256 UNDERGROUND WATERS OF NORTH-WEST YORKSHIRE. 
Tlie Millstone Grit is a porous sandstone, and rain falling 
upon its surface is readily absorbed and percolates through 
its mass until the underlying shales are reached. Here its 
downward progress is arrested and it breaks out as a series 
of springs, the waters of which course down the steep slopes 
of the Yoredale Rocks until they reach the great jDlateau of 
the Mountain Limestone. 
This limestone is hard, crystalline, and quite impervious 
to water in the ordinary sense, but. owing to the fact that it 
is highly jointed, all water flowing on to its surface is at once 
absorbed into the cracks and fissures. 
The manner in which the joint fissures are enlarged by 
the solvent action of the rain water so as to form a series of 
fissures will be better understood by a reference to PL XXXVI., 
Figs. 1 and 2. 
Fig. -A. 
riON THROUGH MOl'caiTON" FKO.M X. TO S., TO SHOW RID(;ES ON THE SILURIAN FLOOR. 
1. Oi'dovician and Silurian. 2. Carbonifei cms Limestone. 
The Ordovician and Silurian rocks consist for the most 
part of hard grits and slates which are practically impervious, 
the result being that the waters which flow into the Mountain 
Limestone on its upper surface are arrested in their downward 
course, and issue as sj^ rings either upon or immediately above 
the line of junction between the limestone and the underlying 
impervious rocks. 
It must be noticed in passing that the surface of older rocks 
on which the limestone rests is by no means horizontal, but 
consists of a series of ridges with intervening vaUeys having 
a general X.W. to S.E. trend {Fig. 3 and PI. XL.). 
These ridges beneath tlie limestone have, as the sequel 
will show, an important effect upon the direction of flow of 
tlie subterranean streams. 
I 
