GLACIAL DEPOSITS. 45 



There is no evidence to suggest that the great ice sheet ever 

 crossed the present Thames Valley, but glacial gravels, probably 

 newer than the Boulder Clay, occur on Kingston Hill and 

 Wimbledon Common ; besides the northern material they contain 

 a large proportion, increasing from north to south, of ' Southern 

 Drift.' It seems a fair inference that these deposits mark the 

 bottom of the valley at the end of the Glacial Period, since there 

 alone would the waters from north and south mingle. 



THE BOULDER CLAY. 



The Boulder Clay or Northern Drift is recognised as a direct 

 product of land-ice formed during the Glacial Period. 



It is for the most part a tough bluish-grey clay with many 

 pellets and pebbles of hard chalk, rolled and unrolled flints, 

 together with fragments of rocks and fossils derived from many 

 different formations, notably from the Jurassic and Cretaceous. 

 Lenticular beds of sand and gravel occasionally occur in the 

 clay. Both rocks and fossils exhibit glacial striae, and they are 

 especially noticeable on the pebbles of hard chalk. 



The Jurassic materials include oolitic limestone and sundry 

 ammonites ; also Lima gigantea from the Lower Lias ; Belemnites 

 abbreviatus and Gryphcea dilatata from the Corallian and the 

 Oxford Clay; Orbiculoidea latissima and Lucina minuscula (a 

 conspicuous white bivalve shell) from the Kimmeridge Clay. 

 Pieces of Red Chalk and occasional Eocene materials, notably 

 Venericardia planicosta (a Bracklesham fossil) have been obtained. 



At the surface the Boulder Clay becomes decalcified, and 

 appears as a brown stony loam 3 or 4 ft. thick, which merges 

 into the unweathered Boulder Clay, or occupies pipe-like 

 cavities in it. 



In thickness the Clay is very irregular, but in our present 

 district is not known to exceed 35 ft. 



It occurs on the uplands in the north-eastern tracts around 

 Navestock and Lambourne, at Havering-atte-Bower and Chigwell 

 Row, as well as at Stapleford Tawney, on the northern side of the 

 Roding Valley. It thus overspreads certain tracts of Bagshot 

 Beds. Sometimes it rests directly on London Clay, and 

 elsewhere is based on Plateau gravel and sand. 



To the south-west of Brentwood 1 , (which lies to the east of 

 South Weald), the Boulder Clay descends into the Thames 

 Valley, and was exposed in the railway-cutting at Hornchurch, 

 between Upminster and Romford, beneath the Thames Valley 

 Gravel, where its presence was made known by Mr. T. V. 

 Holmes. 2 



It occurs to the west of Enfield and in outlying patches 

 towards East Barnet, and a large tract extends from near 

 Whetstone to Finchley and the summit of Muswell Hill. 



1 See Prestwich, Quart. Journ. Geol. Soc., vol. xlvi, 1890, p. 164. 



2 Quart. Journ. Geol. Soc., vol. xlviii, 1892, p. 365. 



