RAISED BEACHES. 553 



inches in diameter under Portland. They consist of Chalk-flints, Greensand chert, 

 Portland rocks, quartzite (from the Budleigh Salterton pebble-bed), Devonian 

 grits, quartz, porphyry, granite, jasper, etc. ; all have probably come from the 

 west, and chiefly from strata between Exmouth and Lyme Regis. 



Several theories have been proposed to account for the formation of the Chesil 

 Bank. That the Isle of Portland, acting as a natural groyne or breakwater, was 

 essential to the collection of the material, all admit : the eddy tide produced 

 by that promontory serves to divert the travelling of the shingle, so that the 

 pebbles are driven westwards again along the shore, and by "wear and tear" 

 gradually diminish in size until we reach Bridport Harbour and Eype. West of 

 Eype the shingle ordinarily travels from west to east. The partial separation of the 

 bank from the land has been the source of much discussion. Messrs. H. W. Bristow 

 and W. Whitaker consider that the Chesil Bank may have been formed at first in the 

 same way as the ordinary shingle beaches of our coast, and that what was once an 

 ordinary beach, banked up against the land, has been since separated, as a bank 

 or bar, by the denudation of the land behind it. This they consider due to the 

 action of the streams which originally filtered through the shingle, turning east- 

 wards before doing so, as do other streams further west on the Devon coast. 

 In time these streams gradually united and formed a long channel (the Fleet) 

 between the beach and the mainland : and this water may have been materially 

 widened by tidal action, which would also assist it in cutting back the land. ^ 



Prof. Prestwich considers that the Shingle is chiefly derived from the materials 

 of the Raised Beach, of which a remnant still exists on the Bill of Portland, and 

 from the submerged portions of the beach to the west. The pebbles, however, on 

 the Chesil Bank at Portland are much larger than those which make up the mass 

 of the Raised Beach at Portland Bill. Some have thought that the larger pebbles 

 were moved more readily than the smaller ; but the diminution in size of the 

 pebbles is probably due to wear and not to sorting by the sea, for tiny Budleigh 

 pebbles may be observed in the beach at Bridport Harbour which must have 

 travelled westwards from Portland. The fine shingle east of this harbour is 

 extensively dug and sent away for making paths, for concrete, etc. ; but the 

 material taken away is said to be readily replaced by the sea. 



Eaised Beaches. 



These Beaches consist of accumulations of sand and shingle 

 which have been formed by the sea, but yet occupy such a height 

 above the present reach of the breakers as to necessitate the 

 opinion that changes of level in the land have taken place since 

 their deposition. They generally contain recent marine Mollusca, 

 and are often well stratified and compacted. 



As Mr. Kinahan has pointed out, littoral deposits may be 

 accumulated at varying levels at the present day, owing to the 

 variations in tide heights in different localities. Therefore if we 

 find raised beaches occupying different elevations in a certain 

 district, it does not necessarily follow that the land has been 

 unequally uplifted since the formation of the beaches:^ moreover, 

 the horizontal nature of most Raised Beaches suggests uniform 

 upheaval. 



1 Bristow and Whitaker, G. Mag. 1869, p. 433 ; Sir John Coode, Proc. Inst. 

 Civ. Eng. 1853 ; Pengelly, Trans. Devon Assoc. 1870 ; Kinahan, Q. J. xxxiii. 41 ; 

 Codrington, G. Mag. 1870, p. 23 ; Prestwich, Proc. Inst. Civ. Eng. xl. 1S75, 

 Q. J. xxxi. 33 ; Damon, Geol. Weymouth, 18S4, p. 167 ; see also P. Geol. Assoc. 

 ix. 205. 



2 G. Mag. 1876, p. 78. See also Harkness, Proc. G. S. iv. 178. 



