DRIFT-STAGES OF THE DARENT VALLEY. 127 



not pass beyond the first outworks, beinj^ shut out by the range of 

 the Lower Greeusand from the central Weald. To tho south of 

 the Chalk ranj^e, the valley branches westward in the line of tho 

 main stream to near Limpsfield, and eastward to near Ightham in 

 directions parallel with the ranges of the Lower Grecnsand and the 

 Chalk, and is terminated by watersheds which separate it in the one 

 case from the Oxted stream, and in the other from the Ightham 

 stream (the Sliode), both of which run from the foot of the Chalk 

 hills, and flow into the central or Medway drainage-area of the 

 Weald. The valley is thus isolated, and its basin is of very limited 

 extent, though at one time it would appear to have been larger, in 

 consequence of the greater importance of the affluents from the 

 Tertiary area (see Map, PI. VII.). 



The first indent of the Darent Valley was, for the reasons given 

 in two papers referred to in the last page, clearly subsequent to 

 the deposition of the Lenham Sands, which are of Pliocene age, 

 of the Red Clay-with-flints, and of the Southern Drift, while it com- 

 menced with the general great denudation of the Weald. It is there- 

 fore of late pre-Glacial or very early Glacial date *. At the former 

 of these periods, the great valley separating the Chalk and Lower- 

 Greensand ranges of hills was still bridged over by the Chalk and 

 overlying strata, and it is to the denudation of these latter that 

 both valley and escarpment are due (PI. VI., figs. 1, 2, 3). 



As the Lenham Sands are only of local occurrence, our object 

 will be best answered by taking tlie lied Clay of the Chalk Plateau, 

 with its sprinkling of Southern Drift, as our base-line. Without at 

 present going into the question, of the origin of the Red Clay- 

 with-flints f, beyond mentioning that it is of local derivation, I may 

 state that it is newer than the Tertiary strata, the outliers of which 

 it encircles, while it seems to be older than the Southern Drift, with 

 which it is closely associated. 



Besides the main valleys of the Medway and Darent, the Chalk 

 Plateau, with its " Red Clay," is intersected by a system of lesser 

 valleys, which, starting near the crest of the escarpment, run north- 

 wards into the main valley of the Thames. These valleys commence 

 on the Red Clay in very slight deflections on the surf;xce, which 

 rapidly increase in depth, and enlarge into the deeper valleys above 

 which the Red Clay is left high on the adjacent plateau. These 

 valleys, therefore, like the larger ones before named, are posterior 

 in time to the Red Clay, as well as to the implement-bearing old 

 drift with which the latter is associated. The difference of level 

 between this older drift and the drifts of these other valleys, though 



* In further proof of the sub-glacial action before noticed at the time of the 

 Southern Drift (Quart. Journ. Geol. Soc. vol. xlvi. (1890) pp. 157, 174), I. should 

 mention tliat ^Ir. B. Harrison has since found on the summit of tlie Chalk 

 escarpment at Terry's Hill above Wrothain, and at a height of 770 feet, some 

 small angular boulders (' as large as quart measures') of the Oldbury chert, 

 and several smaller blocks of Irou-sandstone from the Lower-Greensaud range 

 to the south. 



t For an account of this Red Clay, see Mr. W. Whitaker's 'Geology of 

 London,' vol. i. chap, xviii. ISSD. 



k2 



