5o6 



PLEISTOCENE. 



Along the borders of the Thames Valley it is often a matter of difficulty to 

 distinguisli between the Middle Glacial gravels and those formed by the river in 

 great measure from their destruction ; such is the case near Great Marlow, also at 

 intervals along the valley as far as Southgate and Enfield. 



At Danbury, in Essex, the Middle Glacial gravel occupies a high elevation, but 

 the hill is coated with gravel and not formed by it, as Mr. W. H. Penning has 

 traced the London Clay in gullies near the hill-top. Traces of gravel occur 

 further south in Essex on Langdon Hill, in Middlesex on Hampstead Heath, 

 in Kent on Shooter's Hill, etc. They are usually made up largely of flint and 

 quartz pebbles, and are seldom of great thickness, varying from 2 to 12 feet. 



The Tertiary hills of Berkshire, Surrey, and East Kent, between Reading and 

 Canterbury, are capped by gravels of uncertain age.^ In these gravels are found 

 subangular pieces of quartz or rock-crystal called ' Bagshot Diamonds.' ^ 



Deposits of loam and brickearth are associated with the gravels on the Chalk 

 tracts of Buckinghamshire and Hertfordshire (see p. 449), and sometimes with 

 those on the London Clay ; but it is not possible always to fix the ages of these 

 beds. Thus, at Chelmsford there is a deposit of brickearth, which may be of 

 Glacial age or Post-Glacial. 



Chalky Boulder Clay. — The Great Chalky or Upper Boulder Clay of the 

 Eastern counties is a clayey bed full of pellets or pebbles of Chalk, containing 



Fig. 89. — Section at Writtle, near Chelmsford. 



2. Chalky Boulder Clay. (Upper Glacial.) 

 1. Sand and Gravel. (Middle Glacial.) 



also Chalk-flints, pebbles of flint, and blocks of various rocks (Septaria, Carbo- 

 niferous Limestone, etc.), many of them scratched or grooved by ice-action. In 

 thickness it varies from a few feet to 150 feet. It contains numerous fossils 

 derived from different formations (chiefly Lias, Oxford Clay, and Kimeridge Clay), 

 including Grypkiea incuiva, G. dilatata, Bdemnitcs abbrcviatiis, etc.^ 



1 Wood, G. Mag. 1870, p. 19. 



2 T. R. Jones, P. Geol. Assoc, iv. 442 ; see also Buckland, T. G. S. v. 519. 



^ See list by John Brown, of Stanway, of specimens identified by J. de C. 

 Sowerby and S. P. Woodward, Proc. G. S. iv. 165 ; Q. J. viii. 191, P. Geol. 

 Assoc, i. 32 ; also Mag. Nat. Hist. ix. 42; J. Morris, G. Mag. 1868, p. 411 ; 

 S. Woodward, Geol. Norfolk, p. 39 ; H. Walker, Glacial Drifts of Muswell Hill 

 and Finchley, 1874, P. Geol. Assoc, ii. 289. See also E. Spencer, Proc. G. S. 

 ii. 181. 



