The Geology of Coll is most interesting. The most common local rock is Lewisian Gneiss (pronounced 'nice'), the oldest rock in Britain and Europe, probably 2500 million years old. On Coll and Tiree it forms extensive outcrops of banded, granular rock, giving way in South Coll to a more varied outcrop of granite, Marble and Quartzite. The Gneiss has a long and complex geological history involving several periods of deep burial in the earth's crust where it was metamorphosed (cooked) and compressed before long erosion finally unearthed it. Although it is the oldest rock in Europe Gneiss is essentially 'American', part of the ancient Canadian Shield and once joined to Greenland, before the Atlantic Ocean opened.

Our excursions took us to Uig, where the Coll Marble outcrops as thin bands of attractive green and white rock, rich in serpentine and mica minerals. At Acha is a notable ridge of tough, white Quartzite, used by Iron Age peoples to build their fort on, surrounding it with walls of Gneiss boulders. At Breachacha Bay the tough Granulite rocks exhibit beautiful pink feldspars - called pegmatites - and red-brown garnets dotted throughout the rocks. Both minerals were concentrated into these rocks because of the intense heat and pressure of deep burial in the crust.

Today's Coll landscape was produced by recent ice sheets which swept over the area from Mull and the mainland during the past million years. The last ice to cover Coll can be dated to the Devensian advance, some 18000 years ago. Ice scoured and eroded the ancient Gneiss bedrock to a considerable degree, leaving a much reduced landscape notable for its spectacular perched erratic on Ben Hogh. The most recent events involved higher sea levels as Devensian ice melted 15000 years ago. The highest seas would have completely inundated Coll, but the island steadily emerged as part of a broader Scottish emergence with the removal of the weight of ice. This emergence has left raised beaches (around Chad, Arnabost) high and dry. Peat deposits eventually formed along with the blown sands and machair along western shores.

This piece was kindly contributed by Julian Hill who organised Workers' Educational Association summer schools to Coll in 1986 and '87, and who was geology tutor on both occasions.