 The lower boundary of the Islay Subgroup and hence the Argyll Group as a whole is defined at the base of the Port Askaig tillite, a diamictite which displays limestone clasts overlain by quartzite and granite clasts.The tillite is thickest in its type area and on the Garvellachs.The Schiehallion Boulder Bed of the eastern Grampians correlates with this tillite.