THE STORY OF THE ISLAND RIVERS 95 



faster rate, till it cut the course of the old river which ran 

 by Salisbury to Southampton, and, having a steeper fall, 

 diverted the upper waters of this river into its own 

 channel. 



Frost and rain and rivers cut down the valleys of the 

 river system for hundreds of feet ; the sea which had 

 broken through the chalk range gradually cut away the 

 south side of the main river valley from Purbeck to the 

 Needles ; and eventually the valley itself was submerged 

 by a subsidence of the land, and the sea flowed between 

 the Isle of Wight and the mainland. 



A gravel of somewhat different character to the rest is 

 the sheet of flint shingle at Bembridge Foreland. It 

 forms a cliff of gravel about 25 feet high resting on 

 Bembridge marls, and consists of large flints, with lines of 

 smaller flints and sand showing current bedding, and also 

 contains Greensand chert and sandstone, which must 

 have been brought from some district beyond the Chalk. 

 The shingle slopes to north-east. To the south-west it 

 ends abruptly, the dividing line between shingle and marls 

 running up steeply into the cliff. This evidently marks 

 an old sea cliff in the marls, against which the gravel has 

 been laid down.* 



One or two comparatively recent deposits may be 

 mentioned here. At the top of the cliff in Totland Bay, 

 about 60 ft. above the sea, for a distance of 350 yards, is a 

 lacustrine deposit, consisting in the main of a calcareous 

 tufa deposited by springs flowing from the limestone of 

 Headon Hill. The tufa contains black lines from vege- 

 table matter, and numerous land and freshwater shells 

 of present-day species many species of Helix, especially 

 H. ncmoralis and H. rotundata, Cyclostoma clegans, Lim- 

 ncea palustris, Pupa, Clausilia, Cyclas, and others. 



On the top of Gore Cliff is a deposit of hard calcareous 

 mud, reaching a thickness of about 9 feet, and forming a 

 small vertical cliff above the slopes of chalk marl. It 



*Fig. 9, p. 79. 



