114 COLE: PHYSICAL GEOGKAPHY OF THE EAST RIDING. 
thousands of g-uillemots and razorbills, whilst pigeons, jackdaws, 
kittiwakes and puffins find here also a congenial home for the 
rearing of their young. 
As a contrast to this may be mentioned the curious sandy 
promontory of Spurn, some 4 miles long, and 200 yards wide, at the 
mouth of the Humber, raised only a few feet above the sea level, 
which, though once supposed comparatively devoid of life, has proved 
of extreme interest both to the botanist and zoologist. 
Here, too, on the Yorkshire Wolds, you meet with the narrow 
ramifying dales, extending for miles, with their steep sloping sides, 
and level grassy bottoms, a sight unique in England. Coloured on 
a map they look like branches of a tree. No water flows in them, 
either above or below the surface, yet in all probability their 
excavation is due to the rainfall, acting partly in lines of faults, and 
partly in fissures, caused by the drying and contraction of the Chalk 
when elevated above the sea. It is well-known that the rainfall acts 
with more denuding effect on the bottom and sides of a valley than 
on a level surface, and consequently that valleys deepen faster than 
a level surface is lowered ; hence the inequality of surface is slowly, 
but continually on the increase. At present the top of some of the 
dale sides is 200 feet above the bottom. 
The high land out of which these are carved attains a height of 800 
feet. This elevation is i-eached at Garrowby Hill top. Eastwards, 
towards the sea, the elevation drops to about 400ft. at Bempton, 
whilst southwards, towards the Humber, the top of the chalk quarry 
at Hessle is little more than 80ft. above sea level. The Wolds 
before the Inclosure Acts, some 90 years ago, were the resort of 
numerous rare birds, notably the Great Bustard which is now 
extinct in England. 
As may be supposed the Wolds form a watershed, or water- 
parting, between the Vale of Pickering, the Vale of York, and 
Holderness, but the rain which falls upon them re-appears under 
different conditions. Chalk is very porous, and holds water somewhat 
like a sponge ; so much so, that the line of saturation underground 
actually follows the contours of the hills. This has been proved by 
Mr. J. R. Mortimer by a careful measurement of the wells in dry 
