376 SPENCER : THE YOREDALE AND MILLSTONE GRIT ROCKS. 
stream from Dean Head on the south, and the combined streams 
then flow in an easterh^ direction, by Hebden and Sowerby Bridges, 
to Elland. The valley of the Calder divides the whole area into 
two halves, and, being the lowest part, it drains the whole district, 
receiving from the north its tributary streams, the Calder at 
Mytholm, the Hebden and its affluent from Horse Bridge Clough 
at Hebden Bridge, the stream from Luddenden valley at Ludden- 
denfoot, and the Hebble from Halifax, while the streams from 
St. John's Valley, the Ryburn, and the Black water join it from 
the south. 
The Millstone Grit rocks form three natural groups, named 
after the three principal rocks, viz., (1.) Kinder Grits, (11.) the 
Third Grits, and (III.) the Rough Rock. 
I. and III. have the greatest range and most uniform thick- 
ness, while the Third or middle grits are the most variable, both in 
thickness and range. In fact, according to the Geological 
Surveyors, the Third Grits form a series of lenticular beds in the 
great mass of Shales which divide the Kinder Grit from the Rough 
Rock.^ 
The Kinder Grit forms the subsoil of the high and extensive 
moorlands forming the boundary between the counties of Yorkshire 
and Lancashire, and reaches the height of 1,550 feet on Blackstone 
Edge and on Black Hambledon. It has suffered denudation on 
a vast scale, areas of many miles square having been swept away, 
and the underlying Yoredale rocks exposed. 
Before proceeding further with the description of the Kinder^ 
it will perhaps be advisable to include a brief sketch of these 
Yoredale beds. In the north, at Widdop, is an oval basin about 
miles long by a mile broad, where the Yoredale shales are 
exposed. This basin is surrounded by bold perpendicular escarp- 
ments of Kinder Grit a hundred feet or more in height, and 
is composed of the grandest masses of conglomerate we have in 
the district. 
Memoirs, Geo. Survey, Burnley Coal Field. 
