GEOLOGY 



Whitaker gives expression to the same view from other evidence when 

 he says : ' From its occurrence on the tops of the hills, whilst the 

 Middle Glacial gravel often lies at their base or on their flanks, it would 

 seem that the pebble-gravel is the older of the two, and was deposited 

 long before those hills were cut into their present form a process 

 which must have been somewhat advanced before the other gravel was 

 laid down.' ] 



The Westleton Shingle caps the London Clay hills between Hat- 

 field and Hertford Heath, where they form a conspicuous range between 

 320 and 380 feet high, and rests on the London Clay at Shenley Hill 

 towards the south-west. Most of the Tertiary outliers, whether of 

 Reading Beds and London Clay or of Reading Beds alone, are also 

 capped by this shingle. It may be well seen on the Reading outlier at 

 Bernard's Heath, St. Albans (406 feet), where it is from 8 to 10 feet 

 thick, and on the Reading and London Clay outliers of Ayot Green 

 (406 feet) and Datchworth (407 feet). At a lower level it caps the 

 outliers of Collier's End (348 feet), and Sacombe Green, north of Ware 

 (362 feet), and at a much higher level the small outlier at Bennett's End 

 near Hemel Hempstead (465 feet), which is partly covered by brick- 

 earth. On the borders of Hertfordshire and Middlesex the Westleton 

 Shingle rests on the London Clay ridge which extends from Potter's 

 Bar to Bell Bar (380 to 400 feet), and a little to the west caps the London 

 Clay in Mimms Wood, a mile and a half north of South Mimms (400 

 feet). Within a mile of our border the Reading and London Clay 

 outlier of Tyler's Hill or Cowcroft has a small capping of this shingle at 

 a height of about 600 feet above sea-level, and much farther to the 

 south-west, on what was once an outlying portion of our county, the 

 shingle caps the Tertiary outlier of Penn near Beaconsfield, at the same 

 elevation. It is thus seen that the Westleton Shingle generally occurs 

 at a higher level as we proceed from east to west, showing that the 

 existing elevation of the land in that direction took place after its 

 deposition. This inference would not follow with Glacial deposits which 

 may have been dropped from icebergs, and occur at very different 

 levels. 



Nearly all our London Clay hills and Tertiary outliers are thus seen 

 to be capped by gravels of pre-Glacial age, remnants of a bed once of 

 great extent. Although at one time a continuous sheet, the Westleton 

 Shingle varies much in its composition at different places, but the 

 greater part of it in our district is composed of well-rounded Tertiary 

 flint-pebbles ; white quartz-pebbles and subangular flints come next in 

 different proportions, but together usually about equal in quantity to the 

 flint-pebbles, and the rest consists of subangular fragments of chert and 

 ragstone of Lower Greensand age, and pebbles of white and yellow 

 quartzite, Lydian stone, etc., with a few old-rock pebbles. 



In the foregoing description of the hill-gravels of the south of 



1 Guide to the Geology of London, yA -d. p. 57 (1880). 



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