180 LAMPLUGH : NOTES ON THE WHITE CHALK OF YORKSHIRE. 
feature, so that its true course from Bempton eastward is somewhat 
problematic. But the clift-sections from Thornwick eastward clearly 
indicate the existence of the buried valle}^ invariably showing the 
solid rock to be higher above the sea on the headlands than in 
the recesses. In the deeper bays the surface of the chalk is fre- 
quently only slightly above high-water mark, the remainder of the 
cliff being composed of clayey or gravelly drift. One result of this is 
that when the sea has once managed to break through some weak 
place in the outer part of the valley- wall, the rock being lower and 
more shattered by ancient weathering within, gives way at an 
accelerated pace until the gash across the drift-filled hollow becomes 
deep enough to afford some shelter to the inner walls of the recess. 
The succession of ridge and inlet, so well shown in plates 4 and 5 
illustrates the structure. In the second of these plates Great 
Thornwick lies just below us, divided by a narrow barrier from 
Little Thornwick, beyond which, across a slightly broader ridge, 
is Chatterthrow. The dividing ridges have a heavy capping of drift, 
consisting chiefly of tough boulder clay, though with many inter- 
bedded patches of gravel and silt or sand. Where narrow^ they 
weather down from both sides into picturesque aretes. Fragments 
of marine shells occur rather plentifully in this drift, but seem to have 
been introduced with the other pebbles, and are not a trustworthy 
indication that the deposits are marine." 
There is a feature in the nearer ridge which affords an excellent 
small-scale illustration of what often occurs on a large scale in 
mountainous regions. It is the existence of a short dry valley or 
col crossing the ridge, to be observed near the left-hand margin of 
the picture. This col is the abandoned channel of a little stream 
which trickled down into this corner of Great Thornwdck, until inter- 
cepted by the wearing back of Little Thornwick, where it now 
discharges itself to the sea. Marine erosion has in this instance done 
what in hilly districts is usually done by another stream, and by 
tapping the valley has diverted the drainage and left the dry channel. 
6. Great Thornwick. 
The chief features of this view, showing the eastern side of 
* Further information with regard to these sections may be obtained in 
Quart. Journ. Geol. Soc, vol. xlvii., p. 403. 
