Coll's geology is based on very old and very hard heterogeneous Lewisian Gneiss. That should spur any geologist's hand-lens & hammer into action but, sadly, that's about all Coll can offer. Compared to Mull & the Treshnish Isles, Coll's geology is quite boring and singular - except to the migmatitic specialist, who would be in heaven! All that exciting lava activity, and consequential basalt formations so obvious just across the waters to our east, were eroded from Coll well before Coll's rising displacement along the Camasunary-Skerryvore fault.
However, one can take solace as Coll's visual presentation of how gneiss has been attacked and moulded over the last two to three billion years, can only be envied.
Much of Coll's coastline exhibits stunning examples of weathering & erosion, basic dykes , foliations & Bandings and, in the dune slacks, there are some of the finest Indurated aeolian calcarenites (Caliche outcrops) you're likely to find - - although, being 'young' and not strictly 'rock', perhaps they're geography and not geology?
Anyhow, here's some photos to give you a taste of the wonders of an exposed Coll.
Click images to enlarge
© All photos, Tony Oliver.