1878. J 
355 
Superficial Gravels and Clays . 
had been subjected in pre-glacial times. There would be 
the patches of Bagshot sands, only small remnants of which 
are now left on some of the hills, and other tertiary sands 
and gravels where they came to the surface. To these local 
materials would be added all those brought by floating ice 
before the first lake was discharged, and constituting the 
Lower Boulder-clay. 
These materials would be caught up by the mighty flood, 
swept away, mingled together, and ultimately spread out 
over the lower lands, when the violence of the debacle 
abated. If it had not been checked it would have probably 
carried all its spoil into the sea. The lessening of the fury 
of the flood, when it reached a level of about 200 feet — and 
still more below 100 feet — above the sea, must have been 
due to the outlet of the water being contracted or obstructed. 
The channel across Languedoc to the Mediterranean may 
not have been broad enough for the rapid discharge of the 
great lake below 200 feet above the sea. Whatever was the 
cause, the evidence shows that the violence of the flood was 
checked, though not entirely stopped, and the materials that 
had been swept off from the upper slopes were spread out 
over the lower ground. 
At Finchley we find the Middle Sands and Gravels, and 
even patches of the Lower Boulder-clay, at heights from 
which they are entirely absent on the flanks of the hills 
bordering the Thames Valley, as, for instance, the southern 
slopes of Harrow and Hampstead Hills. The preservation 
of these deposits at Finchley is due to the faCt that to the 
north and north-west there is a ridge of high land rising to 
over 400 feet above the sea, which would protect them from 
the violence of the flood when it was moving from the 
northward ; and when the water was lowered to below 
400 feet the current would be from the westward, and 
Finchley would be completely sheltered from it by the high 
chalk ridge running south-westward from Tring. The 
southern slopes of Harrow and Hampstead would, on the 
other hand, be exposed to the full violence of the torrent 
rushing down the Thames Valley, and if, as is likely, gravels 
had been deposited on them whilst the current was from the 
north, they must have been utterly washed away later on 
when it was from the west. 
Below 200 feet above the sea, when the violence of the 
flood was somewhat checked, the outspread of the materials 
it was hurrying along began ; at first in sheltered places ; 
then, as the water was still further lowered and its velocity 
still more lessened, more generally and continuously. 
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