The Middle and Upper Devonian Catskill facies of New York and Pennsylvania interfingers westward with marine rocks. It was derived from a geologically complex source area that lay to the east and included low-grade metamorphic with some sedimentary and volcanic rocks. Three subsidiary facies are recognized in the Catskill. Two are devoid of marine faunas and show varying proportions of conglomerate, sandstone, and siltstone. Their sedimentary textures and structures, and also details of stratigraphical succession, indicate deposition in alluvial environments when compared with modern fluvial sediments. The third subsidiary facies, which generally occupies a more distal position in the Catskill dispersal system, contains current-worked marine faunas, and on the basis of comparative studies, may have been formed in a tidal environment.

The Catskill is similar in major respects to the Old Red Sandstone of southern Britain and Spitsbergen. Assuming continental drift, a rational Devonian paleogeography is developed for the North Atlantic region that involves the symmetrical occurrence of marine-influenced coastal plains in relation to a unified Caledonide fold-belt and, within the limits of the fold-belt, a number of intermontane basins apparently unconnected with the sea.