VI.— The Stratigraphical Evolution of the Midland Valley

Abstract

Summary The Midland Valley, though not now a rift valley, had its origin in a rift defined by the Highland Boundary Fault and the Southern Uplands Fault. Its northern margin may be clearly traced from the North Sea to the Atlantic; its southern margin is well expressed in Scotland but becomes hidden in central and western Ireland. In its earliest recognisable form the region of the rift appears to have been a positive tract, elevated to significantly higher erosion levels than the region of the Highlands to the north: during the greater part of Ordovician and earlier Silurian times it was a floor on which neritic sediments, showing repeated discontinuities in sequence, were accumulating on the margins of a geosyncline to the south. Arenig rocks on the shelf may perhaps rest on thin Cambrian or on Pre-Cambrian, but are thrown along the Highland Boundary Fault against thick Dalradian—Cambrian sediments to the north: a circumstance best interpreted as due to dislocation with downthrow to the north in pre-Arenig times. The Highland Boundary Fault may thus in origin be regarded as a “normal” fault and not a thrust, though later movements alternated repeatedly along it. Lower Old Red Sandstone mostly rests (in present outcrops) discordantly on older rocks. It was deposited in a major basin situated approximately along the rift, but a basin probably established by swelling uplift of the Highlands and the Southern Uplands, rather than by powerful movements along marginal faults. It extended without interruption or great reduction in thickness This 250-word extract was created in the absence of an abstract