JOWETT AND MUFF : GLACIATIOX OF BRADFORD, ETC. 221 
Thus the glacial deposits in Airedale are found abundantly 
in the main valley and the lower parts of the tributary valleys, 
but they do not generally extend to the heads of the tributary 
valleys. On the north side of the dale the glacier coalesced 
with that in Wharfedale, and the summit of Rumbles Moor 
alone projected above the surface of the ice. On the south 
side, where the hills attain to greater elevations, there was 
a larger unglaciated area. The upper limit of the distribution 
of the drift, though seldom marked by any moraine, has been 
traced from an altitude of nearly 1,400 feet on Combe Hill to 
one of 700 feet to the south of Bradford. The distance between 
these two localities being 12 miles, the average rate of fall of 
the surface of the glacier was about 60 feet per mile. 
VI. — Glacier-Lakes and their Overflow Channels. 
The results of the preceding section show that even at 
the period of maximum glaciation, the highest parts of the 
district were unoccupied by ice. Water draining off this more 
elevated land was unable to escape normally, as the main valley 
and the lower parts of the side valleys were full of ice, but it 
collected in the unoccupied heads of the side valleys, which 
were converted by the ice-barrier into basins. 
Thus a series of lakes was formed fringing the edge of the 
glacier. 
The surplus water from one of these lakes had to discharge 
either over, through crevasses in, or along the margin of the 
ice, or over the lowest part of the surrounding watershed. Under 
the first two conditions of drainage no permanent traces o^ 
such an overflow would be left after the glacier disappeared ; 
under the last two conditions, the water would cut a channel 
for itself, which might be identified when the lake had been 
drained by the melting away of the glacier. 
The valleys eroded by the escaping waters of the glacier 
lakes have the foUowdng characteristics* : — 
* Prof. Kendall in describing a magnificent series of overflow channels 
in Cleveland has given their characters in great detail, and classified them 
according to the relation they bear to the ice front. Q.J.G.S., vol. Iviii., 
1902, p. 471, and Proc. Yorks. Geol, and Polyt. Soc, vol. xv., 1903, p. 1. 
