Origin of the Chesil Bank. 229 



from which the shingle of the Chesil Bank could have been derived 

 was between Lyme Kegis and Budleigh, and that it was propelled 

 eastward along the coast to the Chesil Bank by the action of wind- 

 waves, due to the prevalent and heaviest seas. The objection to this 

 view urged at the time by the Astronomer Boyal was, that the 

 largest shingle occurred at the Portland end of the beach, or the 

 most distant part from which it had travelled. 



More recently an old "raised beach," standing from 21 feet to 

 47 feet above tire present beach, had been discovered on the Bill of 

 Portland, and Professor Prestwich showed that this beach contained 

 all the materials found in the Chesil Bank, including also numerous 

 chert pebbles from the Upper Greensand of the cliff between Brid- 

 port and Sidmouth. This raised beach was not due to any existing 

 agency, but to causes in operation at a geological period so remote as 

 the end of the Glacial period, and before the land had assumed its 

 present position and shape. Bemnants of this beach could be traced 

 in or on the present cliffs, at intervals from Brighton to the coast of 

 Cornwall, being more numerous in Devon and Cornwall, as the rocks 

 were harder, than among the softer strata of Dorset and Hants, 

 where, with few exceptions, the old line of cliff had been worn back 

 and deeper bays formed. The travel of the shingle of this old 

 beach was generally like that of the present beach from west to east. 



The Author considered that the action of the "Race" off Portland, 

 and of the tidal waves during storms, combined to drive the shingle 

 of the old beach at the Bill, and of that portion of it which must be 

 spread on the sea-bed westward of Portland, on to the south end of 

 the Chesil Bank, whence the shingle was driven northward to 

 Abbotsbury and Burton, by the action of the wind-waves, having 

 their maximum force from the S.S.W., a direction which he showed 

 to be the mean of the prevalent winds. Here, these wind -waves 

 became parallel with the coast, and the westward movement ceased 

 about Bridport, beyond which point the shingle travelled in the 

 opposite direction, viz. from west to east, or from the coast of Devon 

 to that of Dorset ; the quartzite pebbles from the conglomerate beds 

 of Budleigh Salterton, which travelled from that part of the coast 

 eastward to and beyond Sidmouth, gradually diminishing in numbers 

 as they approached Lyme, very few, if any, reaching Bridport. This 

 conclusion was in accordance with the facts: — 1. That the pebbles 

 of the Devonshire and Dorset strata, which formed the shingle of 

 the "raised beach," constituted also the bulk of the Chesil Bank. 

 2. That there were also, in that bank, pebbles of the rocks and flint 

 of Portland itself. 3. That the largest pebbles occurred at the 

 Portland end of the bank, the pebbles decreasing gradually in size 

 to Abbotsbury. The large dimensions of the bank he attributed to 

 the great accumulative and small lateral action of the waves. 



Professor Prestwich next discussed the questions connected with 

 the shingle of the south coast generally, and showed that the greater 

 part of it was derived indirectly from beds of quaternary gravel 

 and debris, from the wreck of the " raised beach," and partly from 

 the strata of the chalk and other cliffs, and not altogether or directly 



