GILLIGAN : ALLUVIAL DEPOSITS AT WOODLESFORD, ETC. 263 
of organisms and concretions, but beyond the rootlets mentioned 
above nothing was found. 
Looking at the 1-inch Geological Map it is seen that Oulton Beck 
meanders across this deposit of alluvium from west-north-west to 
east -south-east, and it occurred to me that this beck may have iu some 
measure contributed the material which is now found here. The 
deposits are undoubtedly of deltaic origin, laid down in a lake or pool, 
and are not such as are ordinarily formed by rivers in such a part of 
their course. The lacustrine conditions may have been due to the 
blocking of the mouth of the Humber by the North Sea ice at one 
stage of the last Glacial period. This ice held up a great lake in the 
central valley of Yorkshire. As Professor Kendall* points out, the 
presence of finely laminated muds is indicative of lacustrine conditions, 
and such clays were present in the great system of lakes which he 
proved to have formerly existed during the Glacial period on the 
northern flanks of the Cleveland Hills and in the Vale of Pickering, 
which latter in Glacial times was a lake receiving practically all the 
drainage of the Cleveland area. In the Vale of York itself, these 
laminated muds or clays also cover a very large area, and are found to 
have exactly the same characters as these at Woodlesford. 
How are we to interpret the presence of the two clays with the 
intervening sands and gravels ? The phenomena must mean either 
that the level of the lake was lowered after the formation of the first 
clay and that in consequence the rivers could sweep out their coarser 
material further than previously ; or, that there was an increase in 
the carrying power of the water caused by uphft of the source or an 
increased volume of water. The irregular surface of the lower clay 
points to the former explanation, and to the river or stream having 
meandered over its surface after its formation. 
As stated above, Oulton Beck suggests itself as a hkely vehicle 
for the conveyance of some of the material in this delta, and support 
is given to this view by the fact that it is seen to cut through or rather 
pass between two very large patches of sand and gravel capped by 
laminated clay and boulder clay. These occur at a height of 175 to 
250 ft., capping the high ground which forms the spur between the 
* P. F. Kendall, Proc. Yorks. GeoL Soc. Vol. XV. pt. 1, pp. 38-39. 
