Mid-Devonian tectonics and sedimentation in the Bristol Channel area

Red Sandstone ’ formations. The Trentishoe Formation is some 1OOOm thick and consists of lithic arenites which petrographically match closely with those in the higher parts of the Lower Old Red Sandstone in South Wales. This formation was derived mainly from erosion f the highest Lower Old Red Sandstone in early mid-Devonian times as a result of end-Caledonian uplift and erosion in S Wales. The Rawns Formation which follows contains 120 m of conglomerates and pebbly sandstones with angular clasts of porphyry, tuff, quartzite and lithic arenite which do not match with rocks in possible source areas in Wales, but could have been derived from the Lower Palaeozoic and Precambrian rocks believed to lie at shallow depth to the north of the Bristol Channel Fault Zone. Fault movements along this zone are believed to have generated relatively short-lived coarse clastic supply at various times and places around the Bristol Channel in Lower to early mid-Devonian times, in response to end-Caledonian movements. The later production of late Namurian to Westphalian sands from the Bristol Channel area was unrelated to these Devonian events, and represents uplift in response to Variscan events in SW England. Evidence is gradually growing for the existence in the late Palaeozoic of a positive area in the region of the presen