Osmington Mills is near Weymouth on the famed Jurassic coast and this visit was arranged so that the members could be shown the geology and also collect fossils which are abundant here.
The walk started in Ringstead and we travelled westward along the beach, stopping at each outcrop and considering the rock. The first was the Kimmeridge Clay, grey, sticky but containing 10cm oysters.
Further west, ledges of white rock, a limestone, appeared and had 8 cm thick bands of large fossil shells – Myophorella – which covered some surfaces and were distinguished by lines of white bumps.
Next came the Osmington Oolite – white, fairly fine-grained, and full of minute spheres (oolites) when examined with a lens. Solid muddy tubes running through the rock in several directions were borings by a shrimp-like animal before the rock was consolidated. These are known as Trace Fossils and tell us about the living and feeding habits of their makers. They have wonderful names like Thalassinoides, Orphiomorpha and Skolithos.
After a pleasant morning, with rapidly reducing cloud, the group were able to eat their lunches in sunshine, with the sea lapping on the rocks and splendid views along the coast and across to the Isle of Portland.
A walk along the boulder beach brought us to the final exposure of the day, the Bencliff Grit – a yellow, fine-grained sandstone, which, when broken, gave a strong oily smell. This was a clue to some industry along this coast. Oil wells were drilled here, at Kimmeridge and at Wytch Farm near Poole and Wytch Farm at its best produced 100,000 barrels a day. Also in the Grit were seen beautifully rounded spheres of sandstone up to 2 m diameter and time was spent discussing their origin.
All along this coast, loose fossils lying on the beach could be found and these included curly marine snails, small sea urchins (echinids) and a wide variety of bivalve shells, all to be identified and collected.
This thirsty group of geologists now walked the short distance to the Smugglers Inn, where thirsts were slaked with the local brew, together with much discussion and examination of fossils.
The Reading Geological Society holds regular lectures and field meetings. The next field meeting is on Saturday 17th August, when we will visit the Greensands in Bedfordshire. Our next indoor meeting is a mineral and fossil identification workshop in Reading on Monday 5th August.
We welcome visitors to our meetings. For further information, please contact us at readinggeology.org.uk
David Ward RGS
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