Setting the Scene
Since the early nineteenth century, professional and amateur geologists alike have been inspired by the spectacular landforms, geological structures, rocks, minerals and fossils of Skye. Generations of students have learned their practical geology here and many fundamental theories of worldwide significance have been developed and tested.
The oldest rocks on Skye are found on the Sleat Peninsula. These Lewisian gneisses are also some of the oldest in Europe. They were formed around 2,800 million years ago from a wide variety of even older rocks which were modified by the effects of heat and intense pressure deep in the earth's crust.
By about 1,100 million years ago, these Lewisian rocks had been raised to the surface of the earth by powerful earth movements and had been worn away by wind and water to form a hummocky land surface. This surface was then gradually buried beneath several thousand metres of gritty and pebbly sandstones, known as the Torridonian - the products of fast-flowing rivers which swept across an otherwise hot, dry landscape.
At this time, the Lewisian gneisses and their Torridonian cover were part of a huge continent, the remains of which now form large parts of Canada and Greenland. North-western Scotland lay on the southeastern margin of this continent which, by 550 million years ago, had been eroded to a gentle, low-lying landscape bordered by a broad, shallow continental shelf. Beneath its tranquil seas, a sequence of sands, silts and limy muds were laid down during the Cambrian and Ordovician periods.
At the same time, far to the south-east, dramatic earth movements were afoot.
These movements caused intense folding, alteration by heat and pressure, and even melting. The resultant metamorphic and igneous rocks were lifted up (between 470 and 400 million years ago) to form the Caledonian Mountain Belt whose roots now constitute the bulk of the mainland Scottish Highlands. In scale this was probably similar to the present-day Alps and extended from the Appalachians, through Ireland, northern Britain, and Norway. The north-western limit of these effects passes through the Sleat Peninsula, where the Lewisian gneisses, the Torridonian sandstones, and a group of metamorphosed sandstones from the south-east called the Moine have all been sliced up and folded by the earth movements.
Over the next 200 million years, there is no record of any geological events on Skye. Any rocks which may have been deposited during this time have either been removed by erosion or covered by younger rocks. It is in the Triassic Period, some 250 million years ago, that the record resumes.
By this time, Skye was part of a hot, dry desert area in which silts, sands, and gravel were deposited periodically on wide floodplains. By the beginning of the Jurassic Period, this continental area had been eroded to a low-lying plain, the sea began to encroach and sands and mud were laid down on a shallow marine shelf.
The warm seas teemed with life, and the resulting sedimentary rocks are the most fossil-rich in the Hebrides.
Subsequently, the land surfaces stayed very close to sea level, so that sediments were deposited in estuaries, deltas, mudflats and lagoons, which ranged from freshwater to brackish or, more rarely, marine. The climate was tropical, and coal-forming coastal swamps developed from time to time.
At the end of the Jurassic Period, the sea once again crept across the area, depositing a thick layer of mud.
Another gap in the record follows - from about 140 million to 95 million years ago when, later in the Cretaceous Period, most of the Hebridean area is believed to have been covered by an extensive sea. Widespread deposits of organic oozes, rich in lime, built up on the sea floor. These strata we now recognise as the 'chalk' found mainly in southern and eastern England.
At the beginning of the Tertiary Period, some 65 million years ago, the area of the Hebrides was raised above sea level and most of the rocks of Cretaceous age were eroded away to leave an almost level surface of Jurassic and older rocks. At this time, movements deep inside the earth were beginning to split the earth's crust into huge 'plates', and Greenland began to split away from north-western Europe along the line that is now occupied by the North Atlantic Ocean.
As a consequence of these plate movements, an extensive system of fractures began to develop in the earth's crust. Through these fractures magma welled up and erupted in what was probably the most extensive volcanic episode ever experienced in north-western Europe. The products of these eruptions spread over both sides of the widening North Atlantic. Today they are found in the Inner Hebrides, Northern Ireland, The Rocknall Bank, the Faroes, Jan Mayen Island, Iceland and east Greenland.
The initial outgoings rapidly built up the vast plateau of northern Skye. Later activity was more localised and large central volcanoes developed. The volcanic cones and craters have been eroded away, but their roots remain - they form the Cuillin and the Red Hills.
By 52 million years ago, the igneous activity was all over. Sedimentary rocks of Tertiary age are known to have been deposited on top of and, in places, between the lavas, although they are now largely preserved offshore beneath the Sea of the Hebrides. The whole area was then gently stretched and titled towards the west. This caused faulting, and the faults governed the position of several valleys and the shape of large stretches of the coastline.
For most of the next 50 million years, the area around Skye experienced weathering and erosion in a mostly warm, sub-tropical climate. It was during this long period of erosion that the main elements of Skye's dramatic landscape were developed. The landscape which we see today is determined essentially by the structure and resistance to erosion of the rocks, but there were further - and considerable - modifications of the landforms by glaciers during the Ice Age.
At the beginning of Pleistocene times some 2.4 million years ago, the climate cooled dramatically. Major cycles of climatic fluctuation began. Initially cold episodes, during which mountain corrie glaciers probably first appeared, were separated by shorter warm episodes every 40,000 years or so. The climatic fluctuations, however, became more pronounced about 900,000 years ago. The changes in climate became even more extreme about 450,000 years ago, since when there have been four long, intensely cold, 'glacial' episodes separated by short warm 'interglacials' at roughly 100,000 year intervals. Most of the soils, sediments and weathered rocks which had formed during Tertiary and early Pleistocene time were swept away by ice in the first major widespread glaciation, and it is after this event that Skye probably first became an island.
During the last widespread glaciation (30,000 to 14,700 years ago), Scotland was covered by an ice-sheet. It enveloped all but the highest pinnacles on Skye and swept away most of the older glacial deposits. Following a sudden climatic warming about 14,700 years ago, the ice cap started to melt. The freshly exposed stony soils were soon colonised by pioneer vegetation such as grasses, sedges, clubmosses, 'alpine' herbs, and dwarf willow. Next came a mosaic of juniper scrub, crowberry, heather, grassland, and birchwood.
Summer temperatures similar to those of today occurred until about 13,500 years ago, when the climate cooled again. By 12,500 years ago, a tundra environment had returned and arctic conditions prevailed.
During a final, relatively short, glaciation 12,500 to 11,500 years ago, corrie glaciers and substantial areas of ice and snow accumulated in the mountains. The surrounding lowland areas were frozen wastes, similar to the conditions found today in parts of arctic Europe and Asia.
Some 11,500 years ago, the climate warmed rapidly. The glaciers melted. The initial pioneer vegetation was soon replaced by birch and hazel woodland, ferns, tall herbs, and scattered willows. Oak and elm arrived on Skye by about 10,000 years ago, together with some pine. Alder arrived much later. Mixed birch-hazel-oak woodland flourished until the climate became wetter some 6,000 years ago. At this time, people first began felling trees to plant crops - the beginning of human influence on the landscape.
Grass and heather moorland began to expand at the expense of the woodland, and blanket peat started to accumulate. Pine stumps, regularly found in the peat, are proof of a short-lived expansion of 'Caledonian' pinewood about 4,500 years ago.
As we approach modern times, large areas of woodland survived in the south of the island until about 300 years ago, when cattle grazing became more widespread. As the woods were felled, the human factor became still more telling with increasing use of peat as fuel.