Two billion years of geology in one road cutting
Автор: OneMinuteGeology
Загружено: 2021-03-13
Просмотров: 1754
Just outside Laxford on the A838 is the road cutting known to geologists around the World as the Multi-Coloured Rock Stop. Here you can see fresh exposures of Lewisian Gneiss cut by successive dyke intrusions of Scourie basalt and Laxfordian granites.
The Lewisian Gneiss was metamorphosed in the roots of a mountain chain about 3 billion years ago, but it was originally a granodioritic igneous rock before that. The Sourie and Laxfordian dykes are about 2.5 and 1.7 billion years old respoectively. The cross-cutting relationships which show the relative ages of the intrusions are still clear but the whole sequence has then been deformed by later shear. This shows as semi-brittle deformation in the granite dykes which have stretched out into discrete pods resembling a string of sausages (boudinage), allowing us to estimate the sense of shear deformation quite easily here.
So we have at three periods of igneous intrusion and at least two mountain-building episodes spanning about 2 billion years of geological history here.
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