Introduction Around the close of the Silurian, but with associated igneous and volcanic events lasting well into the Devonian, occurred the major phase of the Caledonian Orogeny. The main mountain belt resulting from this, stretched from the joined area of Spitsbergen, E Greenland and Norway SE for some 4000 km, the belt being up to 600 km in width. Detritus from the erosion of this belt produced the Old Red Sandstone deposits of Ireland, Wales and Scotland with extensions in the early Devonian into southern England. Later, south of the Bristol Channel to Thames Estuary line, elastics were laid down mostly under marine conditions with a transitional zone in N Devon. Mudrocks in the Devonian thus occur in several distinct sedimentary regimes: in the north, terrestrial facies (Old Red Sandstone) formed as intermontaine basins and lacustrine areas in Scotland, and as alluvial and pedogenie facies of extensive coastal plains in southern Wales and, whilst usually thought to consist of coarse and red arenites, do, in fact, include in aggregate significant proportions of usually poorly-sorted, fine-grained rocks. The near-shore marine deposits (Rhenish facies) include mixed arenite/pelite/carbonate sequences in which the pelites may be thick. But it is in the offshore and basinal areas (Hercynian facies) that mud-grade sediments reach their finest development, seen especially in Cornwall, where successions of little else total several thousand metres. These are now in the form of indurated and cleaved argillites forming most of the ‘killas’ of southern Devon and Cornwall. The areas of outcrop of