Fracturing

When rock formations are brittle (for example near the Earth's surface where the temperatures are lower) they will crack or fracture rather than fold. Fractures which are large are called faults. One of the largest faults in Scotland is called the Great Glen Fault, and the part of Scotland to the northwest of this fault has moved tens of kilometres (perhaps more than a hundred kilometres) relative to the part of Scotland to the southeast of the fault. Although the inset diagram shows the top part of Scotland moving to the northeast, the Great Fault has a long history of activity and it is believed that there has been substantial movement in the other direction, too. Along this fault several deep and narrow lochs have formed - Loch Ness, Loch Oich, Loch Lochy and Loch Linnhe. These were later joined up in an ambitious engineering project which constructed 35km (22 miles) of man-made canal to form the Caledonian Canal, which opened in 1822.

Great Glen

Part of the Caledonian Canal

Most fractures are smaller than the Great Glen Fault, and the relative movement along the fault may only be a few metres.

Fault

Bedded sedimentary rocks which show displacement by faulting

Sometimes fractures and faults are filled with quartz. Water continually circulates through open fractures, and just as stalagmites and stalactites are formed drip by drip on the floors and ceilings of caves, so silica is gradually deposited from the circulating water within fractures. Infilling of these fractures takes a long time. These quartz veins, as they are called, can be all sizes from microscopic to tens of kilometres long. Many quartz veins near Tyndrum are large, and one of them contains substantial gold which, like the quartz, was precipitated from solution.

Vein

Large white quartz vein on Meall Odhar, near Tyndrum. This vein does not contain significant gold.
The trench which runs parallel to it was apparently dug many years ago in an effort to find lead deposits.

As discussed in earlier sections, sometimes fractures and faults are filled with uprising magma, usually dolerite, which is a member of the gabbro family. These are called dykes because they are usually nearly vertical and sometimes they weather to form upstanding ridges a bit like a stone dyke. A swarm of dolerite dykes occurs on the shore of Loch Fyne just southwest of Strachur.

plate 13

PLATE 13 FRACTURING IN ROCKS, WITH LATER INFILL VEINS
A. Fracture in schist later filled with quartz which was deposited from solution
B. Quartzite with fracture first filled with quartz from solution, then re-fractured and filled with molten magma,
which has chilled to a fine grained basalt. An unusual specimen. C. Drill core with schist to left and quartz to right;
part of an underground quartz vein within schist.

Sample 13A (Plate 13, sample A) shows a narrow vein of quartz which has infilled a fracture in schist.

Sample 13B shows a quartzite rock which was fractured and then infilled with quartz, which was gradually deposited from solution.. Subsequently the fracture re-openened and was infilled with a thin ‘dolerite' dyke (although when the grain size is so fine we would normally call this a basalt dyke). The dyke was a magma from depth.

Sample 13C is a short piece of drill core from diamond drilling at Tyndrum. The core shows part of a quartz vein in schist. (The term ‘diamond drilling' usually has nothing to do with the search for diamonds. It is a drilling technique which uses a circular bit tipped with diamonds and which can cut through the rock to produce a continuous core of rock.)

quartz

Vein of white quartz on shore just south of Creggans Inn, Strachur
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