128 PROF. PRESTWICH ON THE AGE, FORMA TIO If, AND 



necessarily small at the head of those valleys, equally indicates, as 

 ■when the difference becomes greater, a marked difference of age. 

 It will be our object to show the relation of both these valley- 

 systems, with their drift-beds, to the Clialk-Plateau drift, and also 

 to the Glacial and post-Glacial drifts of the Thames Yalley *. 



§ 2. The Cualk-Plateau Drifts and the associated 

 Flint Implements. 



Since the publication of my Ightham paper, Mr. Harrison has 

 traced the rude Palaeolithic implements of the Chalk Plateau to 

 West Yoke, 1 mile N.W. of Ash, and very near to the line where 

 the Red Clay with flints ends abruptly on the brow of the hill over- 

 looking the lower plain of bare chalk, which, except where the 

 Swanscombe Hills intervene, extends to the Thames (PL VI., fig. 2). 

 The relation that the Eed Clay with the associated Palaeolithic im- 

 plements here bears to the adjacent valleys is shown in the 

 following section (fig. 1): — 



Fig. 1. — Transverse section of the West-Yohe plateau. 

 w. E . 



Speedgate Fawkham West LongBeld 



Hill. Valley. Yoke. Valley. 



490. 270. 460. 280. 



a. Eed Clay-with-flints. 



e. Low-level valley- gravel. 



2. Chalk. 



V. Site of Palieolithic implements of the Plateau-type. 



(The vertical scale of all the general sections in the text, unless mentioned 

 otherwise, is i\- inch = 100 feet. The base-line represents the sea-level, 

 and the figures which give the height above it are taken from the six- 

 inch Ordnance maps, which are indispensable in work of this sort.) 



The valleys on either side of the plateau at West Yoke are about 

 180 feet deep, while the small central depression (*), which eventually 

 joins and belongs to the same valley-system, is here in an incipient 

 state (10 to 20 feet in depth), showing that the mere question of 

 level is not always conclusive in determining the relative antiquity 

 of these drifts. 



The great antiquity of the plateau-drifts can, however, be better 

 realised by the IS", and S. section (PI. YI., fig. 2), which extends 

 from the Lower-Greensand hills to the Thames, and shows the 



* I use these terms for convenience, meaning to embrace the whole of the 

 cold period from the earliest pre-Glacial to the latest post-Glacial times. 

 The pre-Glacial, Glacial, and post-Glacial cycles pass one into another in a 

 continuous series marked only by different and fluctuating degrees of intensity 

 of cold. The term 'post-Glacial' conveys an incorrect meaning. 'Later- 

 Glacial ' would be a better term. 



