Natural Heritage - Landscape in the Western Highlands

The geology of the Area is highly complex and encompasses part of the most geologically diverse area of Scotland, the western seaboard. A wide variety of sedimentary, metamorphic and igneous rocks occur and represent nearly all the geological periods dating back to the pre-Cambrian, with some rocks dating back 3000 million years. These are some of the oldest rocks in Britain.

The Torridonian sandstone rocks in the far north west were originally sediments deposited 1000-800 million years ago. They are divided from neighbouring rocks by the Moine thrust, which is of international importance as it was from here that it was possible to develop thinking on plate tectonics. From evidence at the Moine thrust it became apparent that older rocks could lie on top of younger rocks.

400 million years ago the future Britain lay near the equator and had a very different climate and shoreline from that which it has today. The land surface of the area that was to become the West Highlands was then c1 km above the current height of Ben Nevis, with mountains and volcanoes rising higher still; massive earth movements having created the Caledonian mountain belt, mountains to rival the alps. The Great Glen Fault, which runs through Loch Linnhe and up into the Moray Firth was active at this time. This major line of weakness in the earth’s crust effectively splits the Highlands in two. Although there has been considerable movement along the fault in the past, current tremors are smaller and more infrequent.

It was at this time that deep under the ground, the future Ben Nevis was forming, Beneath the former land surface of volcanic lavas overlying schists and magma, a large block of schist subsided, allowing molten magma to flow into the space that it had created. The block of schist then sank again, allowing a second mass of molten magma to fill this second space. Soon afterwards, a ring fault developed in the upper rock layers around where the magma injections had been, and a cylindrical mass of rook sank into the underlying magma. Over the following millions of years, erosion removed the surrounding land surface to expose the sunken plug of rock beneath. It is this plug, with its original cap of lavas still intact, that now forms Ben Nevis.

Glencoe also formed by the development of a ring fault and the subsequent sinking of the rock plug it created. In Glencoe, however, this ‘Cauldron subsidence’ was on a far greater scale than at Ben Nevis. Glencoe is actually the world type locality for this feature, as it was from here that the feature was first described in detail.

In more recent times, about 65 million years ago, during the Triassic period, when dinosaurs roamed these parts, as evidenced from fossils found on Skye, the West Coast again saw considerable volcanic activity. The remote and rugged lands of Ardnamurchan are composed mainly of volcanic gabbro; a volcanic centre on Skye created the renowned Skye Cuillins; and a volcanic centre on Rum spewed lavas that helped form the small Isles. Later erosion of these basalt layers helped to give the islands their distinctive appearance: the islands’ most prominent natural feature is the Sgurr of Eigg, a ridge of columnar pitchstone lava nearly 400 meters high, the largest single amount of this rock type in Britain. The lava formed steep sea cliffs too. Similar ‘stepped’ landscapes also occur on Skye, and where these are underlain by weaker sediments spectacular landslips have occurred, as at Trotternish and on Raasay.

In East Lochaber mountain ranges such as Ben Nevis and Bidean nam Bian, which sits brooding over Glen Coe and Glen Etive, dominate the landscape, and ancient rocks that have been affected by heat and pressure in the earth’s crust dominate the geology. They are mainly quartz- and mica-rich with pockets of limestone and slates. They have a strong NE-SW structural grain running parallel with the Great Glen Fault. There are also large intrusions of igneous rocks, particularly granite.

Throughout time, the erosive power of nature has been working away at the rock, with the final major reworking of the landscape occurring relatively recently, in the last two million years. The coming and going of ice sheets and glaciers accompanied by the forces of moving ice and water and the rise and fall of sea levels has resulted in the formation of a plethora of glacial features, many of which were formed during the latter stages of the last ice age, about 12,000 year ago.

The hills have been heavily affected and the Skye Cuillin contain the most ‘alpine’ type scenery in Britain, with superb examples of corries, arêtes, troughs, rock basins and ice-moulded bedrock. Rannoch Moor is a high-level amphitheatre where ice built up before breaching watersheds and spilling out to gouge out adjacent glens. Some of the Area’s ice-carved U –shaped valleys have been flooded to form spectacular fjord-like sea lochs such as Loch Nevis and Loch Hourn, or the deep, narrow freshwater lochs of Morar and Maree. Ice scouring is extensive and glacial deposits are well represented, with lateral, terminal and hummocky moraines. Deposits, of sands and gravels, laid down by ice and glacial rivers, can be found throughout the Area, with notable depositions in e.g. Glen Tarbert, Glen Spean and Glen Docherty. In and around Glen Roy can be seen the internationally renowned and much studied Parallel Roads - shorelines of former glacial lakes that are etched into the hillsides. And around Arisaig and near Acharacle there are sites which are important for studies into the history of sea level change.

The rocks of the Area, and the past geological processes that have formed, changed and moulded them, have profoundly influenced the development of the landscape and the life that it supports; in more recent times, the presence of humans, has also made its mark.