LAMPLUGH : NOTES ON THE WHITE CHALK OF YORKSHIRE. 189 
able for rapid denudation in the area, and streams were everywliere 
busy in carving out the intricate valley systems of the Wolds, amon^^- 
which we may reckon the Preglacial hollows revealed in the cliff- 
sections of Flamborough Head. Whatever rain falls on the W^olds 
now-a-days sinks at once through the pervious soil into the interstices 
of the porous Chalk, so that no running water exists in any of the 
valleys, except in a few instances where the valley bottom lies tem- 
porarily or permanently below the saturation-level '.n the Chalk. But 
during the erosion of these valleys the conditions were such as to 
produce an abundant flow at the surface. Not only must there have 
been a much heavier rainfall and snowfall (a " Pluvial Period," in 
fact), but also, probably, a permanently frozen subsoil which formed 
an impervious cover to the porous Chalk, and thus prevented the 
absorption of the surface waters, •'■ Such conditions appear to have 
been more or less continuous in the Wold district throughout the 
Glacial Period, and to have persisted into Post-Glacial times. 
There is clear evidence of an elevation of the land shortly before 
the deposition of the Glacial deposits. This may have occurred either 
before or after the accumulation of the Sewerby and Speeton Infra- 
Glacial marine beds, the data in hand being insufficient to determine 
the point, though the latter supposition seems the more probable. 
One proof of this elevation is that most of the valleys draining east- 
ward to the present coast are excavated considerably below the 
present sea-level. At Scarborough, for example, there is a drift-fided 
valley the bottom of which is 100 feet below mean-tide, t and at 
Bridlington the valley of the Gypsey Race goes considerably below 
sea-level. In a minor degree this is true also of certain of the 
buried hollows which trench the Chalk of Flamborough Head. This 
elevation probably continued throughout the earlier stages of the 
Glacial Period. It favoured the accumulation of that great ice-sheet 
which afterwards gradually filled up the basin of the North Sea until 
an icy plateau linked our islands to Scandinavia and Germany. 
The flank of this ice-sheet rose high against the coast-line, but 
* For further discussion of these conditions see "Glacial Sections near 
Bridlington," partiii., in Proc. Yorksh. Geol. and Poly tech. Soc. (1883), vol. 
viii., p. 251. 
+ Geol. Survey Mem. "Country south of Scarborough," p. 31. 
