JOWETT AND MUFF : GLACIATION OF BRADFORD, ETC. 231 
This is caused by the valley cutting down into a bed of shale, 
whilst previously it was in sandstone. The sudden descent 
marks the position of an old waterfall. 
Two dry valleys run along the outer face of the Stony 
Ridge escarpment from Shipley Moor Head. Recent excava- 
tions show that they are cut in part in shale. The higher one 
seems to have a double intake, the levels of which are 480 feet 
and 440 feet. The intake of the lower one is at 430 feet. 
The ridge bounding the Cottingley Valley on the west is 
notched by four overflow-channels, which taken in descending 
order are as follows : — (1) Gaisby Hall, about 875 feet ; (2) 
Swain Royd, 855 feet ; (3) Salter Royd, 840 feet ; and (4) 
Coplowe Hall, 660 feet. 
The first three channels are streamless and comparatively 
shallow winding valleys, whose floors slope steeply in the direction 
of the Cottingley Valley (see PI. XX.). The Coplowe Hall 
channel is a wide, deep channel with a broad floor. 
The succession of events in the Cottingley Valley during 
the shrinkage of the ice was probably as follows : — 
When the Harden Lake was discharging by Bell Dean into 
the Bradford basin, and before it had cut down to its lowest 
point, a slight retreat of the ice-front opened the Gaisby Hall 
channel. This gap just cuts through the 875-foot contour, and 
terminates at its lower end just below the 850-foot contour. 
It is obvious from the conformation of the ground that when 
the glacier was standing up to 875 feet on the ridge between 
the Cottingley and Harden Valleys it would stand at nearly 
the same height against the hillside at the head of the valley, 
owing to its opening broadly almost in the direction whence the 
glacier was coming (see PI. XXI.). Instead therefore of forming 
a lake in the Cottingley Valley, the stream from the Gaisby 
HaU gap flowed between the glacier and the hillside, forming the 
terrace and shallow valley near Prune Park, and then ran over 
the watershed, where Chellow Dean was cut out at a later period, 
and so into the Bradford Lake. 
A shght advance then closed the Gaisby Hall channel, and 
the Harden Lake discharged again for a short period by Bell 
Dean. On retreat again taking place the Swain Royd channel 
