WESTLETON BEDS TO THOSE OF NOKFOLK, ETC. 145 



They have there been disturbed and faulted (PI. YII. fig. G), before 

 the intrusion of the Glacial Beds, which have deeply eroded them. 

 As they rise to higher and more exposed levels, the Westleton 

 Beds become greatly diminished in importance and are commonly 

 reduced to a mere shingle-bed. On the hills near Epping, where 

 they attain a height of 350 or 400 feet, they form a thin bed of sand 

 and shingle, capping the London Clay, and quite apart from the 

 Boulder-clay, which lies from 80 to 100 feet lower down on the slope 

 of these hills at They don Bois and North Weald. 



In Hertfordshire the distinction between the two deposits is very 

 marked, as on the hills between Hoddesdon and Hatfield the 

 Westleton Shingle forms a plateau-gravel high above the plain of 

 the Lea, over which is spread the Boulder-clay with its associated 

 sands and gravel. At Bell Bar and Mimms Wood the hills are 

 also capped by this Shingle, while in the valley between them 

 the Boulder-clay in the railway-cutting is at a level of from 100 

 to 150 feet lower. The Boulder-clay again enters the valley of 

 the Colne at Bricket Wood and Aldenham., whilst Westleton Shingle 

 lies on the neighbouring heights of Shenley and Bennett's End. 



Here the Boulder- clay ends, and the Westleton Shingle alone is 

 prolonged westward at intervals on the higher summits of the Buck- 

 inghamshire, Berkshire, and Oxfordshire Downs, the Chalk-plain on 

 which these hills rise being in some places denuded, and at others 

 thinly covered by Glacial Drift-beds. The Shingle-bed may, in 

 this way, be traced to the northern edge of the Chalk Downs and 

 to the extreme western end of the London Basin, which shows 

 how extensive the submergence of these districts must have been 

 at a time immediately antecedent to the Glacial period. 



The height (500 to 600 feet) to which the Shingle-bed attains 

 is sufficient to carry it over the summit of the Chalk and Oolitic 

 hills of the Warminster and Bath districts. There may thus have 

 been a Pre-Glacial sea bounded on the south by the Wealden anti- 

 clinal, and spreading northward over great part of Hertfordshire, 

 Buckinghamshire, and Oxfordshire (and possibly beyond), while it 

 stretched in the other direction from the jN'orth Sea to the Severn 

 Basin or the Bristol Channel (see Part III. and Map). 



6. Orirjin of the Shingle. 



The distinctive features of the AYestleton Shingle are of so marked 

 a cliaracter and so different from those of the overlying Glacial 

 series, that it is at once evident that their origin must be different. 

 The characters of the latter are so well known that it is unnecessary 

 to repeat them here; suffice it to say that they are essentially 

 Drifts from the north and north-east, whereas that of the Westleton 

 Beds is from the south and south-east, for the reason that the 

 sources whence the sevor."] different component pebbles are de- 

 rived is to be traced in those directions. 



Thus: — 1. The fiint-pcbbles are doubtlessly derived from the 

 shingle-beds of Diestian, Bagshot, or Lower Tertiary (Woolwich) 



