A HISTORY OF SURREY 



map). The Tertiary deposits thus lie in a shallow trough or basin of 

 Chalk, termed by geologists the London Basin. A trough of this kind 

 formed by strata dipping towards a medial line from both sides is tech- 

 nically known as a syncline. This arrangement of the strata is of 

 course the reverse of an anticline such as we have traced out in the 

 Weald. The dome or anticline of the Weald and the trough or syncline 

 of the London Basin, taken in conjunction, are the governing factors in 

 the geological structure of Surrey. 



To return to the description of the Chalk at its outcrop ; we shall 

 find that when closely examined its successive parts exhibit slight differ- 

 ences of character, which enable us to distinguish the divisions given in 

 the table on p. 3. 



Of these, the lowest or Chalk Marl consists, as the name implies, 

 of a marly admixture of calcareous and clayey material. Above this 

 comes rather hard greyish chalk, slightly clayey, and then i oo to 1 50 

 feet or more of white chalk. The beds up to the top of this ' Lower 

 Chalk' division are marked by the absence of flint; but from this horizon 

 to the top of the formation in Surrey flints in scattered nodules and 

 occasionally in thin continuous layers are everywhere present ; and the 

 whole formation is thus roughly divisible into an upper part containing 

 flints and a lower part without flints, of which the former is always thicker 

 than the latter. As previously mentioned, it is however from the suc- 

 cessive appearance and disappearance of the different species of fossils 

 in the slowly accumulated sediment that we are best able to divide the 

 Chalk into zones ; but at present it is only in certain parts of the county 

 that this has been done, 1 and further research on these lines is greatly to 

 be desired. 



In the Hog's Back ridge the Chalk has scarcely sufficient width to 

 display the typical down-land scenery, but farther east where the outcrop 

 begins to widen it exhibits the rounded steep-sided hills and deep dry 

 winding valleys which are everywhere so characteristic of a chalk country. 

 The elevation of the Downs also increases eastward through Surrey, 

 their highest ground, 876 feet above sea level, occurring near the eastern 

 boundary of the county. 



The dry thin soil of the Downs where the Chalk immediately under- 

 lies the surface is generally treeless, and covered only with smooth short 

 turf. But these conditions are largely modified in the Surrey uplands 

 by the presence on the hills of an irregular surface-deposit of clay or 

 clayey earth containing many flints derived from the Chalk, hence termed 

 the ' Clay-with-flints.' This material occurs as a variable sheet which 

 fills all the little pits and hollows in the weathered surface of the Chalk 

 wherever the ground is not too steep for it to rest. Its origin bears a 

 simple explanation. We know that the calcareous matter of the Chalk is 

 slowly taken up and carried away in solution by the downward percolation 



1 Consult C. Evans' paper, On some Sections of the Chalk between Croydon and 

 Oxtead, with Observations on the Classification of the Chalk,' Proc. Geol. Assoc., supplement 

 to vol i., 1870 ; also that of G. E. Dibley, ibid. vol. xvi. (1900) pp. 489-496. 



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