The Younger Sedimentary Rocks : records of life
The oldest rocks on Skye, the Lewisian gneisses, occur mainly on the south-east coast of Sleat where they form low, well-rounded rocky hills and produce poor, acid soils. The ancient gneisses are characteristically streaked and banded and any remaining trace of the original rock types have been obliterated by repeated earth movements during their long history. They have been thrust over the top of the younger Torridonian rocks, here predominantly red and brown, gritty and pebbly sandstones, and rocks of the Moine Group, similar to those of the Torridonian but metamorphosed by the heat and pressure of the earth movements. These rocks form the moderately high, heather-clad hills with predominantly terraced slopes. To the north of Loch Eishort, the Torridonian rocks are thrust over the top of still younger rocks of Cambrian to Ordovician age at the north-western edge of the thrust zone.
Some of the best agricultural land on Skye is found in the Strath area between Broadford and Loch Slapin. Here the limestones of Cambrian to Ordovician age have created smooth, well-drained, grassy slopes ideal for crofting. Important small shelly fossils have been found in these rocks. These fossils are similar to some of the first hard-shelled organisms to appear worldwide in early Cambrian times. They are, however, very rare and unlikely to be found by the casual visitor. More easily found are the infilled burrows of worm-like organisms which are quite common in parts of the quartzites (hard, very pure sandstones) which lie under the limestone. Some of these burrows are trumpetshaped and the rocks are commonly known as 'Pipe Rock'.
Much of the area between Broadford and Loch Eishort, the Strathaird Peninsula and the northern and eastern coastal areas of the Trotternish Peninsula consists of sedimentary rocks of Jurassic age. Smaller outcrops occur between Portree and Broadford.
Lower Jurassic rocks were deposited mainly in shallow seas and are a varied sequence of limestones, shales and sandstones. On Raasay, this sequence includes the Raasay Ironstone which was mined during the First World War.
Middle Jurassic rocks were deposited mainly in estuaries and are predominantly sandstones with some shales and a few thin limestones. The quartz grains of the sandstones are often cemented together by calcium carbonate which can result in some very strange and fascinating weathering characteristics. At Valtos, for example, the sandstones have been weathered by the elements into large cannonballs which can be up to a metre in diameter. In other places the rocks 'weather-out' in an irregular manner to give a curious 'honeycomb' effect which can be seen in the sea cliffs near Elgol.
Some of the shales from the Jurassic rocks were found to contain up to 17 gallons of crude oil per ton of shale. Although they were never mined commercially onshore, the presence of these oil-rich shales has promoted recent searches for oil in the sea areas off the coast of Skye.
Virtually all of the Jurassic rocks contain fossils. These fossils enable geologists to sub-divide the strata locally and compare them with established Jurassic age successions throughout the world. The limestones and shales are particularly rich in fossils which commonly make up a large part of the rock itself.
The most common fossils are different kinds of oysters and other two-shelled organisms, gastropods (sea snails), ammonites (elaborate coiled shells of squid-like creatures), belemnites (like cuttlefish but with a hard, pointed internal shell), crinoids (sea lilies which, despite their name, are actually animals related to sea urchins, and appear to grow from the sea bed like a plant) and corals. Fossils of microscopic organisms are abundant and are particularly helpful to geologists in sub-dividing rock successions. At the other extreme of size are the remains of reptiles such as ichthyosaurs and plesiosaurs - the sea living relatives of the dinosaurs which roamed the land at this time. Fossil plants and indeterminate fragments of fossilised wood show that there was a landmass close by and give an impression of the tropical nature of the vegetation.