INTRODUCTION. XV 



Yedding, and other brooks. The presence of water in these gravels 

 has attracted a popuhxtion which is remarkably deficient in many 

 parts of the county — as, for example, the line of the Yedding brook 

 between Harrow and Uxbridge, where in many square miles scarcely 

 a habitation is to be found. (Journal R. Ap-icidtural Society, second 

 series, vol. v. p. 4.) In dry seasons these watercourses are nearly dry. 

 Valley Drifts. — Gravel, brick earth, and alluvium are one or more 

 present in the valley of the Thames and its tributaries. They are of 

 a later age than the gravel and brick earth of the high grounds, and 

 are found not only at the bottoms of the valleys (low-level gravels) 

 but often reaching some way up the flanks, and even capping the 

 tops, of the low hills bounding them (terrace gravels). Each terrace 

 forms a more or less flat tract sometimes worn through by side valleys. 

 As many as three admit of being traced, including the low-level 

 gravel. The gravel and brick earth of the Thames Valley is a very 

 important feature in the geology of Middlesex, occupying as it probably 

 does a third of the whole county. It covers all the southern part 

 bounded by the Colne, Thames, and Lea, and to the north, approxi- 

 mately, by a line drawn from Ililliugdon to Hanwell through Acton 

 and Paddington to Stoke Newington. The first part of this line 

 runs nearly parallel vAth. the turnpike road from Uxbridge to London, 

 and about half a mile to the north of it. The whole breadth on both 

 sides of the river of this tract, in the western part of the county, is 4 

 miles or more. According to Mr. Prestwich, the great bulk of the 

 gravel is composed of subangular chalk flints derived directly from the 

 chalk, with a considerable number of rolled flint pebbles from the 

 tertiary beds, and pebbles of quartz, slate, and other older rocks, from 

 the conglomerates of the new red sandstone of "Worcestershire and 

 Warwickshire. The whole deposit is more or less arenaceous, and 

 often contains beds of sand ; it is rarely more than 20 feet thick. Gravel 

 spreads over Uxbridge Common, and forms the top of the hill. It is also 

 found between Southall and Hanwell, and at Staines and Ilounslow 

 Heath. There is a gravel flat, about a quarter of a mile broad, 

 between Greenford and Perivale. ' At the top of Netting Hill there 

 is a high terrace of gravel, separated from the lower terrace of Iven- 

 .«ington by a narrow strip of London Clay. This terrace is the cause 

 tif the springs and moist ground in the southern part of Kensington 

 Gardens ' (p. 8G). The gravel has been denuded through for a short 

 v.'ay down the valley of the Serpentine, the water of which is held up 

 liy the London Clay. Along the Fleet Valley the gi-avel has also 

 bten denuded, and the springs which have been thrown out at its 



