308 
ME. PEESTWICH ON THE GEOLOGY OE THE DEPOSITS 
Explanation of the Plates. 
PLATE IV. 
The uncoloured ground-plan shows the chief geological divisions of the country. 
The coloured lines indicate the course of the debris derived from the several principal 
formations. The lines are carried through from the parent rocks to the sea; but 
it is to be observed that the debris of each of these lines becomes less and less abun- 
dant as they recede from their source, so that all traces of some of them sometimes 
nearly disappear before the end of the river-valley is reached. In places, also, the gravels 
formed by these debris meet with long interruptions, those on the higher levels especially; 
the lower-level gravels are more continuous, though they are constantly hidden by recent 
alluvia. They also vary materially in width. These more minute details are repre- 
sented for small portions of two river- valleys in Plate V. The scale of the Map 
does not admit of the delineation of the lines of debris of each valley. The chief river- 
valleys and their principal tributaries are therefore selected, but the same law of 
the occurrence of high- and low-level gravels, and of the local limitation of the various 
rock debris to the several river-basins, is applicable to all the river-valleys in the area 
comprised in the Map. The authorities for France are the several authors mentioned 
in the text, with a few observations of my own. The English part of the Map is given 
from my own personal observations. The commencement of some lines of debris is 
not unfrequently higher up the valleys than marked on the Map, or from rocks beyond 
the present range of the river. This arises from the presence of outliers of certain 
formations more or less beyond the limits of the main mass, which outliers are not 
represented on the Map : in other cases the debris are derived from secondary sources, 
like the ‘ palaeozoic, oolitic, and cretaceous debris of the Boulder Clay; and in a few 
cases, especially in the Wealden area, they arise from the originally greater length of 
the rivers. 
PLATE V. 
Figs. 1 & 2 show the distribution of the high- and low-level valley-gravels, and of the 
Loess in parts of the valley of the Somme adjacent to Abbeville and Amiens. 
The relative position of the high-level gravels and of the plateau Loess 
is not always quite clear. In many cases I consider the latter to be newer 
than some of the former ; but, at the same time, there is a quaternary argil- 
laceous deposit on parts of the chalk hills which I believe to be older than 
any of the valley-gravels. This is generally more removed from the river- 
valleys than shown in these plans. 
The valley-gravels are here divided into two stages only. Each stage, 
however, must be considered to represent not one exact level, but the several 
nearly allied terraces formed during a particular time. It may be assumed, 
for example, that the high-level gravels include all the terraces at heights 
