R. M. Deeley—The Thames Valley Gravels. 57 
IJ1.—Tar Frouvio-etactan GRAVELS oF THE THAMES VALLEY. 
By R. M. DEELEY, M. Inst.C.H., F.G.S. 
(WITH A FOLDING MAP, PLATE IV.) 
CAREFUL examination of the distribution of the high-level 
gravels and sands of the Thames Valley, especially those 
associated with the boulder-clays, shows that in pre-Chalky Boulder- 
clay time, not only were the main valley lines marked out, but many 
even of the smaller valleys, such as the Brent, were in existence. 
Of course, during Pleistocene time, denudation has been very active ; 
for over large portions of this area the boulder-clays, gravels, and 
sands now only occur as scattered patches. 
Denudation has been most active in the valleys of the main 
watercourses, less so in the smaller watercourses, and least in the 
neighbourhood of ridges and high land. However, even in the main 
valleys, where the deposits were chiefly gravels and sands, the gravels 
and sands have protected the softer rocks below them, with the result 
that the gravels which once occupied the old valley bottoms now cap 
the hills and higher lands. In many situations the most elevated 
masses of boulder-clay rest directly upon the older rocks, whereas in 
lower positions the boulder-clays, especially the Chalky Boulder-clay, 
generally rest upon sands and gravels. We have here an indication 
that these sheets of gravel were laid down as extensive fluvio-glacial 
deposits, their heights above the sea depending upon the slope of the 
land and supply of material coming from the margin of the ice; or, in 
the case of valleys up which the ice was advancing and blocking the 
natural exits, upon the heights of the cols at their ends. 
It must be remembered that both during the advance and the 
retreat of the ice-sheets vast volumes of water were thrown off at 
their fronts. When we consider that the ice which entered the 
Thames Valley came from the Scandinavian Peninsula, and that the 
greater portion of the precipitation on the ice-sheet which occurred 
between England and Scandinavia was liberated as water near the ice 
fronts, it is clear that an enormous volume of water was given off 
at the ice-sheet margins. Such melting took place both during 
advances and retreats of the ice. During the advances some of the 
precipitation was stored up as ice, and during the retreats some of 
the ice was melted, and increased the volume of water thrown off ; 
but when we consider the slow nature of the advances and retreats, 
and the great length of time the ice-sheets persisted, it is clear that, 
as far as flood-water from the ice is concerned, periods of retreat did 
not differ greatly from periods of advance. 
In the Thames Basin, with the exception perhaps of the estuarine 
portion, the whole district has been above the sea-level ever since 
the deposition of the Lenham Beds; consequently subaerial denudation 
had been active over the district for long ages before the Glacial 
Period, and there must have been an old river system in existence 
before the advent of the ice. However, as the directions of the main 
valleys in the unglaciated areas have always been largely as at 
present, in spite of the increasing development of subsequent streams, 
