i8yS.) 
Superficial Gravels and Clays. 
345 
points, or a little over 3 feet per mile, and yet, according to 
the theory of the post-glacial origin of the valley, have had 
power to spread out a continuous sheet of gravel, without 
at the same time being able to wash away the loose Middle 
Sands and Gravels with their thin covering of clay on the 
slopes at Finchley. 
These seem to me formidable preliminary and general ob- 
jections to the theory ; but let us suppose that they have 
been overcome or explained, and try to follow in our mind 
what would happen if the valley below the 150-feet contour- 
line had been left, at the end of the Glacial period, an un- 
dulating plain, covered more or less with boulder-clay and 
underlying sands and gravels, through which the streams 
began to cut their channels. We have no great body of 
water to do this in the valley of the Brent below Finchley, 
if between Finchley and Hendon the stream was not pow- 
erful enough to wash away the glacial beds. The drainage 
area of the Brent is also too small a one to have supported 
a large stream. To excavate the wide valley from below 
the “ Welsh Harp ” to the Thames, it must therefore have 
often changed its course and wandered from side to side, 
so as to operate at different points. It would first have to 
clear away the glacial beds, and then cut down into the 
London Clay. We find, however, no bluffs in the latter 
such as we might expert, but a long gentle slope, from 
Ealing down to the stream at Hanwell, covered continuously 
with a mantle of gravel and clay. The superficial beds 
could not be deposited without break or overlap during 
the excavation of the valley, but only after the escarpments, 
formed whilst it was being cut out by the stream, had 
been sloped down by subaerial denudation. Every stream 
that is lowering its bed leaves precipitous banks, but the 
sides of the valleys of the Thames and its tributaries had 
been bevelled off into gentle slopes before the deposition of 
the gravels. 
We may next inquire if the gravels and clays in question 
resemble those that are now being deposited by streams. 
When travelling in Russia I examined the sands and gravels 
of the beds of the large rivers. These rivers are frozen over 
for several months every year, and it seemed likely that the 
conditions would be somewhat similar to those that are 
supposed to have existed in the valley of the Thames at 
the close of the Glacial period. Much of the sand and 
gravel from the rivers of Southern Russia was used for bal- 
lasting the railways, so that I had many opportunities of 
examining it. I found it in every case full of river-shells, 
