Foundations of the Landscape - Formation of the Rocks
The making of Glen Roy began in Precambrian times (some 1000 million years ago) when sandy and muddy sediments were deposited in shallow water at the margins of an ancient continent to the northwest. In the older parts of these sediments (the Grampian Group), some of the sandstone beds record the effects of storms, and pebbles in a few of the muddy beds may have been dropped from icebergs. Later, there was an increase in the amount of muddy sediments and of lime-rich muds and limestones, which now form a younger suite of rocks (the Appin Group). Precambrian sedimentary rocks up to several kilometres in thickness are preserved in Glen Roy, but undoubtedly they were at one time overlain by many kilometres more of rock which has since been removed by erosion.
Between about 750 and 400 million years ago the Precambrian sediments of the Highlands were involved in the formation of the Caledonian Mountain Belt, a chain of mountains extending through Scotland from Scandinavia to eastern North America. The rocks of the Glen Roy area were probably buried about 10 kilometres deep, in the roots of the mountain belt, where they were subjected to intense compression at temperatures over 600°C. The result was complex folding and the 'cooking' (metamorphism) of the rocks; the sandstones recrystallised and became tougher, the limestones became crystalline marbles and the constituents of the mudstones reacted together to form mica schists, sometimes with garnet (a semi-precious mineral).
Molten material (forming 'igneous rocks') was injected towards the end of the period of folding and metamorphism. It formed bodies of grey and pink granite, pegmatite veins with large crystals of quartz and feldspar, and dark green basic rock (appinite) characterised by well-formed crystals of the mineral hornblende. Later, in early Devonian times, about 400 million years ago, dark grey igneous dykes were intruded into the Glen Roy area. These were associated with a volcano which developed where Ben Nevis is now situated.
By this time the Caledonian mountain chain had been much reduced by erosion, and movement was taking place on the Great Glen Fault, a major line of weakness in the Earth's crust. A narrow basin or valley developed there and fans of gravel and sand were deposited in the basin by streams flowing from the highlands on either side; the gravels and sands consolidated to form the Devonian conglomerates and sandstones that occur today along parts of the Great Glen.