Past Environments
seann àrainneachd
1000 million year old ripples in Moine quartzite at Fassfern
Lochaber had an arid climate in the Triassic 200 million years ago
West Lochaber in Late Jurassic times 150 million years ago
The land which lies on the north-west side of the Great Glen belongs to what geologists call the Northern Highlands Terrane. Many of the rocks in this part of Lochaber were originally laid down in a shallow-water marine shelf environment between 1000 and 870 million years ago.
Later this same group of sediments were subjected to two major periods of deformation - the Knoydartian (830 million years ago) and the Grampian (460 million years ago). The original sediments were heated and folded and turned into what are known as Moine metamorphic rocks.
The land which lies on the south-east side of the Great Glen belongs to what geologists call the Grampian Terrane. The rocks here are younger than the Moine. They were originally deposited as a more varied group of marine sediments (muds, silts, sands and limy deposits) between 850 and 510 million years ago. Some of this group were old enough to be affected by the Knoydartian deformation, and all these rocks were folded and heated during the Grampian mountain building event. They are known as Dalradian metamorphic rocks and were once buried over 20km below the Earth's surface in the roots of the Caledonian mountains.
Marine sediments
Despite being metamorphosed by these mountain building events some of the Moine and Dalradian rocks still retain evidence of their sedimentary origin. For example ripple marks formed on the sea floor when the sediments were deposited are still preserved in some rocks such as quartzite.
Over 700 million years ago ‘Scotland’ lay near the South Pole, but as plates on the Earth’s surface moved around (see Mountain Building section) it drifted slowly northwards. On its journey north 'Scotland' experienced a great range of different climates.
The Caledonian Mountains
Sedimentary rocks are not very common in Lochaber, but those there are allow us to find out some of the different environments Scotland has experienced since the end of the Caledonian mountain-building event. The sedimentary rocks found along the Great Glen and in western Lochaber indicate that Scotland has experienced a great range of environments from tropical forests, hot deserts and warm seas to full glacial conditions.
Hot deserts
Small exposures of red sandstone and conglomerate of Triassic age found in western Lochaber tell us that the climate was very dry some 200 million years ago.
Warm seas
Limestones and shales of Jurassic age found near Lochaline and Kilchoan contain numerous fossil remains. These tell of a time when creatures such as ammonites were once plentiful in the sea. Later deposits of Cretaceous age include an interesting rock called greensand and an exceptionally pure quartz sandstone which is mined at Lochaline.
