228 JOWETT AND MUFF : GLACIATION OF BRADFORD, ETC. 
side of the col. Two dry channels cut through the col at about 
the same level. The southern one is a deep and well-marked 
notch, with a broad floor sloping to the east, and covered with 
peat (see PL XIX., Fig. 1). Its intake level is slightly higher 
(1,130 feet) than the parallel channel to the north (1,120 feet). 
These two valleys belong to Prof. Kendall's type of " direct 
overflows." Of these he remarks, " They generally occur singly, 
one overflow^ serving for the drainage of a considerable area, 
but when the watershed is uniform in height, and the ice has at 
one stage actually surmounted it, then several parallel gutters 
may be trenched on the outer slope by the water floAving from 
the ice itself."* No surprise need therefore be felt on account 
of there being two channels at nearly the same height. They 
continued to operate whilst the Worth Lake sank to a still 
low^er level. This will be referred to again (p. 238). 
The highest overflow channel in the district is situated 
on the south-west shoulder of Combe Hill, and is known as 
the Great Nick. This dry gap carried off the water from a lake 
on the north side of Combe Hill and from the ice itself. Its 
intake level is 1,325 feet above O.D., and it is nearly 25 feet 
deep on the watershed. The valley terminates suddenly on 
the hiU side at its lower end, and a gently sloping fan of detritus 
is traceable under the heather at its foot. The level is about 
1,250 feet — that of the Worth Lake at the time when its dis- 
charge poured through the gap at Harbour Hole. 
Thus at the period of maximum glaciation there stretched 
along the southern border of the Airedale glacier a series of 
six lakes, the surface levels of which fell from about 1,325 feet 
in the north-west to about 700 feet in the Bradford basin. The 
overflowing waters from these lakes discharged into the head 
of the Spen Valley, and so into Calderdale. The Spen Valley, 
as compared with the other valleys opening on the left bank 
of the Calder is marked by a relatively broad and continuous 
strip of alluvium, which stretches from its mouth almost up 
to its head. This suggests that the volume of the stream was 
formerly greater, and that as its volume diminished the stream 
aggraded its bed. 
* Proc. Yorks. Geol. and Polyt. Soc, vol. xv., p. 9, 1903. 
