Physical Features 17 



Isolated patches of sands on Shotover, and at Brill, indicate apparently 

 river-deposits of fresh water of the Wealden horizon; the sands being mixed 

 with clays, and showing patchy deposits of river-sediments. Sandy deposits 

 of similar character, also mixed with clays, are also left in isolated pockets on 

 higher ground, more particularly on Boar's Hill and Cumnor Hurst, where they 

 yield no fossils and rest directly on Kimeridge Clay. These appear to be of 

 marine origin, and the intervening Portland beds must have been denuded 

 before their deposition as Lower Greensand. 



Small traces of a still higher clay at Boar's Hill, Toot Baldon, and Cumnor 

 have been identified with Gault, which is predominant away to the SE., as the 

 third great succession of an estuarine clay. 



No trace remains of the Upper Greensand which represents the next 

 clear- water deposit, nearer than Chalgrove, 10 miles to the SE. ; nor does the 

 Chalk come any nearer than 12 miles at Wantage and Wallingford, with an 

 outlier as Wittenham Clumps ; though it is probable that the whole district 

 was once covered with a great depth of Chalk-formation of warm and 

 shallow sea. 



Since this time, it would appear that the land has been wholly out of 

 the water, and subaerial denudation has wiped off these later deposits, leav- 

 ing only ridges and patches of the Middle Cretaceous beds as the caps of 

 the low hills of the district. Even before the exposure of the vestiges of 

 Gault and Greensand on the adjacent hills, possibly 800 ft. 1 of a general 

 Gault clay bed, with superimposed Chalk hills, had been cut away by 

 erosion over a period of possibly 100 million years. This marks a wide 

 gap in the geological story ; but the record, continued with warm seas, coral- 

 reef, estuarine beds and the mighty rivers of the older continental area, 

 over Mesozoic times, implies a land-surface of continental connexion with 

 a vegetation far surpassing that of present times for the same locality, of 

 which no trace remains. Of the land-conditions extending into the Early 

 Tertiary Period (Eocene, Miocene), again, nothing is known ; only in the 

 late Pleistocene is the tale picked up again with the changes effected at the 

 last * Glacial Epoch ' which is responsible for the surface-modelling of 

 the district, as the Jurassic period has afforded the main material. 



Surface - Modelling. 



Post-Glacial Erosion : Only in the latest deposits of the Pliocene period 

 is there any definite indication of the Pre-glacial flora of Great Britain, as 

 presented more particularly in the Cromer Forest-bed, 2 as a formation of 

 estuarine swamp and lakes, with a temperate climate, to all botanical intent, 

 identical with that of the present age. Large mammals still remained, and 

 t the Oak was a dominant tree, 3 just as at the present day. This fact, as also 

 the way in which the Oak is replaced in a slightly colder climate by Pinus 

 sylvestris, and in a slightly warmer one by the Beech, 4 very exactly defines 

 the general nature of the flora, as essentially ' English ' in aspect. With 

 such a climate, and continental land-connexion, there can be little doubt 

 that man of low-grade types, at the limit of the northern dispersal of the 

 human race, had followed the plants and animals on which he subsisted. 



These pre-glacial plants of the latest Pliocene, found on the coasts of 

 Norfolk and Suffolk, are still characteristic of the British flora, with only 

 a few additions suggestive of a more continental range (Picea excelsa^ Trapa 

 natans). Recognized examples include : Coniferous trees, as Juniper us y 

 Taxus, Pinus, Picea ; Angiospermous forest-trees, Oak, Beech, Alnus, Betula, 



1 Pocock (1908), loc. cit., p. 107. 



2 Clement Reid (1899), Origin of the British Flora, p. 35. 



3 C. Reid (1913), Submerged Forests (Post Glacial), p. 106. 



4 Warming (1909), Ecology of Plants, Eng. Trans., p. 203. 



B 



