246 JOWETT AND MUFF : GLACIATION OF BRADFORD, ETC. 
the main valley. The striae to the south of Shipley were un- 
doubtedly produced by ice moving from tlie north-north-west, 
and pushing into the mouth of the Bradford Valley. The 
strise near Cullingwortli were produced by ice moving from the 
north-west, and not by a glacier flowing down the Harden 
Valley from the hills above Denholme. 
The average rate of fall of the surface of the Airedale glacier 
as indicated by the upper limit of the drift was about 60 feet 
per mile. 
The glacier occupying the main valley of the Aire and the 
lower parts of its tj ibutary valleys obstructed the normal courses 
of the water flowing from the unglaciated area on the Pennine 
Hills, and impounded it in the upper parts of the tributary 
valleys. In this manner a chain of six lakes was formed fringing 
the south-western margin of the ice. The surplus waters 
discharging from each lake passed into the next lake to the 
south and east, whilst the discharge of the lowest (the Bradford 
Lake) escaped across the main watershed into the Spen Valley, 
and so into Calderdale. The outlets and surface-levels of each 
of these lakes are identifled by the channels which the discharging 
streams eroded. These channels exist as an anomalous series 
of dry valleys, which trench through the hills and are entirely 
independent of the natural drainage of the country. As the 
ice melted away the lakes were enabled to discharge at lower 
levels, and during the temporary halts and short re-advances 
of the ice, several series of overflow-channels were eroded. The 
lelative lengths and heights of the spurs separating the lakes 
occasioned sometimes the coalescence of two lakes previously 
distinct— sometimes the formation of a new lake. This irregu- 
larity of the topographical features, combined with the oscillatory 
retreat of the ice, has rendered the system of overflow-channels 
eroded during the retreat of the ice very complex and their 
correlation a matter of great difflculty. 
On the northern margin of the Airedale glacier a chain of 
lakes was in existence for some time during the shrinkage of 
the ice. 
Evidence for one period of glaciation only has been obtained. 
The yellow clay passing down into the blue boulder-clay can 
