HA RD WICKE'S SCIENCE- G SSI P. 



Ill 



on the North Downs at altitudes of 500 feet to 600 

 feet as belonging to the Bagshot or later Eocene ; 

 and these lie either on the Chalk or on the Reading 

 Beds. This has been discussed by the author 

 elsewhere.* If, as the hypothesis requires, some 

 accentuation of the synclinal during the latter part of 

 the London Clay period and during the Bagshot 

 period, took place (against which no h priori reason 

 can be alleged), the facts just enumerated would 

 receive a rational explanation. The Upper Bagshot 

 Sands, however, retain remains of a fauna, which 

 indicates the conditions obtaining in a marine 

 estuary ; and as these have now been identified as 

 far west as Highclere it would seem that towards the 

 close of the Eocene period a narrow arm of the sea 

 extended from the open sea towards the east, 

 westwards as far as the place just mentioned, and 

 ' perhaps somewhat further. This will be better 

 understood by reference to the section (Fig. i), 

 which is, of course, like the next, only a diagram- 

 matic representation, drawn through the longtitude 

 of Windsor, of what was probably the relation of 

 things in the Tamisian area towards the close of the 

 Eocene period. 



The Pliocene Period. (See Fig. 85.) 



In the shallowing of the waters of the Hants and 

 Seine Basin during the Oligocene period, we seem to 

 have the initiation of a more general upward 

 movement, which perhaps continued through the 

 Miocene ; the failure to identify any deposits of the 

 latter period in the London and Paris Basins going to 

 show that the south-east of England (as w-ell as the 

 north of France) were for all that length of time dry 

 land, the superficial strata therefore undergoing 

 destruction by atmospheric agencies. The chalk 

 especially must have suffered enormous waste 

 through the removal of its carbonate of lime by 

 carbonated atmospheric waters, leaving its flints (in 

 some places largely mixed with the clay residue of 

 the chalk) to accumulate upon the uplands, both on 

 the north and south sides of the Tamisian valley, 

 just as we see them at the present time accumulated 

 upon the chalk downs above Ventnor.f 



With the Pliocene movements the Weald probably 

 attained its maximum elevation ; though there are 

 reasons for supposing that this elevation was not 

 commensurate throughout the whole distance, but 

 somewhat in excess towards the western part, which 

 was lifted above the sea, while further east marine 

 waters still encroached upon it to a much greater 

 extent, depositing strata of which the well-known 

 Lenham deposits are the remnants.^ This is further 

 borne out by the fact that on the north side of the 



* " Geol. Mag." for September, 1890, pp. 403 et seq. 



t See the author in " Proc. Geol. Assoc," vol. viii., "On 

 the Bagshot Beds of the London Basin, and their Associated 

 Gravels;" also the "Journal of the Geol. Soc," vol. xlvi., 

 P- 558. 



: See " Geol. Mag.," loc. cit. 



Tamisian area there must have been towards the east 

 some depression, with the consequent encroachment 

 of the waters of the Anglo-Germanic Sea, laying 

 down the Crag of the Eastern Counties, and filling 

 up the valleys and hollows which had been cut into 

 the London Clay during the preceding Miocene 

 elevation of the area. The great Pliocene elevation 

 of the Weald, which, working independently, both 

 Professor Prestwich and the author have come to 

 regard as the most important factor in determining 

 the present surface-geology of the south-east of 

 England, which also probably had its counterpart 

 in a more regional elevation on the Mercian side, 

 would seem to have two most important results : (i) 

 it gave a general tilt to the north of the Eocene 

 strata of the Tafnisian area, and so threw the main 

 line of drainage further north, initiating the present 

 actual Thames Valley, while erosion was facilitated 

 along that line by the weakness of the strata 

 themselves ; (2) a higher gathering-ground for tri- 

 butary waters from the Wealden hill-range, and a 

 general declivity to the north of the Eocene area, 

 furnished the requisite conditions for the transport of 

 the flints of the Chalk region, the flint pebbles 

 washed out of the Eocene beds as their destruction 

 on the north flank of the Weald advanced, and the 

 Neocomian chert fragments, which, together with the 

 flints and a few quartz pebbles, constitute the 

 materials of the Plateau -gravels.* The erosion of 

 the uplands of the Weald had evidently by this time, 

 made such deep incisions into the strata, as to lay 

 bare to the action of denuding agencies considerable 

 portions of the Neocomian or Lower Greensand ; 

 though this proceeded slowly enough for the trans- 

 verse drainage to cut down its valleys through the 

 Chalk escarpment, which even then must have begun 

 to take shape. 



The one incision of this kind across the strike of 

 the chalk on the Mercian side of the Tamisian area, 

 that of the Pangbourne and Goring gorge, was in all 

 probability initiated too in Pliocene time, but the 

 area of drainage concentrated upon this line was 

 much smaller than at present. It would appear that 

 the head-waters of the present Ouse are cut off from 

 those of the Cherwell and the Thame by a watershed 

 largely composed of glacial drift ; at least the 

 author's own observations of that district have led 

 him to regard this as probable. If this were so, we 

 should have to date the outlining of the present 

 Upper or Oxford Basin of the Thames rather late in 

 Quaternary time, the Pangbourne gorge having 

 been no doubt deepened considerably during the 

 Glacial period. 



The tilting to the north of the Eocene strata, as a 

 result of the great Pliocene elevation of the western 

 portion of the Weald, perhaps affected the more 



* "Journal of the Geol. Soc," loc. cit.; also Prestwich, 

 ibid., vol. xlvi., on the Southern Drift. 



