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Raised beach |
Immediately after a short lunch break on a small lawn like area of grass in front of a snack kiosk; fortunately there was a hedge surround which did offer some shelter from the cold wind, the group visited L’Eree headland. This headland is renowned for its 1.2 Ma Pleistocene 8m Ipswichian Raised Beach (1), which consists of poorly sorted very rounded pebbles. It was difficult to realise it was 8m high because the present pebbly beach obscured the true depth by approximately 5-6m.
The Raised Beach was surmounted by a narrow layer (~60cm) of loess (2), a very fine silky grained brownish yellow wind blown deposit. A curious feature of these strata was a single line of fairly large rounded pebbles. Above the loess was the solifluction periglacial head (3) containing large poorly sorted angular pebbles this was topped by a shallow layer of periglacial loess (~16cm) (4) and finally a thin covering of recent soil deposits covered by grass vegetation (5). (see diagram below) The strata above the Raised beach are Devensian.
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Raised beach |
Also at this headland, away from the Raised Beach towards the sea, were marvelous examples of L’Eree granite (circa 650 Ma) that contained megaphenocrysts of feldspar, the slow growth being highlighted by concentric rings. A dolerite dyke with columnar jointing also indicated the phenomenon of deep, slow cooling. The whole area was cross cut by a series of aplite and dolerite dykes. One green coloured dyke caused much deliberation and close inspection to determine what the soft dark green fine grained crystalline rock could be, the general consensus was that it was a rock called vogesite.
Whilst all this was going on the sun appeared from behind a cloud and transformed the scene from monochrome to wide screen technicolour, what a difference sunshine makes. The cold wind did not abate but the sun shone for the rest of the afternoon. In the welcome sunlight the enthusiastic geologists clambered back into the cars for the short journey to last site of the day, Cobo Bay, to study the now exposed diorite and Cobo granite (circa 570 Ma). The Cobo granite was mainly feldspar with approximately 20% quartz and two types of mica, the lighter muscovite and the darker biotite, unlike the L’Eree granite there were no phenocrysts. Through the large block of granite exposed in the bay were several aplite veins. An interesting feature found at the top of the granite outcrop was what looked like a blob of granite in a bowl full of aplite. This generated plenty of discussion on how this came about.
All the time more and more rocky outcrops and sandy beach were being exposed due to the falling tide. It was beautiful to behold as one walked back to cars at the end of an enjoyable day, the blue sea with white crested waves and enhancing the scene were brightly painted lobster boats bobbing at their moorings together with bright orange marker buoys dotted around the bay. What a lovely memory to have of Guernsey
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Lithou Island |
Val Freeman