The Chesil Bank
The formation of the Chesil Bank has puzzled scientists for a very long time. Mr Poole who lives locally has made it a life long study. His approach to the problem has been from three ways, and together solve the problem. Firstly, the land geometry, from Lyme Regis to Portland. Beginning at Lyme in the west, the soft Lower Jurassic grey clays and shales have been eroded away quite rapidly to West Bay, where a fault brings in the hard Mid Jurassic Bridport Sands that form vertical cliffs and act as a bulwark. Further along the coast these are truncated by another fault at Burton Bradstock which brings in the Chalk and Greensand, which give a smooth gentle slope to the shore as far as Abbotsbury. Another fault here brings up the Upper Jurassic clays and limestones of the western edge of the Weymouth Anticline. These again are faulted and give the landscape of bays and headlands as seen at the back shore of the Fleet as far as Weymouth. This was the old coastline before Chesil Bank was formed. The Isle of Portland is a remnant of the hard limestone beds at the top of the Jurassic, and here slope southward out to sea. It is the southern limb of the Weymouth Anticline. So, from Lyme to Portland it is a coastline of much faulting in hard and soft rocks, but very little folding.
Secondly, the environment. During the glacial and interglacials of the last two million years, the English Channel was often dry land, acting as a large catchment area draining into the Atlantic ocean. Today’s small rivers such as the Axe, Char, Brid and Bride would have been just tributaries. The greatly enlarged volume of melt water in the interglacials would have brought much debris from inland, especially flint and chert from the Chalk and Greensand. When the sea level rose for the last time a large amount of pebbles lay on the sea floor. It was only 10,000 years’ ago that the Straits of Dover were cut as an overflow channel, making Britain an island.
This leads to the third parameter, that of wind and wave power. The power of all waves is controlled by the distance and strength of winds blowing over them. This is called the fetch. The Weymouth Bay area is quite remarkable, since from the Chesil Bank in a south-westerly direction a patch can be traced between Cornwall and Brittany right out into the Atlantic ocean, a distance of over 2,000 miles. Yet from Weymouth Bay in a south-easterly direction, it is a mere 70 miles to the French coast. Storms approaching from mid Atlantic meet no resistance until Portland Bill, so here they come to rest. Over the years this has produced a storm beach, maybe the largest in the world, stretching 18 miles westward from the bulwar of Portland to West Bay near Bridport. Consisting of 98% flint and chert it extends in a perfect curve, in equilibrium with the wave power. The wave power too is responsible for the grading of the pebbles, from fist size at Portland to shingle at West Bay.
The storms of winter 1979-80 were interesting since those in December were destructive and removed tons of pebbles especially from the promenade end of Portland, while the storm waves of February were constructive and came in carrying tons of pebbles which swept over the lower part of Portland causing much damage. These waves came in with no signs of an approaching storm. The whole coastline of Chesil Bank is regarded as a constructive one, as it is being built up by marine deposition. Recent boreholes have shown that the beach rests on Pleistocene sands, gravel and peat, and below them the Jurassic Kimmeridge clay. The beach dips steeply out to sea to a depth of 150 feet.
On the opposite side of Portland in Weymouth Bay, conditions are just the reverse. It is a wide shallow bay with sand building up near the harbour wall, and gradually changing to shingle further away to the east. The depth is a mere 50 feet. This is in response to the fetch of the waves from a south-east direction, no large waves can develop since the French coast is only 70 miles away.
During the field trip taken in conjunction with the lecture, the raised beach on Portland Bill was studied. This consists of three types of deposits resting on a wave cut platform of Portland limestone. First there were boulders and pebbles, slightly orientated as a beach would be, then, this was covered by well rounded shingle with some cross bedding and lenses of well-cemented pebbles. This in turn was covered with an irregular rubble of limestone fragments in a sandy loam, which suggested solifluxion from the last Ice Age. All of this denoting more than one phase in the Raised Beach deposit.
After stopping at the view point on Portland Heights to see the whole scene of the Weymouth area, we then traversed the Weymouth Anticline to Abbotsbury, walked along the Chalk and Greensand outcrop and saw old beach cliffs. Inland several deep valleys cut in the soft clays were noted.
The last call was at West Bay where because of the harbour entrance, (man made) the base of the Bridport Sands is banked with shingle, 20 feet higher than on the opposite side of the harbour.