302 PEOr. J. PRESTWICH ON THE EAISED 



others common in the Glacial drifts. The E-aised Beach moUusca 

 agree therefore pretty closely with the molluscan fauna now living 

 in the British seas, and this accords with the stratigraphical evi- 

 dence which leads us to place the Beaches with the latest of the 

 Biver-valley Deposits as described in the next section of this paper. 



§ 5. The Eelation^ oe the Raised Beaches to the 

 Yalley Driets. 



The evidence on this point, though not so definite as could be 

 wished, is still sufficient to prove with tolerable certainty that the 

 Beaches are contemporary with the lowest and therefore the newest 

 of the fluviatile drifts of the valleys, and consequently that the 

 high-level valley gravels are older than the Beaches, though these 

 higher river-drifts rarely extend to the coast. For example, the 

 low-level fossiliferous drifts of the Thames at Grays and Crayford, 

 and the high-level drift gravels of Purfleet, Chadwell, and Dart- 

 ford, are represented at the mouth of the Thames only by the 

 small bed at Chislet, and by the circumscribed bed of gravel which 

 caps the cliffs between Heme Bay and the Beculvers, at a level 

 about 100 feet higher than the Chislet deposit.^ In this latter we 

 meet with indications of the proximity of a coast-line, in the 

 presence of Balanus and a species of Olobulina, with which are 

 associated the Cyrena fluminalis and a few freshwater shells such 

 as are found at Crayford and Grays. Over these are 3 to 8 feet of 

 Bubble-drift or Head, derived from the gravel and clay of the 

 adjacent range of hills. This shell-bed is, like the Beaches on the 

 coast, at a height of about 15 to 20 feet above sea-level. The 

 Chalk on which it rests is in one part festooned, in the manner I 

 found the Chalk to be at Dartford before the deposition of the Cray- 

 ford low-level drift.^ 



But in order to obtain better evidence of the relation between the 

 Raised Beaches and the Valley Drifts, we must step for a time 

 beyond our assigned limits, and cross to the valley of the Somme. 

 The estuary of that river, which now does not extend beyond 

 St. Yalery, formerly extended to Abbeville, where a number of 

 marine shells are met with at the base of the fluviatile beds of 

 Menchecourt.^ This estuarine bed is 24 feet above the mean level 

 of the sea, or approximately on the same level as the Baised Beach 

 at Sangatte, and contains the Buccinum undatum, Littorina littorea, 

 Nassa reticulata^ Purpura lapillus^ Cardium edule, Ostrea edulis, 

 and Tellina halthica of our Baised Beaches, together with a large 

 proportion of the fluviatile shells and the Cyrena fluminalis of the 

 low-level fluviatile drifts of Grays and Crayford. There are high- 

 level gravels on both sides of the valley at Abbeville, at a height of 

 98 feet above the marine shell-bed at Menchecourt, and again at the 



' Quart. Journ. Geol. Soc. vol. xi. (1855) p. 110. 



2 Ibid. vol. xlvii. (1891) p. 153, fig. 11. 



3 Phil. Trans. Roy. Soc. vol. cl. (1860) p. 286. 



