The study of clast lithology within the Hornelen sequence has been used to understand the exposed geology in the areas that provided a sediment source for the basin. The currently exposed bedrock to the north and east of the basin consist of metamorphic rocks of the Western Gneiss Region (WGR) that have been affected by ultra-high-pressure metamorphism, which are everywhere in faulted contact with the basin. No clasts have been found that have evidence of high-pressure metamorphism indicating that the WGR did not become exposed at the surface until after the highest preserved levels of the basin were deposited. The clasts do match well with the rock types observed within the Caledonian thrust sheets on which the Devonian sediments lie unconformably to the west of the basin.