Landscape Cameos
By way of a series of eight landscape cameos, the relationship between geology and landscape is explored. The profound influence of rocks and landforms on the development of soils, habitats and land use is readily apparent. The most obvious distinction is between Highland and Lowland Scotland. The generally acidic rocks of the Highlands, for example, support only poor soils, so only rough grazing and traditional Highland sporting pursuits are possible there. However, this mountainous landscape provides a major economic asset, especially for the tourism industry. The Midland Valley, by contrast, is underlain predominantly by sedimentary rocks, so the countryside is softer and more undulating. In most cases these different landscape types are determined by changes in the underlying bedrock which, in turn, are a legacy of Scotland's eventful geological past.
Southern Uplands and the Tweed Valley
The undulating countryside of the Scottish Borders is perhaps less dramatic than the rugged lands to the north, but it has its own particular appeal. The Southern Uplands are built almost entirely of the alternating sequences of sandstones and shales which accumulated on the floor of the Iapetus Ocean separating Scotland from England for many millions of years. The rocks were baked hard when they were buried and heated during this mountain-building process. However, over time, the high peaks were worn down by ice, wind and water, creating the gentle undulations which are now evident.
Midland Valley
The term Midland Valley describes the major geological terrane between the Southern Uplands Fault to the south and the Highland Boundary Fault to the north. The term 'valley', however, is misleading as the topography reflects many different rock types as well as the effects of ice and water erosion and more recent sea-level changes.
The predominant rocks are sandstones and conglomerates with local concentrations of coal from Ayrshire to East Lothian. For the most part, these rocks provide gently undulating topography rising generally to around 100m above sea level. More prominent is the landscape of the igneous rocks. The basaltic lavas form major upland masses, predominant being the Pentlands, Ochils, Sidlaws, Campsies and Kilsyth Hills. The remnants of volcanoes are a singular form in the landscape, for example, the 'laws' of North Berwick, Traprain and Largo, along with the seven hills of Edinburgh. Smoothing and plucking by ice are very pronounced on all of these hills and similar ones, and all show a very distinct east/west orientation. Interestingly, Glasgow is built on a field of drumlins made of till.
The three major river valleys, Clyde, Forth and Tay, have been deepened by the ice and are now infilled with great thicknesses of sediment. Around their shores are fine sequences of terraces left when the sea level fell. The more exposed coasts have rock platforms formed at an earlier period of the Quaternary.
Eastern Highlands
The landscape of the Eastern Highlands is dominated by the mountain massifs of the East Grampians, predominantly the Cairngorms and the Monadhliaths, sandwiched between the Great Glen Fault and the Highland Boundary Fault. The area comprises generally red granites which have been exposed at the surface since at least the early Tertiary period, surrounded by predominantly Dalradian crystalline rocks. A few significant faults have created lines of weakness which ice and water have exploited, for example, Glen Tilt.
The landscape owes much to weathering during the humid climates of the Tertiary period and the later effects of ice and meltwater. The rolling plateaux with relict tors and the easterly flowing river systems contrast with the steep sides of corries and glacial troughs. The slopes, covered with debris from the ice, have been reworked into many forms since the end of the Ice Age and are still on the move under extreme circumstances such as flash floods and avalanches. The valley basins reflect weathering and erosion over some 50 million years of Earth history. During the last glacial period the valley bottoms were choked with glacial debris which has subsequently been reworked into terraces right down to the present river levels.
Western Highlands
The Western Highlands are Scotland's premier mountain and loch landscape, formed predominantly of Moine schists. The rocks of the angular peaks and ridges are very resistant to erosion. The mountain topography is rugged as a result of ice action and later periglacial activity.
Steep-sided and very deep valleys penetrate the mountain areas. Formed originally by streams flowing into the newly opening Atlantic, they are short and have very steep gradients. Most have been deepened and straightened by ice action. Some are now filled with sediments deposited during the Ice Age, some filled with fresh water forming numerous lochs, and others filled with sea water where their beds extend well below sea level. Around their fringes are remnants of beaches from periods of higher sea level and in places large accumulations of sand.
On the western seaboard and on the islands of Mull, Rum and Skye, rock types have a more pronounced effect on the landscape. Here the basaltic lavas, best seen on Mull and Skye, give a stepped landscape and the remains of volcanoes, especially the Cuillin of Rum and Skye, form significant mountain groups.
Moray Firth and Caithness
Chanonry Point guards the inner Moray Firth. It has developed in post glacial times in response to the movement of sand and gravel along the coastline. Here and elsewhere along this coast are massive accumulations of sands (e.g. at Culbin) and the coast is still extending seawards. The firth itself was excavated during the Ice Age by glaciers moving north-eastwards and later occupied by the sea as water levels rose in response to the melting of the ice. Inland, the bedrock of red sandstones gives a gently undulating landscape subtly moulded by the ice as it scoured the bedrock.
Further to the north the red sandstones give Caithness its predominantly flat appearance with the sentinenl quartizite hills of Scaraben and Morven on its southern edge. This is the land of the blanket bog - thick carpets of sphagnum and many pools which have built up over the last few millennia. High humidity and rainfall along with the poor drainage characteristics of the subsoil and bedrock produce conditions which are ideal for this habitat to develop and thrive.
Northwest Highlands
This area of Scotland is known the world over for the grandeur of its scenery. Rugged peaks of Torridonian sandstone, sometimes capped by Cambrian quartzite, rise from an undulating base of Lewisian gneiss, ground down to a characteristic lochan-strewn surface by the passage of ice. The isolated sandstone peaks were at one time part of a continuous cover which buried the gneiss to a depth of a kilometre or more. This is described as an 'exhumed' landscape. Although heavily glaciated, some of the landscape features of the Lewisian surface are those which were present when the Torridonian sediments were deposited some 800 million years ago. Slioch, on the north shore of Loch Maree, illustrates one of the most obvious examples of a landscape which developed during Lewisian times. A 'valley' cut into a bedrock of gneiss was later infilled by Torridonian sediments, so providing the keen observer with a glimpse of an earlier landscape.
Hebridean Islands
The Outer Hebrides are founded almost entirely on the ancient and hard Lewisian gneisses. The landscape which has developed is, however, quite varied. The high ground of Harris and South Lewis and the spine of hills which runs from North Uist to Barra are rounded and smoothed and have very little debris or soil on their surfaces.
Much of Lewis is low lying, and high humidity, high rainfall and poor drainage have resulted in the formation of extensive areas of blanket peat, similar to those in the Flow Country.
The Hebrides are perhaps best known for the 'machair'; the plain of calcareous shell sand which stretches down the west coast. The rich 'soil' and the particular type of cultivation have resulted in very rich flora and outstanding flowering plants during the summer. This is a cultural rather than a natural landscape. Looking more closely at the landscape, we find an amazing succession of types: the offshore rock reefs, the sandy floor of the sea bed, the small dune ridge now being eroded because of stormier conditions, the machair itself, a substantial number of small lochs (usually freshwater but occasionally brackish water), then an area of blanket peat, and finally the high ground which is interspersed with sea lochs.
Northern Isles
The geology of the Northern Isles is similar to that of the Scottish Highlands, and the archipelagoes of Orkney and Shetland which were extensively glaciated during the Ice Age. The igneous and metamorphic rocks of Shetland have a predominantly north/south trend which gives the landscape a pronounced ridge and valley topography. The glaciers deepened the pre-existing valleys, which were later flooded to form the distinctive sea lochs, known locally as voes, when the ice melted around 14,000 years ago and the sea level rose, Weisdale Voe is typical of the coastal scenery on Shetland. As well as moulding the landscape into gentle curves, the ice also left a thin mantle of rock debris, consisting of boulders, stones, sand and clay which smoothed out irregularities in the bedrock on the lower slopes of the valleys.