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18789 Superficial Gravels and Clays. 
Bosphorus that was thus formed, and not the Dardanelles. 
The deposits that were spread out during the continuance 
of the second lake were not subjected to the action of any 
sudden and tumultuous outpouring of the water, but only 
to its gradual subsidence. They have been but little de- 
nuded, and are still to be found, nearly everywhere over the 
area that was submerged. 
During the rise, greatest extension, and subsidence of the 
water, sediments, varying greatly in character, were formed. 
The earliest of all in the Ealing district — the first sign of 
the second rise of the flood — is the bed of silt enveloping 
the stumps of the small trees of the buried forest. The 
similar silty clays at Brentford and elsewhere, with land 
shells, mark the same event. There are, in the alternations 
of gravel and sandy clay in some of the sections at Ealing, 
signs of small oscillations of the level of the water, during 
which the subangular gravels were partly denuded, and fur- 
nished some of the materials for the new deposits. Above 
this we have a bed of clay, showing a greater depth of 
water, and stones scattered through it, indicating the agency 
of floating ice. 
In addition to the single pebbles, and sometimes larger 
stones, there are patches of gravel and sand that appear 
to have been dropped in a frozen state into the bed of clay 
as it was forming. They are often angular, and show the 
lines of their original stratification, now lying at all angles 
or turned completely on end. They occur still more fre- 
quently in the Upper Boulder-clay of Norfolk and Suffolk 
than in the clay of the Thames Valley, and form one of the 
many evidences of the identity of the two deposits. The 
most probable explanation of their presence in the clay is, 
that the rising of the second lake took place wholly or 
partly in the winter season, and during an extreme frost ; 
that ice was continually forming along the ever-widening 
shore, and, being broken up and floated off by the rising 
water, bearing masses of frozen gravel and sand with it ; 
that thus a wide area of the lake was covered with gravel- 
and sand-laden ice, and that, on the melting of the latter, 
the frozen masses fell to the bottom and were imbedded in 
the clay. 
Very little detritus that can be traced to a northern source 
has been found south of the Thames, and this has led some 
geologists to hold the opinion that the country south of the 
river was not submerged when the Upper Boulder-clay was 
spread out. It is difficult to believe that this can have 
been the case, when we find the drift covering the northern 
