232 JOWETT AND MUFF : GLACIATION OF BRADFORD, ETC. 
was opened and the Harden Lake at a level of 855 feet discharged 
into the Cottingley Lake. The overflow of the latter escaped 
over the col at Chellow Dean at about 800 feet and began to cut 
out that gorge. Before, however, it had cut down to below 
780 feet, the ice again retreated. This lowered the Harden 
Lake to 840 feet by opening the Salter Royd channel, and allowed 
an escape to the north of Chellow Heights for the overflow of 
the Cottingley Lake. Thus the Xailer Rough channel was cut, 
and the level of the Cottingley Lake sank to 770 feet. An 
advance of the ice-front, which now took place, closed the Nailer 
Rough channel and turned the overflow of the Cottingley Lake 
through Chellow Dean again. This time it was cut down from 
780 feet to 720 feet. The above advance required to close the 
Nailer Rough channel need only be a very slight one, on account 
of the position of the head of the channel facing up the valley 
of the Aire whence the ice was coming (see PL XXL). The 
ice simply pushed up the Cottingley Valley, but did not close 
the Salter Royd chaimel, which was operating as the outlet of 
the Harden Lake. However, it pressed in against the lower end 
of this channel and caused the stream to flow towards the head 
of the Cottingley Valley close in against the hillside. The 
stream cut into the hillside and produced the strongly-marked 
scarp and shallow valley which runs from the Salter Royd gap 
obliquely across the contours towards Chellow Dean (see 
PI. XVIII. , Fig. 2). This scarp and valley end in a flat at 
720 feet — the level of the intake of Chellow Dean. 
Had the ice in this oscillation advanced sufficiently far to 
close the Salter Royd channel as well as the Nailer Rough one, 
the Swain Royd channel would have been brought into operation 
again and would have been cut down to 720 feet. But it is not 
so cut down ; it distinctly terminates on the slope of the hill at 
780 feet. This is the level to which Chellow Dean was cut down 
just before the ice retreated to open the Nailer Rough channel. 
After this advance, during which Chellow Dean was cut 
down to 720 feet, the ice retreated further than it had done 
before. The Nailer Rough channel was now too high to operate 
as the outlet of the Cottingley Lake. The channel commencing 
at Noon Nick (695 feet) was cut, whilst the ice-margin must 
