JOWETT AND MUFF : GLACIATION OF BRADFORD, ETC. 225 
A short distance below the lower end of Chellow 
Dean there is a thick deposit of stratified gravel, com- 
posed chiefly of small pebbles of sandy micaceous shale, 
very like that which forms the sides of the Dean. The 
pebbles are mostly subangular or rounded, but angular fragments 
are not uncommon. Small flaky bits of black shale, gannister, 
and a few rounded pebbles of grit also occur. The lines of 
bedding in the gravel have a slight dip away from the mouth 
of Chellow Dean. A small exposure of this gravel is seen about 
400 yards below the point where Duckworth Lane crosses the 
valley. In the next hollow to the west yellowish sand onl}^ is 
to be seen. The deposit has a gently-inclined surface and 
appears to end off southwards rather abruptly in a steep bank 
at an altitude of 550-525 feet above O.D. Its altitude corre- 
sponds with that of a dry gap which gashes the eastern side 
of the Bradford basin at Laisterdyke. 
It is believed that Chellow Dean was cut out by a large 
stream discharging from a lake held up in the Cottingley Valley 
by the Airedale glacier, and that this stream deposited the 
gravel delta at its lower end, on entering a lake held up by 
the same glacier in the Bradford basin ; at the same time the 
latter lake had its outlet at Laisterdyke. 
It must be observed that Chellow Dean lies for part of its 
length along a line of fault, as indicated on the Geological Survey 
Map, but the theory that it is merely a hollow produced by 
ordinary sub-aerial weathering along the fault is not tenable, 
because it does not account for that portion of the ravine which 
is not coincident with the fault. As, however, Chellow Dean 
and the fault are coincident where they cross the summit of 
the ridge, it is very probable that a slight original depression 
in the watershed on the line of the fault determined the place 
where the Cottingley Lake was first able to find an outlet. 
THE GLACIER LAKES AT THE PERIOD OF MAXIMUM GLACIATIOX. 
The lakes which fringed the glacier at the time of its greatest 
extension -wall be described first. 
The Bradford Lake held up by the ice-front which extended 
across the valley from Allerton by Leventhorpe and Clayton to 
