The Islands of Islay, Jura, Colonsay, Tiree and Coll

The Hebridean islands of Islay, Jura and Colonsay are mainly composed of Neoproterozoic metasedimentary rocks of the Dalradian supergroup, whereas Tiree and Coll are underlain by Archaean rocks of the Lewisian Gneiss Complex. Most of the terrain is low-lying; only the three peaks of the Paps of Jura exceed 600 m. Successive episodes of ice-sheet glaciation have produced areas of ice-moulded topography, and retreat of the last (Late Devensian) ice sheet was accompanied by the deposition of outwash deposits and moraines, including the longest medial moraine in Scotland. The most impressive and internationally renowned landforms on the islands are those related to coastal erosion and deposition: high-level glacimarine deposits; ancient (possibly Pliocene) glaciated strandflat; extensive areas of high rock platforms and intertidal platforms that demonstrably pre-date the last ice sheet; an isostatically tilted rock platform that formed under Lateglacial periglacial conditions; magnificent flights of Lateglacial shingle ridges extending up to 37 m above present sea level; and lower Holocene raised beaches locally backed by sand dunes and areas of machair (calcareous aeolian shell-sand) deposits. Inland, the quartzite flanks of the Paps of Jura are scarred by paraglacial landslides possibly triggered by seismic activity that accompanied rapid glacio-isostatic uplift.