YORKSHIRE PRIFTS. 
291 
and clay ; then the strata deposited above were laid horizontally and 
evenly above it. In other instances, the sands and clays appear to 
hOiVe been pushed or folded by a lateral force ; and these may be due 
to icebergs borne by the tide and left stranded. Mr. .Sorby considers 
that at this time the chalk wolds were above the sea, and if so, there 
must have been a kind of bay at Bridlington somewhat similar to the 
present. He very carefully investigated the directions of the currents 
in this bay at the period of the drift, as shown by the ripple marks 
and other evidences, and was led to conclude that a tolerably strong 
current swept round its shores, and that in the centre it was more 
tranquil, and had a returning eddy. Comparing the directions of the 
currents with these contortions, he found that the side on which the 
pushing force acted in the different parts of the ancient bay agrees 
with that which would result from the drifting of an iceberg in the 
line of the current which prevailed. As the contortions are underlaid 
and surmounted by undisturbed beds, the time when they were formed 
is fixed within narrow limits. They occur throughout the whole series, 
and whatever produced them acted during the deposition of every 
part. If, therefore, it is requisite to consider them to be due to the 
action of icebergs, it leads to the important theoretical conclusion 
that they were present during the whole drift period, and not, as has 
been supposed by some distinguished geologists, merely at its close. 
In the following year the Rev. W. Thorp contributed a paper to 
the proceedings of this Society on the Diluvial and Gravel Beds of 
Yorkshire and Nottinghamshire, in which he took much the same 
line of argument as Mr. Clay had done some years previously. The 
diluvium he stated to consist of an upper and lower deposit, the lower, 
consisting of clay with boulders scratched underneath, on their long 
axes, and the upper, of fragments rarely scratched and having much 
false bedding, indicating rapid currents in water having no great 
depth. Marine shells, when found, are in a broken state, and lie 
above the beds containing the bones of mammalia, and the limestone 
pebbles are in no case perforated by boring lithodomi. In Norfolk 
the drift reposes upon the forest bed ; it also covers up the remains 
of mammalia at Kirkdale Cave ; hence it was deposited over pre- 
existing dry land, upon which plants were growing and animals 
