The Lowther Hills are composed of resistant Ordovician shales, heavily compressed and contorted in the Caledonian mountain-building orogeny. The headwaters of the River Nith incise them deeply on the south, as the Nith falls much more steeply (to the nearby Solway Firth) than does the Clyde on the north. The flanks of the heavily glaciated Dalveen Pass and of the deep, steep-sided Enterkin Pass are both lined with rock slope failures (large landslips and landslides), with a deep V-shaped notch linking these passes being a probable overflow channel cut by glacial meltwaters. The original source of the Clyde is inferred to have been here, captured to the Nith by the Dalveen trough-head.