GLACIAL GRAVELS. 47 



Since Mr. Holmes made known his discovery, he has observed 

 another patch of Boulder Clay beneath the gravel on the eastern 

 side of Romford, in a cutting of the Romford and Upminster 

 Railway. 1 



During the 6-inch geological survey of the area by Mr. T. I. 

 Pocock, a number of additional patches of Boulder Clay were 

 mapped on either side of the Ingrebourne River, to the south of 

 Maylands and Havering-atte -Bower. 



The evidence so far appears to suggest that a tongue of 

 Boulder Clay was accumulated in a hollow along what is now 

 the Ingrebourne Valley. As Mr. Pocock has observed, ' Several 

 outliers of stony clay have been traced in this neighbourhood, 

 some of which still contain a little chalk, and others are probably 

 decalcified remnants of the Chalky Boulder Clay.' One of these 

 outliers was found on Tyler's Common, to the west of Great 

 Warley Street. 



THE GLACIAL GRAVELS. 



The Glacial Gravels occupy a large area to the north and west 

 of the Colne, crossing that river in the neighbourhood of Watford 

 and Harefield. Dr. Sherlock finds that these deposits differ from 

 the older ' pebbly clays and sands ' in four particulars : 



(1) There is hardly any clay, the waters which laid them 

 down being more or less torrential. 



(2) The flints are usually subangular, unworn chalk 

 flints being very rare. 



(3) They contain a small proportion of far -travelled 

 stones brought into the district by the ice. 



(4) They lie more horizontally. 



By these criteria it is comparatively easy to distinguish the 

 glacial from the older Plateau gravels described in the previous 

 chapter ; but, on the other hand, it is almost impossible to draw 

 a definite line between the Glacial and the newer River Gravels. 

 In the main valley, as, for instance, near Richmond, erosion had 

 lowered the river -level some 80 ft. between the deposition of the 

 Glacial and the earliest River Terrace Gravels (see p. 63); but 

 as we trace the tributary streams to their sources this figure 

 gradually decreases to zero, and the distinction lies between 

 gravels in the state in which they were left by the flu vio -glacial 

 waters and those which have been resorted by the rivers at 

 approximately the same level. Many of the patches of Glacial 

 Gravel shown on the map should have an indefinite margin, 

 towards the river, coloured as River Gravel. The point would 

 be of small importance were it not for the fact that palaeolithic 

 implements frequently occur at such sites; among the best 

 known is Croxleygreen, near Rickmansworth (p. 68), which 

 should be assigned to the Boyn Hill Terrace, but, on account of 



1 Quart. Journ. Geol. Soc., vol. 1, 1894, p. 443. 



