48 LONDON DISTRICT. 



the difficulty of drawing a boundary line, appears on the map 

 as ' Plateau Gravel.' The thickness of the deposit here is 20 to 

 25 ft., and blocks of quartz and quartzite up to 10 by 8 by 6 in. 

 occur. 1 Further down the valley, at Iver Heath and Hillingdon, 

 a feature can usually be seen between the Glacial Gravels and 

 those of the Boyn Hill Terrace, and the two deposits can be 

 separated as on the map. 



On the east of the Colne the Glacial Gravels approach the 

 older ' High Level ' deposits and reach almost the same height 

 but can be distinguished by the presence of Bunter quartzites. 

 At Moor Park, Clement Reid noted a gravel at 359 ft. (surface 

 level) consisting of some 70 per cent. Eocene material, but 

 yielding also quartzites and Palaeozoic grits, while the Oxhey 

 Wood gravel and its outliers descend to little above the 300 -ft. 

 contour and contain no far-travelled stones. Further north by 

 Battlersgreen and Aldenham, the Glacial Gravel is 15 to 20 ft. 

 thick; the western margin of this spread, towards the Colne, is 

 probably river-gravel. 



At Hendon, as pointed out by Hicks, 2 the gravel contains an 

 intercalated and irregular mass of brown clay like London Clay, 

 and the same feature was observed by Mr. Whitaker at Finchley. 3 

 There, beneath the Boulder Clay, he noted 17 ft. of brown clay, 

 blue-mottled and with much ' race,' somewhat like London Clay ; 

 it rested on about 9 ft. of sand and gravel, largely composed of 

 flint pebbles, beneath which was London Clay. 



These beds of brown clay appear to be re-constructed masses 

 or boulders of London Clay, and it was noticed at Hendon that 

 in places the brown clay extended through the under -lying gravel 

 down to the London Clay floor from which it had not wholly been 

 severed. 



The outlying patches at Dollis Hill and elsewhere are 

 probably flu vio -glacial gravels laid down after the formation of 

 the Boulder Clay and the retreat of the ice; they contain a 

 smaller amount of the readily destructible Jurassic material than 

 the main mass at Finchley. The gravels of Southgate and 

 Belmont are, on the other hand, early glacial, but north-west of 

 Enfield the Boulder Clay appears to rest on the ancient ' High 

 Level ' deposits. 



In Essex gravels containing liver-coloured quartzite pebbles 

 occur in Epping Forest up to 370 ft. above OJ). 4 The higher 

 gravel of Barking Side, north of Ilford, shown on the map as 

 River Gravel, has been grouped by Mr. Monckton with the deposits 

 immediately succeeding the Boulder Clay. 5 



1 See also Hopkinson, J., Proc. GeoL Assoc., vol. xx, 1907, p. 96, and Kidner, H., 

 ibid., vol xxi, 1909, p. 244 



2 Quart. Journ. GeoL Soc., vol. xlvii, 1891, p. 575; Proc. GeoL Assoc., vol. xii, 

 1892, p. 334; vol. xiv, 1896, p. 327. 



3 Geology of London, vol. i, 1889, pp. 309-311. 



4 Salter, A. E., Proc. GeoL Assoc., vol. xiv, 1896, p. 394. 

 6 Essex Nat., vol. vii, 1893, p. 116. 



