Geology of Sark
by Felicity Belfield, author of 'Sark Rocks', an illustrated guide to the geology of Sark, available from La Société Sercquaise.
Sark is made up of a collection of metamorphic and igneous rocks, some of which are so beautiful that they attracted the attention of people back in Neolithic times, making the island, for them, somewhere alluring and special. Most of us continue to think of it as somewhere special, and, if proof were needed, the geology certainly confirms it. Below is a description of Sark's rock types.
Gneiss is a German word meaning altered rock. Biotite gneiss is the country rock of most of Sark, excluding Little Sark, and it is about 2 billion years old. It probably originated as sedimentary mud mixed with lava. It has been subjected to many episodes of metamorphism, folding, crushing and weathering, so it varies in appearance in different parts of the island. Often it is a sandy, cracked and crumbling rock but the true colour of a newly exposed surface is a glittery grey, and often has streaks and lenses of pale coloured quartz and feldspar running through it.
Hornblende gneiss is similar in age to the other gneisses. The dark green or black hornblende often has bands of pale granite and pale green epidote running through it. Epidote is usually associated with altered hornblende.
This is altered granite and has large eye-shaped phenocrysts of feldspar embedded within it. On Sark it is only found at Maseline Harbour or on Les Burons. It occurs more commonly in Guernsey and Alderney.
Granite is an igneous rock made of quartz, felspar and dark mica. The type on Sark has a high proportion of dark mica making the rock darker than a normal granite, so it is called diorite. Diorite was intruded over the ancient gneiss about 680 million years ago, in the Pre-Cambrian Age. It makes up all of Little Sark and the northern most end of the island at the Eperquerie. There is also a sheet of granite, rather like ham in a sandwich, running through the central part of the island. It is clearly seen in the cliffs of the east and west beaches.
The Dyke Rocks
All the rocks are criss-crossed by intrusive dykes that seem to run east-west. When country rock has become settled and cold and brittle, large earth movements can cause cracks to form which may penetrate down through the crust till they reach molten or semi-molten magma. This magma will then shoot upwards filling the crack and, when it has cooled and hardened, becomes a dyke rock. What the dyke rock is made of largely depends on the depth. Three types are found here, they are;
Dolerite - A smooth black or dark grey rock, related to basalt and gabbro. A few of them are porphyritic, where extra large phenocrysts of feldspar have had time to grow. This makes it special in more ways than one. The feldspars in it are large and beautifully distributed in the black ground mass, which clearly attracted the attention of ‘stone age’ people here.
Felsite - A whitish rock made of quartz and felspar. These dykes tend to follow after dolerite intrusions, rather like a last gasp.
Lamprophyre - There are comparatively few compared to the dolerites. They intruded all the rocks of the Channel Islands 280 million years ago (during the Permian age) making them the youngest rocks in the Channel Islands. Although they have a rather dull, earthy appearance, they are interesting because they originate from deep below the Earth’s crust and therefore have a different composition. They contain mainly dark mica, hornblende and olivine. They have a high carbon dioxide content as a result of which they rise rapidly upwards almost like a blow-out.