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The boulder clay is from ten to eighty feet thick, but never 
exceeding- this last thickness. Again, this same gravel 
extends across the Humber, on the sea coast of Norfolk and 
Suffolk, to the mouth of the Thames, at Margate. 
When examined carefully, this diluvium consists of an 
upper and lower deposit. The lower consists of clay, with 
boulders scratched underneath on their longer axes. The 
upper, of fragments rarely scratched, and having much false- 
bedding, indicating rapid currents, and carried by water of 
no great depth. Marine shells, when found, are in a broken 
state, and lie above the beds which contain the bones of 
Mammalia, and the limestone pebbles are in no case per- 
forated by boring lithodomi. 
The Geological date of the Cumberland drift is ascer- 
tained as follows : — At Cromer, in Norfolk, this drift reposes 
upon a subterranean forest of trees. It also covers up the 
remains of mammalia, as at Kirkdale, &c. ; hence this trans- 
port was made over pre-existing dry land, upon which plants 
were growing and animals dwelling. Again, there are large 
accumulations of this drift in valleys, proving the latter were 
excavated previously to its deposition. The scratching in 
the lower boulders upon the longer axes, and their accom- 
panying clay deposit, is ascribed to the rubbing action of 
icebergs, or of glaciers upon the parent rock ; while the 
absence of these marks in the upper beds, together with 
their irregular and mixed up stratification, indicate the 
deposit being accumulated under violent aqueous action in 
shallow water. 
The conclusions respecting the Cumberland drift arrived at 
by Cumming, Trimmer, &c., who have studied its pheno- 
mena under all its phases, are — 
1st. At the commencement of the period of the boulder 
clay formation, the relative level of the sea and land in the 
British Isles was not greatly different from what it is now. 
