188 
HUGHES : INGLEBOROUGH. 
We then come to Thornton Force, which, from its beauty 
and geological interest, has been as often sketched, photographed, 
measured, and written about as any place in Yorkshire, and is 
one of the features impressed most vividly upon the memory of 
all who have visited these varied and fascinating bits of mountain 
river scenery. I have quoted above (pp. 178-181) what Playfair, 
Phillips, and Sedgwick said about it. The basement conglomerate 
is rather thicker here. Perhaps, indeed, the hollow in which 
this debris was accumulated determined the position of the 
joints and bedding planes in the limestone which caused the 
subterranean waters to be concentrated here. However that 
may be, we see most of the stream, in fair weather, when the 
water is low, finding its way through the deep-jointed rock, but 
in flood pouring over the rim of horizontal limestone. This 
limestone, with small pebbles occurring sporadically throughout, 
rests with a sharp line of demarcation on about five feet of grey 
conglomerate made up almost entirely of angular and sub- 
angular fragments of the underlying schists. For the lowest two 
feet it is very coarse, often containing blocks up to two feet in 
length. 
Following the base of the Mountain Limestone down the 
east side of the gorge to nearly due west of T\\dsleton Hall, we 
find a spring, from the water of which a large quantity of traver- 
tine has been thrown down and is being still precipitated, proving 
that the subterranean waste of the limestone is going on apace and 
showing how the caves are rapidly formed wherever the water can 
find an outfall near the base. A spring marks the base south 
of Twisleton Hall, and again S.W. of Dale Barn, in Chapel-le-dale, 
but the outcrop is obscured by drift until we get to a point W.S.W. 
of Dale Barn, where there is an exposure of dark grey limestone 
with, quartz pebbles, while due north of the barn there is a junction 
section, in which there is an irregularly horizontal bed of breccia 
about four feet thick, made up of the schists and resting on the 
crushed edges of the green slates, and a little further on, in an 
old quarry N.W. of the sheepfold, the Mountain Limestone is 
seen with a breccia at its base resting on the green slates. 
Near Twisleton Dale House some thick, folded beds of a hard 
coarse quartzose grit in the Green Slate Series resisted denudation 
more than the softer slates, and gave rise to a pre-Carboniferous 
