limestone 80 m. Black Rock Limestone Group Brofiscin Oolite 14 m. Barry Harbour Limestone 32.m Courceyan Cwmyniscoy Mudstone 43m Avon Group (Lower Limestone shale group) Castle Coch Limestone 25m Tongwynlais Formation 40m Fig. 1 Pembrokeshire Limestone Succession (by S. R. Howe) As we climbed the hill we reached the level of quarries which exploited the succession. As we walked north we rose through the succession to the Gully Oolite, a thick bedded grey limestone, then the Caswell Bay mudstone, which had been ignored by the quarrymen. The path led us by the High Tor Limestone, a thick bedded bioclastic limestone with cross bedding. The last big quarry (Fig. 2) exploited the Hunts Bay Oolite, 180m thick and like all the other lime- stones of this succession it was heavily dolomitised. Now worked out and infilled with spoil, the quarry gets its name, Cwarr Glas, from the blue colour imparted to the pool by limestone. Fig.2 Cwarr Glas Quarry where Hunts Bay Oolite was quarried. 7 Severnside went up a hill and came down a mountain (contd.) 12th April 2015 Within the quarry was a large block ignored by the quarrymen as it was fissured and filled with flowstone and stalactites deposited from flowing water (Fig.3). Further along the road we passed the tunnel entrances to iron mines under the hill. One was strongly secured as the cave system behind is extensive and dangerously flooded.(Fig.4) In its time the mine was a source of limonite and haematite, for its iron content, ochre, as a dye, a storehouse for ammunition in WWII and as a Victorian tourist attraction. The mine was served by horse drawn trams and their route was still discernible through the trees. Fig. 3 Examining pothole fissures in block left in Cwarr Glas. Fig. 4 The locked entrance to the extensive iron mine. Across the road we reached Coed y Bedw, a wooded valley floor where Stephen pointed out the clues to a change. Here the beech, ramsons and hart‘s tongue fern had been replaced by holly and Hard fern more suited to the darker acid soil over the Millstone Grit. Streams in the valley bottom also showed signs of the underlying geology. One, which issued from a spring at the junction of the limestone and Millstone Grit, contained lithified white twigs formed by the calcite-rich water. Another (Fig. 5) flowing out of the nearly buried adit to the Coed y Bedw drift mine, was red from the iron ore. As we walked up along the side of the valley we could see further indications of the industrial heritage, stone sleeper blocks from the tramway, the overgrown remains of the mine manager‘s house and waste tips from small coal mines now being exploited by the farmer. These drift mines were exploiting the few thin coal seams in the Upper Coal Measures, Pennant Sandstone Formation. Fig. 5 A red stream flowing from Coed y Bedw drift mine. 8 Severnside went up a hill and came down a mountain (contd.) 12th April 2015 As we ascended Garth Hill [Allegedly the inspiration for the film ―The Englishman who went up a hill and came down a mountain] a cool wind had increased which did not encourage lingering. We paused long enough for Stephen to point out how the landscape had been shaped by the geology and the action of ice (Fig. 6). The last glacial had cut through the Pen- nant Sandstone Formation, which underlies Garth Hill, and accentuated the offset caused by faulting. Ponding of the ice had widened the valley at several points in response to the differences in resistance of the underlying geology. Our walk back down the valley took us through an abandoned village now engulfed in a conifer plantation. We followed the track of another tramway through the shadowy remains of old bell pits. On our return we passed a recently created memorial to the men and boys who died in a mining accident, (Fig.7). This park and the evidence shown us by Stephen in the surrounding area indicates the long industrial history of Taffs Well which is closely tied to the geology. Fig.6 Walking along the top of Garth Hill. Fig. 7 Memorial to the loss of life in the Lan Mine Disaster, 1875 Report by Averil Leaver 9 Traverse of the Black Mountain 24th May 2015 Leader: Geraint Owen Introduction This field trip to the Black Mountain in south-west Wales was led by Dr Geraint Owen, an associate professor in the Geography Department at Swansea University. During the trip we examined the rock types, fossils, rock succession, landscape and industrial uses of the rocks of the Black Mountain area north of the South Wales coalfield. The Black Mountain lies in the western part of the Brecon Beacons National Park, due north of Swansea, on the northern edge of the South Wales coalfield. Note that it should not be confused with the Black Mountains (plural) in the eastern part of the National Park. We explored four separate locations during the traverse, as shown on Fig. 1 We all met at a lay-by on the west side of the A4069 north of Brynamman (Location 1). Once assembled, Geraint described the geology of the area and the proposed itinerary. His original plan was to travel northwards from Location 1, across the Black Mountain summit, finishing the trip at Carreg Cennen Castle, Location 4. This would take us from the younger rocks in the south through progressively older rocks as we headed northwards. However, low cloud and a cool breeze in the morning persuaded us to drive to Carreg Cennen Castle after visiting the first two locations so that we could enjoy our lunch Fig. 1 Locations visited in the comfort of the excellent tea-room at the castle.