"Tunnels constructed in connection with hydro-electric development near the head of Glen Tromie [north-central Scotland] cut Moinian granulites and semi-pelitic schists penetrated by pegmatites and by quartz-porphyry and felsite sills. Deep and very extensive deposits of glacial drift are also present. A remarkable feature of the latter is the presence of hollow chambers well below the surface which are interpreted as partially collapsed moulds of buried blocks of ice."