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Sunday
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In contrast to Saturday, Mayday dawned bright and sunny, just the day to be on the coast. As we left Weymouth for Lulworth Cove, the National Kite Flying competition was just getting started, kites of all descriptions were beginning to ascend above the beach, much too serious an activity for children. The sequence of rocks we saw today are of Jurassic and Cretaceous age , within the Dorset World Heritage Site. Our first location, Stairhole is a spectacular example of marine erosional processes at work. The sea, having breached the relatively stronger Portland Limestone, is steadily eroding the less resistant Purbeck beds, giving access to the softer Wealden clays. In the process forming caves and a small cove, similar to Lulworth. The Purbeck beds are folded, this is the famous Lulworth Crumple, a minor fold on the northern dipping limb of the Lulworth anticline, thought to have formed when an ancient Hercynian / Variscan basement fault was reactivated during the Alpine Orogeny. (25MA) Both the Portland and Purbeck formations are limestones deposited in marine conditions, although sea level changes are apparent in the Purbeck. The Wealden beds were laid down in a fluvial environment, and consist of clays and sandstones a distinctive yellowish brown in colour, containing fragments of lignite, and sulphur from organic decay. A short walk and we were overlooking our next location, Lulworth Cove. Unfortunately the sea mist started to come in, not so good for photography. Here the sea has completely broken through the Portland limestone and Purbeck beds,thus forming the two arms of the cove. Dipping steeply to the north east, the Purbeck beds form the southern arm, and are subject to rock falls,The Portland beds of the north arm are almost vertical. The stream flowing through the cove to the sea,.only a small trickle now, would, during the post glacial period have carried much melt water, and probably initiated and aided the breakdown of the Portland Limestone, and also eroded the softer Wealden clays. The Upper Greensand strata could be seen at the northern end of the cove, overlain by the Lower and Middle Chalk. In the central area of the eastern wall of the cove could be seen the Wealden beds overlain by the Chalk We descended to the beach for a closer look at the rocks. To the west of the stream and in the central eastern wall of the cove the Wealden Beds consisted of unconsolidated reddish/yellow clays and sands, deposited in a fluvial setting, grit sized pebble beds surround lignite bands indicating trees carried along by the river. Sulphur and fossil Oyster beds were also clearly seen. The Upper Greensand, as the name indicates, are greenish coloured, poorly-cemented clays and silty sands, containing glauconite crystals, indicative of a tropical marine environment. Occasional well-cemented nodules of carbonate and cherts indicated a transitional stage, and contained the typical fossil Rotularia The beds of the Lower and Middle Chalk overlying the Greensand, dip steeply to the north and form a barrier to further erosion. Above these, the highest beds are the Upper Chalk. Whereas in the Lower Chalk we saw particles of silty clay, the Upper Chalk contained flint bands, slickensides – evidence of faulting - were also seen. These beds were vertical to overturned. Ascending the steps to the top of the cliffs, we walked to our next locality The Fossil Forest, the wild flowers, including orchids made this a very pleasant stroll. The Fossil Forest is really the fossil moulds of tree roots which grew when sea levels were low, the trees were drowned by a flood of saline water when the sea level rose. What we see now are stromatolites. We continued along the cliff path to Mupe Bay, pausing at Bacon Hole for lunch, this cove is so called because the Purbeck Beds resemble a side of bacon, an appropiate place to eat our ham sandwiches, as Mike Gagan pointed out. At Mupe Bay I read the notice at the top of the steps warning against stability of the steps and cliffs, four of us decided to remain on top, looking down into the bay we could see that a massive landslide had occurred which looked fairly fresh, no one knows when these rocks fell. We returned to Lulworth Cove and a welcome ice cream. Durdle Door our last locality is a famous arch controlled by two sets of joints, cut through Portland and Purbeck Beds; Upper Greensand and Lower Chalk form the cliffs of Durdle cove. The cliffs to the west are Upper Chalk the beds are vertical and form a series of folds, the valley of one such fold is called Scratchy Bottom once a river valley but dry since the last glaciation. A really interesting day for which many thanks are due to our leader Alan Holliday. Following dinner at the hotel we were treated to a firework display in the harbour, watched in comfort from the lounge with appropriate stimulants to hand. |
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