The lower Paleozoic conglomerates of the Southern Uplands, deposited in areas of rapid subsidence, are commonly coarse grained and of varied thickness. They contain a great variety of clasts, including mainly Arenigian volcanic rocks, other igneous rocks not now found locally, and metamorphic rocks of Dalradian aspect. The source area for the conglomerate appears to have been a roughly NE-SW-trending ridge called "Cockburnland," situated north of the geosyncline.

North of Cockburnland, two conglomerate bodies of Silurian age, derived generally from the south, also contain clasts of igneous rocks (Arenigian type) and of Dalradian-type metamorphic rocks.

The Lower Old Red Sandstone conglomerate beds exposed along the southern margins of the Midland Valley contain fragments of lower Paleozoic graywacke derived from the uplifted geosynclinal strata farther south; conglomerates of the northern margin contain Highland rocks derived from the north.

More detailed studies of the Upper Old Red Sandstone in the vicinity of the Firth of Clyde show that the conglomerates there belong to several dispersal systems, three of which are exposed along the coast between Ardrossan and Gourock. Each dispersal unit (shadow) comprises an evolutionary environmental sequence from alluvial fans to braided streams and, finally, to floodplains of meandering streams. There is thus an extension of the source area and the depositional area with time. The southernmost dispersal shadow contains sandstone clasts within the conglomerate; the northernmost contains schist. The intermediate shadow is typified by volcanic fragments.