224 



HARD WICKE 'S SCIENCE- G OSSIP. 



ON THE POST-GLACIAL DEPOSITS OF 

 THE THAMES VALLEY. 



THE Post-Glacial period is beautifully represented 

 by the brick - earths of the Thames Valley. 

 Undoubtedly this is one of the most completely 

 developed deposits which occurred during that period 

 to be found in England. The brick -earth pits at 

 Erith and Crayford afford very good sections of this 

 formation. It is here, as many of my readers will 

 probably know, that remains of the Mammoth and 

 several other extinct mammals have been found. In 

 excavating the loam for the purpose of brick- making, 

 teeth of the Mammoth are occasionally met with, 

 and discoveries of bones of other animals are not 

 rare. The Thames Valley, in which these deposits 

 occur, is a very fair specimen of the effects of denu- 

 dation. It was partly scooped out by the retreating 

 waters of the sea which covered it during part of the 

 Glacial Period. The immense thickness of super- 



bed, called "Lower Brick-earth," contains many 

 beautifully preserved shells. Cyrenajluviinalis may 

 be found in abundance, as well as species oiPlanorbis, 

 Limnaa, Unio, &c. But the discoveries which have 

 excited the most interest are the remains of mammals 

 which have been found in this stratum. The suc- 

 ceeding deposit, "Thames Gravel," is of a very 

 different nature. It consists of rounded pebbles 

 enclosed in a somewhat clayey sand. During the 

 period of this deposit, it seems to me that icebergs 

 passed down the valley. At Crayford I have found 

 boulders and pebbles of quartz in the Thames Gravel, 

 the presence of which appears inexplicable unless we 

 accept this explanation. The Thames Gravel, like 

 the underlying brick-earth, contains bands of shells 

 and mammalian remains, which, however, are of 

 rarer occurrence. Its thickness is from twelve to 

 twenty feet, and it covers a much wider tract of 

 country than the preceding deposit. The third bed, 

 called " Upper Brick-earth," rests conformably on 



Fig. 173. Section illustrating Post-glacial Structure of the Thames Valley. 



incumbent strata was denuded away by water, and it 

 was not till the chalk had been reached and exposed 

 that this abated. It has been imagined by some that 

 the valley lying between the Essex hills and the hills 

 of the north coast of Kent was entirely formed by 

 the river Thames, but I do not favour this belief. It 

 appears clear to me that a river never forms the valley 

 through which it flows ; but that, on the contrary, it 

 is continually helping to raise the level of its bed. 

 As an example I may mention the river Thames, the 

 bed of which is gradually but unmistakably rising, 

 and as often as the tide rises higher than usual, the 

 waters overflow their banks and the results are often 

 disastrous. It cannot be denied that the principal 

 agent employed in the excavation of the valley is 

 water, but I think it was water in the shape of glaciers 

 that acted most powerfully in breaking up the chalk. 

 I think it probable that a glacier commenced forming 

 the valley during part of the Glacial Period. Sub- 

 sequently a change of level flooded the valley with 

 water, and when it had become sufficiently pacific, it 

 deposited the sand and loam held in suspension ; and 

 thus the first deposit commenced upon the denuded 

 chalk. The deposition of the sediment extended 

 over a somewhat lengthy period, as this deposit has 

 a thickness of thirty feet, — no inconsiderable thickness 

 in comparison to the more recent deposits. This 



the Thames Gravel. The thickness is only eight feet, 

 and in some places even less. The loam is worked 

 for brick-making. This deposit at one time covered 

 a large tract of land, but subsequent denudations 

 have materially lessened its extent. 



After the deposition of these strata, oscillations of 

 level occurred which exposed the Upper Brick-earth 

 to the action of denudation, and dislocated some 

 parts of the valley. Upon the Thames Gravel and 

 Upper Brick-earth there has been found a bed of 

 peat containing the stems of trees, and the river 

 Thames now runs over part of this ancient forest. 

 Remains of the following animals have been found 

 in the Post-Glacial strata of the Thames Valley : — 

 Felis spehca, Hyana spehca, Bos primigenlus, Elcphas 

 primigenius, Ovibos moschatus, &c. 



There are few places so near London where a day 

 may be more profitably spent than at Erith or Crayford. 

 At these places shells may be found in abundance : 

 species of /i't'/Zx, Cyrena, Planorbis, i/nio, Sec, inexcel- 

 lent preservation. These shells, with the exception of 

 [/nia, are exceedingly minute ; but a well-arranged 

 collection of them will repay all the trouble which 

 may be spent in procuring them. 



Geo. Clinch. 



IFes^ IVick/iam, Kent. 



