The Geological Evolution of Wales and the Adjacent Regions

Summary The thicknesses of the Lower Palaeozoic rocks in the region are represented by isopachytes, and by summation isopachytes are constructed for the Cambrian, Ordovician and Silurian systems, as well as for the Lower Palaeozoic. In the centre of the geosyncline the rocks may have reached a thickness of nearly 40,000 feet and the slope at the base of the Palaeozoic on its eastern flank was about 16°. The average rate of accumulation of the sediments is estimated for each system. Earth-movements which preceded the deposition of the Old Red Sandstone affected an area extending for some miles on each side of the Irish Sea. Within this area the Old Red Sandstone rests unconformably on all earlier formations, but to the eastward it follows conformably on Silurian rocks. Evidence is adduced that the so-called Caledonian movements which folded the Lower Palaeozoic rocks of Wales occurred after the deposition of the Lower Old Red Sandstone, and, further, that the formation probably attained a thickness of many thousands of feet over large areas in Wales and in England where it no longer occurs. It is inferred that the sediments were derived largely from the products of tropical or sub-tropical weathering and that they were mainly products of river action. The Upper Palaeozoic is estimated to have attained a thickness in South Wales of 20,000 to 24,000 feet. The effects of post-Carboniferous movements and the subsequent history of the area are briefly reviewed.