During late Ordovician time, a thick succession characterized by coarse clastic sediment of litharenite composition accumulated in the Caledonian geosyncline in southwestern Scotland. The surface rocks in the Cockburnland source area north of the geosyncline were largely the basic and acidic lava and tuff, serpentinite, spilite, chert, and granitic to gabbroic intrusive rocks of the lower Ordovician Arenig volcanic belt. Granite and Dalradian schist and gneiss comprised smaller areas in the highlands.

An inshore "shelly facies" of 7,000 ft of shallow-water sandstone, conglomerate, shale, and limestone is preserved only in the Girvan District on the northwestern margin of the geosyncline. The abundant shelly fossils show close affinity with late Ordovician faunas of North America, but only remote relations to assemblages in the Welsh geosyncline. The Ardmillan Group on the coast south of Girvan contains an unusual combination of neritic and deltaic rocks associated with flysch sequences of graded sandstone and limestone. The Whitehouse delta was built southeastward into the geosyncline, whereas the paleocurrents in the sandstone flysch sequences in the Whitehouse Formation and the Barren Flags flowed southwest, parallel with the trend of the geosyncline. Perhaps the graded sandstone beds were deposited by ocean-bottom currents flowing across the regional SE-dipping paleoslope, rather than by downslope, gravity-controlled turbidity currents.

On the south in the Rhinns of Galloway, 12,000 ft of sandstone flysch sequences accumulated farther offshore in the geosyncline. The paleocurrent directions indicated by sandstone sole markings are very diverse, but both NE- and SW-flowing currents were common. The repeated reversals of 180° in current direction are difficult to explain within the framework of a turbidity-current origin for the graded sandstone. A possible alternative is that the graded sandstone was deposited from waning ocean-bottom currents. The thick sandstone flysch sequences thin markedly across the Southern Uplands into axial sections of dark shale and chert.