A HISTORY OF HERTFORDSHIRE 



represented in our county must therefore be considered as open to con- 

 troversy and liable to modification from future research. 



The Quaternary era is usually divided into two periods, Pleistocene 

 or Post-Pliocene, and Recent, the Pleistocene being equivalent to the 

 Glacial period, and being divided into Lower, Middle, and Upper 

 Glacial, and the Recent period being divided into Prehistoric and His- 

 toric. 'The oldest Pleistocene gravels in Hertfordshire and the south of 

 England generally are however of pre-Glacial age ; the Till or Lower 

 Glacial boulder-clay is not represented here ; and later in the Pleisto- 

 cene period arctic conditions did not prevail uninterruptedly. 



Our two chief beds of gravel Professor T. McKenny Hughes long 

 ago distinguished as ' Gravels of the Upper Plain ' and ' Gravels of the 

 Lower Plain,' the former being the older of the two, and having been 

 deposited by the sea which levelled the county into a plain of which 

 we now see the remnants in the highest ground of the area of the 

 London Clay. 1 



These older gravels have been investigated by several other geo- 

 logists, and especially by the late Sir Joseph Prestwich, 2 who has given 

 to the greater part of them the name ' Westleton Shingle,' separating 

 under the term 'Southern Drift' the gravel which caps our most 

 southern London Clay hills and also occurs south of the Thames, this 

 being considered of earlier formation than the pebble-gravel of 

 Westleton and the eastern counties generally. The largest patch we 

 have of this oldest shingle-gravel spreads over Stanmore Heath from 

 Little Bushey to Bentley Priory at a height of 400 to 450 feet, and 

 there are smaller patches on the hill between Pinner and Watford, and 

 east of Stanmore on Elstree and Brockley hills, nowhere less than 380 

 nor more than 450 feet in height. The great ice-sheet of Norway and 

 Britain, approaching from the north-east, does not appear to have 

 extended farther to the south than these hills, but it is more probable 

 that this was due to the melting of the ice than that the hills, or the 

 range or plateau of which they then formed a part, created a barrier 

 against its further progress. According to the views of Professor Hughes 

 they are the remnants of an extensive plain which then existed, having 

 been formed into hills by subsequent erosion of valleys on the north and 

 on the south. 



A little to the north of these hills are others capped by true Westle- 

 ton Shingle. All these are Tertiary hills, either forming a part of the 

 London Basin, in which case the shingle rests directly on the London 

 Clay, or being outliers of Reading Beds with or without London Clay. 

 We have no Westleton Shingle lying directly on the Chalk, which seems 

 to show that the erosion of the Tertiaries from the surface of the Chalk 

 had not taken place when this marine pebble-gravel was deposited. Mr. 



1 ' On the Two Plains of Hertfordshire and their Gravels,' Quart. Journ. Geol. Sac., 

 vol. xxiv. p. 283 (1868). 



8 In three papers on the Westleton Beds read before the Geological Society, Quart. Journ. 

 Geol. Sac., vol. xlvi. pp. 84-119, 120-154, and 155-181 (1890). 



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