Two strikingly different successions of Lower Carboniferous (mainly Tournaisian) sedimentary rocks are closely juxtaposed on the NE coast of the island of Arran, SW Scotland. Near the village of Corrie a thin succession (~ 17 m) of Tournaisian rocks is preserved, whereas in the neighbouring Fallen Rocks–Laggan area, correlative rocks are > 300 m in thickness. These contrasting successions are separated by the Laggan Fault, which is a landward extension of the submarine Brodick Bay Fault, marking the SW boundary of the Northeast Arran Trough. The contrasting thickness and stratigraphy of the two sequences of sedimentary rocks result from juxtaposition of shoulder and trough deposits along the Laggan–Brodick Bay Fault. Although originally a normal, basin-defining fault, later sinistral movements caused significant displacement of the NE Arran Trough, together with a segment of the Highland Boundary Fault, from their original positions. The most northerly occurrence of the Highland Boundary Fault on Arran is thought to be the truncated northern end of the Corloch Fault. To the SW the surface trace of the Highland Boundary Fault is largely obscured by a Palaeogene granite body but it is present on the west side of the island, near Dougrie. The Highland Boundary Fault appears to be displaced to the south, in Kilbrannan Sound, by a series of NW-trending sinistral transcurrent faults. Thus the ‘anomalous’ trend of the Highland Boundary Fault and narrowing of the Midland Valley of Scotland in the Firth of Clyde area may be explained by later fault movements and intrusion of the Palaeogene North Arran Granite Pluton.