184 LAMPLUGH : NOTES ON THE WHITE CHALK OF YORKSHIRE. 
on the slope of the diff, lookiug south-eastward. The Chalk forming 
the prominent cliff comprises the lowermost beds of the Flintless 
Upper Chalk, with the top layers of the Cbalk-with-Flints seen only 
below high-water mark. Above the Chalk there is a considerable 
thickness of glacial drift, and it is in the relation of this to the solid 
rocks that the chief interest of the section lies. A ravine existed in 
this locality in pre-glacial times which has been filled in and obliterated 
by drift-deposits, and this valley with its infilling is intersected by the 
cliff (see key-plate fig. C). A little cave which the sea has excavated 
in the Chalk near the eastern margin of this valley has penetrated the 
rock and reached the less coherent drift material lying within, and 
has rapidly abstracted it through the orifice until at length the whole 
roof of the inner portion of the cave has fallen in. producing the 
crater-like hollow seen in the picture. When I first knew this place 
the cave was smaller than it is now, and whenever the sea w^as rough 
the waves at high-tide choked up the passage to the crater, causing 
the imprisoned air frequently to eject a vertical column of spray to a 
considerable height, like a great whale blowing. Of recent years, 
with the enlargement of the cavern, this phenomenon has been of less 
frequent occurrence. In the old 6-inch Ordnance Map a continuous 
ridge is shown over the site of the blow-hole, so that it would appear 
that the roof of cave has fallen in within the last 45 years. 
Another larger hole of similar character, known as Pigeon Hole, 
occurs in the same valley-course, 400 yards to the northward of High 
Stacks, near the Lighthouse. 
The drift deposits filling the depression are of varied character" 
(see key-plate, fig. C). Between an Upper and a Lower Boulder-clay 
forming the top and bottom of the series occur stratified gravels, with 
a thin band of interbedded Boulder-clay. A marked peculiarity of 
these gravels is that wdiile the upper portion is largely made up of 
chalk-fragments the lower part contains a medley of distant rocks, 
but scarcely a trace of Chalk ; and the line of division between the two 
gravels is quite sharp and distinct. There are several other instances 
of the same peculiarity, in the drift sections on the southern side of 
* For full details see J. R. Dakyns. Proc. Yorksh. Geol. and Polyt. Soc, 
vol. vii. (1879), p. 126 and p- 250. Also Lamplugh " Drifts of Flamborough 
Head," op. cit., p. 401. 
