THE GEOLOGY OF LONDON. 
1 
He believes that the sea depositing the boulder clay extended 
beyond the limits of England, although the denudation has 
been too complete to show any traces of the deposit on the 
southern side of the Thames Valley. 
When the area was elevated at the close of the glacial 
period, some minor traces of gravel were formed, according to 
Mr. Wood, amongst which may be included the thin deposit 
upon the Bagshot sand at Hampstead Heath, the traces on 
Shooter’s Hill, Langdon Hill, &c. 
There are, however, other and more important deposits of 
gravel, sand, and brick-earth, which belong to post-glacial 
times, which we have now to consider. Such are the deposits 
which cover the greater part of the city, and stretch irregularly 
out into the suburbs. They border the Thames at Hammer- 
smith, Chelsea, and stretch northwards to Highbury and 
Hackney. Eastwards, the pits at Ilford are, perhaps, the best 
known, while southwards the beds extend to Camberwell, and 
spread over Clapham and Wandsworth Commons. They are 
called the Thames Valley deposits, and they have been doubt- 
less accumulated by the river itself. When, however, we 
consider the extent of the old Thames Valley, as indicated bjt 
these gravels, and the comparatively small changes that now 
take place, we cannot but imagine that the denuding and 
transporting powers were vastly greater in former times than 
they now are. Professor Phillips* considers that we require 
the agency of shallow ice-rafts, formed in the valley, to move 
some of the large blocks which are found in the gravel. He 
considers that the region was subject to greater extremes of 
cold than now, with more abundant rain and snow — that there 
was, in fact, a pluvial period, as Mr. Alfred Tylor first suggested, 
after which the local climate has been gradually improving, 
and acquiring more of its insular mildness and comparative 
dryness, due in some measure to the operations of man in 
draining and in the cutting down of forests. 
Professor Ramsay is of opinion that the area of the Thames 
drainage has gradually contracted during tertiary and recent 
times. He points out how the escarpments, both of chalk and 
oolite, are slowly still changing and receding eastward, and as 
that of the oolite recedes the area of drainage will diminish, 
and the Thames decrease in volume.f 
Besides many species of land and freshwater mollusca, which 
are found in the brick-earths at Hackney Down, Stoke New- 
ington, and other places, numerous mammalian remains have 
* See “ Geology of Oxford and the Valley of the Thames.” 
t “Physical Geology and Geography of Great Britain,” 3rd Edition, 
p. 222. 
