Many of the pebbles in the late Precambrian Torridonian arkose (Applecross Formation) of northwest Scotland cannot be traced to the underlying Lewisian basement complex. Pebbles regarded as exotic include quartz-tourmaline rock, tourmaline-aplite, quartz-fuchsite schist, pure and iron-rich metaquartzite, fine-grained igneous rocks comparable with porphyritic rhyolite and acid tuffs, and a large variety of orthoquartzite and chert.

The Applecross Formation was deposited as large alluvial fans which spread out from the west. These arkosic sediments, with the exception of the exotic pebbles, probably were derived from a basement complex similar to that of northwest Scotland. The approximate maximum area of the drainage basin of the Torridonian alluvial fans has been estimated by comparison with those of modern alluvial fans. The Torridonian drainage basin, as plotted on Bullard et al.'s (1965, Fig. 7) reconstruction of the North Atlantic region prior to continental drift, is within the continental margins between Scotland and southeast Greenland.

A study of the literature concerning the Precambrian rocks of southeast Greenland indicates that several of the Torridonian exotic pebble types can be matched with Precambrian rocks exposed in that region.