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Saturday morning
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The morning was misty and murky but after a good breakfast, we embarked on the short drive to Osmington Mills where we parked on the roadside near the Smugglers Inn. It was a brief walk from here, to the rear of the pub car park, to look at the Osmington Oolite, part of the Corallian Beds, which outcrop here. This particular exposure exhibits cross stratification and the trace fossils burrows, Skolithus. The Kimmeridge Clay overlies the Corallian and to get to the beach we had to scramble across a landslip of the clay that had occurred several winters ago during a particularly wet period. Within the clays are ammonites but they are very fragile and easily broken. Once on the beach, there is an assortment of rock types. This is due to landslips and mass movement of the cliffs that regularly occur bringing a jumble on material down to the shore. The Osmington Oolite was present, some of the pieces having calcite veins in them and large rounded boulders of sandstone, many of which exhibited excellent examples of cross-stratification. These large boulders were formed in horizons that were richer in carbonates than the surrounding material. As a result, they are better cemented and therefore more resistant to erosion. As the cliffs have been eroded, these more resistant blocks have fallen on to the beach below. There is also plenty of chert which has come from the over lying Greensand. Other blocks from the Trigonia beds contain many body fossils, particularly Gryphaea and Myophorella clavellata. Myophorella was once known as Trigonia and was a shallow inshore burrower and a suspension feeder, filtering the water for microplankton. Further along the beach we found more trace fossils including the horizontal, branching burrows of Thalassinoides and the vertical U-shaped burrow of Diplocraterion parallelum. The bases of the latter burrows often show a concentric pattern. This is thought to represent the adjustment in the position of the burrow in response to deposition or erosion of sediments, moving up at times of deposition and down at times of erosion. Pieces of lignite were present in the cliffs some of which had pyrite present. Pyrite normally forms in anoxic conditions but the abundance of fossil evidence suggests that the sea floor was teaming with life and hence well oxygenated. The formation of the pyrite may be a result of microbes feeding on the plant remains and creating locally anoxic conditions. The plant remains from which the lignite formed may have originated to the west around Dartmoor, which would have been uplifted above sea level at the time of deposition. The original plan had been to walk all the way to Ringstead but there had been so much to see, the state of the tide and as lunchtime was approaching, it was decided to retrace our steps back to Osmington and the Smugglers Inn for some light refreshment.
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