Tuesday 13 October 2015

Saligo's super geology

15 people joined us on Dave's geology walk at Saligo on Sunday, which turned out to be a fascinating and informative afternoon. Dave started off by a quick resume of basic geology including the 3 categories of rock - igneous (has cooled from molten rock), sedimentary (formed in layers) and metamorphic (changed from one form to another) - which in itself was a good start for a novice like me! We then headed down onto the beach for a closer look at the amazing rocks down there.

Dave pointing out the chilled margin of a 55 million year old igneous dyke. This is made of dolerite and trends NW-SE and probably originated in the Blackstones Bank volcanic centre about 40 miles to the NW. These dykes are part of the process of the opening of the North Atlantic Ocean.

Throw it! THROW IT!

The rocks to the south of the beach dipped to the left (SW), the ones to the north dipped right (NE), indicating that the rock layers had folded due to compressive forces (like when continents collide). Dave taught us that a convex-upward fold is called an Anticline (like and Arch) and a convex-downward fold is called a Syncline (like a Saucer).

The rocks here were turbiditic sandstones deposited by turbidity currents in deep water, and Dave reminded us of the school experiment we might have done, filling a jam jar with water, mud and rocks, shaking it up and letting it settle. The bigger bits sink to the bottom and the fine mud setlles out slower - leaving a deposit that 'fines upward'.

A small anticline fold in bedded turbiditic sandstones of the Colonsay Group (deposited about 750 million years ago and folded about 470 million years ago).

another smaller dyke

Many thanks to Dave for a fascinating talk/walk, and to all those who came along!

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