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). 19
