222 JOWETT AND MUFF : GLACIATION OF BRADFORD, ETC. 
i. — Steeply sloping parallel sides, the parallelism being 
strictly preserved where the valley curves. 
ii. — At each curve the concave bank is more precipitous 
than the convex, as is the case with the banks 
of a river, thus indicating that these valleys 
formed the actual channel in which the water 
ran. 
iii. — The valleys' floors are generally broad and flat. In 
Airedale they all slope to the south or east. 
iv. — The valleys are either streamless or possess very in- 
significant streams. They generally cut com- 
pletely through the watershed, quite irrespective 
of geological structure, the great square-cut notches 
being very striking in appearance when seen 
from a distance. 
Some small streamless valleys occur which begin quite 
on the watershed. These may have been produced by water 
escaping from the ice-front at a time when it just reached but 
could not cross the watershed. 
It will be convenient at this point to consider a good example 
of one of these overflow channels in some detail. 
The ridge which runs from Harrop Edge (1,000 feet) to 
Stony Ridge, near Shipley, and separates the Cottingley valley 
from the Bradford basin, is trenched across by a deep ravine 
called Chellow Dean (see Pis. XVI. and XVII.). The floor of the 
Dean is generally 30 to 40 yards across, and slopes to the south- 
east towards the Bradford valley. Its walls, which reach a height 
of 100 feet, are very steep, and are formed of the sandy micaceous 
fihales and sandstones of the Lower Coal Measures. The tiny 
trickle of water, which runs through the Dean, is maintained 
by some springs rising on the hill-side beyond the head of the 
Dean and in the Cottingley Valley, but it is obvious this tiny 
stream could never have cut out Chellow Dean, whose characters 
indicate rapid erosion by a large volume of water. The striking 
peculiarity about the Dean is the way in which it suddenly 
opens out at its upper end into the Cottingley Valley. Here 
its floor is hardly 20 feet above the Cottingley Beck, where 
the latter flows past the entrance to the gorge. 
