50 LONDON DISTRICT. 



exist at the present day. As Prestwich pointed out in 1863, 1 it 

 is necessary to infer river-action of greater intensity, and floods 

 of a more or less torrential character; such conditions may 

 reasonably be attributed to a greater elevation of the land, and, 

 to some extent, to a more rigorous climate. 



The River Gravel, obtained for the most part ready-made 

 from the older gravels that cap the hills on either side of the 

 Thames, is formed chiefly oi pebbles and subangular flints, with 

 many pebbles of quartz and quartzite. Some materials, including 

 flint-casts of Chalk fossils, may, of course, have been derived 

 directly from Eocene pebble-beds or from the Chalk. There is 

 a greater variety of stones in the gravel of the tributary streams 

 on the north than on the south, owing to the fact that the Plateau 

 gravels are there of local composition, while on the north occur 

 extensive tracts of mixed Glacial Gravel as well as older Plateau 

 Gravel. 



Interbedded with the Gravel there is much sand, and in some 

 cases the bulk of the deposit is sand. Peaty seams are occasionally 

 met with. The gravel attains a thickness of 30 or 40 ft., and 

 even more, but is not usually so thick. 



To the Glacial Gravels may be attributed most of the erratic 

 stones, such as the Bunter quartzites, occasional Jurassic fossils, 

 and fragments of older stratified and igneous rocks. Limestone 

 pebbles, such as occur in abundance in the Upper Thames Valley, 

 are not met with, as they would have been ground to powder 

 before reaching the present district. 



Blocks of greywether or sarsen stone have been encountered 

 here and there in the gravel at Bays water, Kensington, and at 

 Grays, just beyond West Thurrock. These were probably 

 derived from the Woolwich and Reading Beds, and may have 

 been dislodged during Glacial times, as examples are met with 

 in the newer Plateau deposits on the Chalk tracts near Rick- 

 mansworth. Specimens figured by Mr. T. V. Holmes from 

 Grays Thurrock are very similar to those obtained at Croxley- 

 green. 2 



The Brickearth or loam, a mixture of sand and clay, was, no 

 doubt, formed largely from the destruction of London Clay and 

 Bagshot Sand, and in part from other Eocene clays and sands. 

 It is of variable character, and passes from a light sandy loam 

 to a clay that seems to be little else than reconstructed London 

 Clay. It appears to have been deposited for the most part in 

 tranquil waters, and has been described as an inundation-mud. 

 In some cases it has been, held that the material may have been 

 wind-drifted like some of the loess of northern Europe; and 

 Clement Reid has suggested that desert or steppe conditions 

 prevailed during the later parts of the Pleistocene period. 3 In 



l Phil. Trans., 1864, pp. 250, 297, etc. 



2 Essex Nat., vol. xiii, 1904, p. 197; Woodward, H. B., GeoL Mag., 1891, 

 p. 119 ; 1900, p. 543 



* Nat. Science, vol. iii, 1893, p. 367 ; ' Origin of the British Flora,' 1899, p. 44. 



