Glen Fruin is a quiet secluded glen, about six miles in length, extending in a north-westerly direction between Lochlomond and the Gareloch, and opening off the neck of land which divides those lochs at between 200 and 300 feet above the sea. To the south-east, it slopes towards Lochlomond, of which the Fruin water is one of the principal affluents. On the west it is divided from the Gareloch by a range of hills 1000 or 1200 feet in height, which at its upper extremity subside into a col or pass of 600 or 700 feet. This upper part of the glen is composed of the mica and clay slates common to the Western Highlands. The lower part is formed of beds of the Calciferous Sandstone series, which have here been faulted down against the older formations.