340 
Superficial Gravels and Clays. 
IJuiy, 
of the distribution of the northern drift in Russia, shows 
that hills and slopes 200 or 300 feet high are often covered 
with large blocks, whilst the valleys between them, often 
wide, are free from them ; and again, that the northern rocks 
are scattered plentifully over the plateaux, and rare on the 
low plains, though not entirely absent.* Prof. James Hall 
has described similar faCts with regard to the distribution of 
the northern drift in America. t 
As the transported stones are most abundant at Finchley 
at a height of about 300 feet above the sea, we must suppose 
— if they were brought by floating ice — that the water at the 
time stood sufficiently above that level to permit of the 
flotation of the ice, and at the same time not too high to 
prevent it stranding on the ridge. The most of the stones 
have been brought from the north, from districts where we 
know there were glaciers, from the marks they have left 
behind them ; and it is probable, therefore, that the carriers 
of the transported stones were icebergs. If it had been 
shore ice that brought them we might have expected to find 
a preponderance of local stones from the hills in the vicinity, 
instead of which there is a great abundance of far-transported 
materials. 
For icebergs, a depth of from 50 to 100 feet at least would 
be required for flotation, giving about 400 feet above the sea 
as a probable depth of water at the time they stranded on 
the hills at Finchley. That this really was about the height 
of the water at that time is probable, for if we colour a map 
of the drainage area of the Thames, so as to show the ground 
that would be then submerged, we shall find that we have 
opened avenues from the north-east connecting the Finchley 
deposits with wide areas in that direction that are covered 
with the same boulder-clay. 
The Upper Boulder-clay overlies at Finchley, as it does 
farther north, sands and gravels with much oblique stratifi- 
cation, generally known amongst geologists as the Middle 
Glacial Sands and Gravels. The gravels contain many 
stones not found in place in the present drainage area. 
Patches of a boulder-clay, known as the Lower Boulder-clay, 
are found beneath them, from which these foreign materials 
seem to have been derived. We are thus led to conclude 
that there was an earlier submergence, during which the 
Lower Boulder-clay was spread out, and that afterwards it 
was subjected to some aCtion by which it was greatly denuded 
* The Geology of Russia in Europe, pp. 510, 513, and 519. 
t Natural History of New York, Part 4, p. 321. 
