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July 6, 2016 at 7:48 pm #8923Anonymous
Here is a brief summary of our field trip on Saturday 2nd July 2016.
We gathered at 10.30am in the car park at Martin's Haven and walked down to the inlet. Here we looked at the basaltic lavas exposed on the beach. The lavas belonged to the Skomer Volcanic Group that were erupted in lower Silurian times (443-428 Ma). They contained many vesicles which would have been filled with volcanic gases when the lavas were molten. Some lavas also showed scoracious or cindery tops indicating that they had been exposed to the air whilst other lavas must have been extruded under water since they showed a pillow structure. At the back of the inlet we discovered evidence of a pyroclastic flow deposit containing fragments of lava set in a fine grained ash.
We walked up on to the Wooltack peninsula, otherwise known as the Deer park to view Skomer Island beyond Jack Sound in beautiful sunshine. Skomer is famed for its puffins and other seabirds but geologically it is formed of lavas similar to those of Martin's Haven. Next we walked around the coast to Renny Slip and back across the barley field to the car park.
A relaxing lunch was taken at the Lobster Pot Pub.
In the afternoon we made our way down the track to Marloes Sands. to examine some of the Silurian sedimentary rocks. Our first stop was in the Gray sandstone Group where we looked at ripple marked limestone with thin shale marking the bedding planes! At Mathew's Slade landslipped strata occurred between two major fault planes. Further along the beach we encountered a basalt intrusion, part of the Skomer volcanics faulted against the Silurian sandstones. Here the lava was vesicular and several veins revealed the green coloured mineral epidote plus numerous calcite veins. Next we observed the sandstone pillars called the Three Chimneys which had been uplifted into a vertical plane by the Caledonian earth movements. Finally we made it to the Coralliferous Limestone where both colonial and single corals could be seen. Also beds of small brachiopods (Eocoelia) indicated that the sea depth was about 20 metres when these limestones were deposited. In fact researchers have demonstrated how a range of brachiopod species can be used to estimate the approximate depth of the Welsh Basin in Silurian times.
Unfortunately the tide was rising quite rapidly by mid afternoon so we retraced our steps along the beach to Sandy Lane, which is no longer sandy since it has now been surfaced with Carboniferous limestone chippings to confuse the geologists!
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