The party of about 18 met at the car park in the National Trust woods between Morcombelake and Chideock. The early frost was being burnt off by the strong autumn sun as we walked through the woods and across the fields climbing to the top of Golden Cap under the leader ship of Mike Hermolle and Bob Christian the local expert from the Charmouth Heritage Centre.

From the top of Golden Cap, at 196m the highest point on the south coast, we could see clear from Portland in the East to Start Point in Devon. Bob explained that the 4 degree southerly dip of the main surface of Portland and the 60 degree dip of the northern section were due to thrust movements from the south which had formed an anticline at the northern end. Looking inland we were able to see the uniform height of the surrounding land surface cut with river channels. These are thought to have been formed as melt water channels by run off from the ice sheet which lay further north across country during the last ice age. These channels had cut down though the cretaceous beds and carried the cherts from the beds out to sea which at the time lay about 3 km beyond the present day coast. As sea level subsequently rose these cherts were fetched back inshore by the prevailing currents from the south west and have been piled up as the Chesil Bank wich separates the sea from the brackish Fleet Lake on the coast a little further towards us from Portland. The cherts have been sorted by this process so that those close to portland are larger than those at the western end of the bank.

The next feature noted were the vertical cliffs in the yellow sand at Bridport. It was explained that a new port breakwater was being built at Bridport and this was already running well over budget. This was in no small part due to the failure to adequately consider the nature of the local geology and the coastal drift. The new breakwater has changed the coastal currents such that extensive underpinning of the cliffs has been required to minimise the risk of slips.

At the eastern foot of Golden Cap is the small village of Seatown. In the headlands on both sides of the river are fine examples of the rotational slips which characterise this part of the coast. The overlaying beds are cretaceous sands which allow rainwater to percolate down. The cretaceous however lie unconformably on Jurassic clays which prevent further drainage. As water accumulates it lubricates the junction until the cliff sides outwards bottom first and the upper land surface drops vertically. The sea removes the toe of the slip and the upper layers slump down further forming a terraced topology.

Looking westward the cliffs of Charmouth and Lyme Regis were visible. These clearly show the 3 degree eastward dip of the south coast of England. Consequently as Bob pointed out as you walk westwards along the beach you are moving past progressively older beds from cretaceous exposures of approximately 80 million years old to the Permian around Exmouth of approximately 280 million years old.The cliffs of Charmouth are predominantly pale grey belemnite marls. Asides from a number of species of belemnites the ammonite Aegeoceras is also widely distributed. Further east was Black Ven the largest landside in Europe and the town of Lyme Regis itself suffering from the effects of slipping and only maintained at some cost.

The group walked down towards St Gabriel's Mouth stopping to take a closer look at a small rotational slip in the cliffs. From here the beds in Golden Cap could be observed.

The final descent to the beach was by a series of steep steps along a ridge top and a short final ladder.

At beach level is the belemnite bed. Above this are the lower and middle lias clays and unconformably on this the cretaceous gault and greensand. Turning east along the beach we were able to observe many belemnites in the clays, The party broke for lunch beneath Golden Cap where belemnite, ammonite, bivalve and crinoid fossils were collected. To the Eastern side of the cap another stop was made at a small sandy patch were the sand was found to be crammed with a microfauna of the above with added gastropods. Again a little further on was a thin exposed layer comprised entirely of crinoid fragments. Infront of the pub at Seatown Bob pointed out a narrow dark band in the golden sands of the cliff. This is thought to have been deposited by the tsunami which washed up the English Channel after the 1755 Lisbon earthquake. From here it was only a half hour climb back up to the car park where we arrived at about 4:00. Thanks were given to Bob for his expert and anecdotal descriptions.

Click here for extra photos. Words: Chris Popham, Photos: Chris Popham

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