J. C. Mansel-P ley dell — Geology of Dorset. 445 



tbey result from fluviatile or marine agency. Mr. Codrington con- 

 siders the gravels covering the table-lands at the highest levels are 

 of far greater antiquity than the valley-gravels of the rivers, which 

 were probably formed previous to the disruption of the Isle of 

 Wight from tlie mainland ; also that the rivers reaching the sea by 

 Poole Harbour, Christchurch, and Southampton, were affluents to au 

 estuary opening into the sea in the direction of Spithead. 



The last movement appears to have been one of subsidence, and 

 the occurrence of several submerged forests around our coasts gives 

 good reason to lead to this supposition. Those of Bournemouth and 

 on. the north side of Poole Harbour, Sir Charles Lyell makes au 

 exception to, and attributes their submergence in modern times to 

 the washing out of the subjacent sandy strata on which they rested, 

 and not to a general subsidence or change of level. In several parts 

 of the county, especially on the chalk-uplands, drift-beds occur of 

 local origin varying from flint-rubble and brick-earths to sandy 

 clays. Sponges, Echinoderms, and Brachiopods (the usual Chalk 

 fossils) are plentiful in these deposits, which represent the redistri- 

 bution of the older gravels during the Pleistocene age. 



SuB-AEBiAL Deposits. — Chesil Bank.^ — This remarkable beach 

 runs from Portland to Burton Bradstock, a distance of 18 miles. 

 Messrs. Bristow and Whitaker consider it to have been an ordinary 

 beach subsequently separated from the mainland by denudation. It 

 is connected with the mainland from Burton to Abbotsbury, where 

 a narrow channel, the Fleet, which extends to Wyke Ferry, separates 

 it. At Abbotsbury its base is about 170 yards in width, and its 

 crest 22 feet above the level of the sea. At Portland its base is 200 

 yards wide, and its crest 42 feet in height. The separation of the 

 land from the beach is, doubtless, owing to the configuration of 

 the coast-line, which is without any high land or cliff from Burton 

 to Abbotsbury ; and the several streamlets which, through the im- 

 perviousness of the subsoil, and eastward trend of the coast, are 

 diverted from their seaward course, succeed each other, and rill after 

 rill by their united power attain sufficient force to excavate a chan- 

 nel, while the influx and reflux of the tide have contributed towards 

 the formation of the Fleet, on whose bosom nearly 1000 swans 

 roam, — the unique and enviable possession of the Earl of Ilchester. 

 The Isle of Portland has acted as a base or breakwater in the 

 formation of the Chesil Bank. Between Studland and South Haven 

 there is a large accumulation of shingle, derived from the submerged 

 beds as well as from the re-formation of older gravels. 



The raised beach at Portland Bill consists of pebbles cemented 

 together with comminuted shells ; it is 30 or 40 feet above the sea- 

 level, and about three feet thick. A projection southward shows 

 another conglomerate with shells, chiefly Littorina littorea, Littorina 

 littoralis, and Patella vulgata. This ancient beach terminates after a 

 course of 250 yards. A bed of shingle caps the cliff at the Bill, and 



^ See a paper "On the Formation of the Chesil Bank," by H. W. Bristow, and 

 W. Whitaker, Geol. Mag. Vol. VI. p. 433 (1869). Also J. Coode in Proc. Inst 

 Civ. Eug. vol. xii. p. 520 (1853). 



