154 PllOF. TRESTWICU Oii THE AGE, FORMATION, AND 



It is therefore evident that the festooning took place before the 

 undermining of the slope and deposition of /', but subsequently to 

 the spread of the gravel b, or during the period intervening between 

 the High-level gravels of Dartford and Limpsfield, and the Shore- 

 ham and Eynsford Mammaliferous drift — a period which corresponds 

 with that of the Chovening drift ; and if the Chevening drift is to 

 be attributed to glacial agency, this festooning may be one of the 

 effects of cold on the surface, either by repeated freezings and dis- 

 integration, or by the puddling of the ground caused by the passage 

 of masses of snow and ice over the surface of the Chalk. It is a 

 feature so common that it must bo due to some very general cause — 

 possibly to the same one that formed the Chalk-rubble described in 

 the next section (§9). 



It follows from the preceding considerations that there are in 

 this district four distinct zones or levels of Drift, and that in three 

 of these Palaeolithic implements have been found, namely : — 



1. That («) of the High plateau on the Chalk hills, and of which 

 the implements exhibit a distinct difference in type and workman- 

 ship from the other two. This is possibly of pre-Glacial or early 

 Glacial age. 



2. That (b) which accompanies an early stage of the existing 

 river-courses, and includes the High-level gravels of the valleys 

 (the " Hill group " of Ightham). 



3. That (e) of a Lower-level valley drift. The implements found 

 in this and the preceding zone, h, are very similar in character, and 

 can only be regarded as variations of the same group *. They both 

 belong to the so-called " post-Glacial " period. 



JN'o organic remains have hitherto been found in the older drift, 

 and they only occur locally in a few places in the second, while 

 they are common in the last. 



§ 9. The Bubble on the Sides and IjST the Bed of the Yalley. 



Another feature connected with this valley, and very general in 

 the Chalk districts of the South, is the debris of broken chalk and 

 flints scattered over the slopes and at the base of the hills. Its 

 origin is obscure. It has been referred to subaerial action and rain- 

 wash — terms which, though applicable in some instances, are too 

 often used in default of a better. Both these terms imply surface- 

 action and effects due to existing causes, and cannot therefore apply 

 to any deposit due to anterior geological causes. 



As the " Chalk- and flint-rubble " frequently forms the surface-soil, 

 it might, without further investigation, be referred indifferently to 

 one of the causes just named, did not the irregularity and absence 

 of sorting of the materials militate against its being rainwash, while 

 the occasional presence of materials foreign to the spot is an objec- 

 tion to local disintegration. But though the surface of the " Chalk- 

 and flint-rubble " is often, or rather is more generally, bare, it is 

 occasionally covered by a bed of red loam or clay with flints and 

 * The main difference seems to be one of size and proportion. 



