The Northern Belt of the Southern Uplands/Longford-Down zone contains two compositionally distinct clastic petrofacies suites each ranging from mid- to late Ordovician age. They extend throughout the length of the belt and are generally separated by a strike fault. The northwestern Metaclast Petrofacies suite is characterized by felsic igneous and metamorphic detritus derived from a continental source to the NW, whereas the southeastern Basic Clast Petrofacies suite contains abundant calc-alkaline detritus derived from an island arc and plutonic/metamorphic root zone complex to the S or SE. A similar source, but located to the north, is indicated for younger clastic sequences of similar composition in the northern part of the Central Belt immediately south of a major sinistral shear zone which defines the boundary between the Northern and Central Belts. It is concluded, in light of other facets of regional geology and possible palaeogeographic configurations, that this pattern reflects a back-arc basin in the Northern Belt, margined by a volcanic arc massif which once occupied the space now represented by the shear zone, and a fore-arc basin to the SE (Central and Southern Belts). The Northern Belt back-arc basin is postulated to have been closed and deformed at the end Ordovician and subsequently overthrust northwestward by the allochthonous, thrust stack imbricated Central and Southern Belts fore-arc sequence at the end Silurian. This, in the Northern Belt, resulted in the development of a major recumbent structure, downward facing on regional (end Silurian) cleavage.