58 R. M. Deeley—-The Thames Valley Gravels. 
the old river gravels have been largely destroyed as the rivers cut 
vertically and horizontally into the land. 
For many reasons, in the process of excavating their valleys, rivers 
at various times bring their valley bottoms to base-level. ‘hey then 
cease to deepen them, but continue to attack the valley sides, and in 
course of time lay down extensive sheets of gravel. When a pause 
in vertical excavation persists for a long period, the surrounding high 
lands, when the rocks are soft, are denuded until we have a country 
of low relief through which the rivers course sluggishly over wide 
plains of gravel and of brickearth formed by floods. 
The gravel deposits of the River Thames show that such pauses in 
vertical erosion occurred several times, for we have the remnants 
of several of these gravel plains at various heights, some of them 
forming terraces along the main river valléys, and others far removed 
from the present watercourses. However, whilst the rivers were 
excavating their valleys from one base-level to another, they also 
formed gravel deposits at intermediate heights. It thus comes about 
that there are gravel patches at almost all heights above the rivers, but 
there are masses concentrated at particular levels. 
During the Pleistocene Period the ice-sheets reached the Thames 
Basin, and threw into the Thames Valley large quantities of gravel 
and sand. At these times gravel beds of exceptional thickness and of 
large area were formed. An endeavour will be made to connect these 
fluvio-glacial deposits with those of the fluvio-glacial fans near the 
old ice margins. 
The suggestion that the pre-Chalky Boulder-clay Thames Valley, 
especially the eastern portion, was deeply excavated and subsequently 
partially filled up with fluvio-glacial gravel has already been made 
by T. I. Pocock.? 
Bearing in mind the considerations outlined above, by the aid of 
the excellent Drift maps and memoirs published by the Geological 
Survey, it is possible to make a preliminary outline sketch of the 
conditions obtaining in the Thames Valley during the Chalky 
Boulder-clay stage. 
To accomplish this object we must proceed from the known to the 
unknown. In the lower Thames Valley a good deal is known 
concerning the nature and distribution of the deposits left by the 
ice-sheet. The unknown is the actual physical condition of the 
valley in Glacial times; but here we can bring to our aid a knowledge 
of the nature of the deposits which are being formed by existing 
ice-sheets and glaciers in such regions as Alaska and Greenland. 
The most profitable course to adopt in studying such an area as that 
of the Thames Valley is to obtain, if possible, a knowledge of the 
deposits formed at some particular stage, and use these as a datum 
with which to compare older or newer deposits. . 
When the ice which formed the Chalky Boulder-clay reached the 
north-eastern watershed of the Thames Valley it discharged into the 
Thames Basin great volumes of water and large quantities of gravel 
and sand. The deposits formed in this way contain numbers of rocks 
foreign to the Thames watershed. During such a time we had 
1 Geol. Surv. Summary of Progress, 1902, p. 201. 
