106 
this blue till and the blue till inside the Bradford basin, will 
be readily discerned on a closer inspection. In the first place, 
it does not efiervesce when tested by acid ; and in the second 
place, it does not contain a single pebble of carboniferous 
limestone or limestone shales. 
The debris found in this till is all from the millstone grit 
series and the lower coal measures ; the shales, calliards, iron- 
stone nodules, and bits of coal being well polished and 
scratched. The contained stones all point north of the Aire, 
between Shipley and Skipton, as the locality whence they 
were obtained. A similar bed of blue clay has lately been 
bared for the Shipley Waterworks reservoir at the head of 
Eldwick Beck, nestling below the edge of Bingley Moor at 
an elevation of 750 feet. The^ section is about 600 feet in 
length and 40 feet in depth. This bed is identical in every par- 
ticular with the one at Thornbury, save that its upper portion 
is not characterised by large boulders; and the brown clay iron- 
stone nodules, so plentiful in the Thornbury clay pit, are entirely 
wanting. Could we determine the locality of the parent rock 
whence these nodules were derived, it would aid us materially 
in mapping the direction taken by the ice-stream conveying 
them ; but upon this point no satisfactory evidence is as yet 
forthcoming : probably the Halifax shales to the west of Hope 
Hill supplied these as well as other Jportions of the contents. 
If this glacial mud had been formed by the grinding action 
of an ice- sheet upon the surface of limestone rocks, it would 
have been highly charged with carbonate of lime, as is the 
Bradford till. This, and the entire absence of limestone 
pebbles, so abundant in the till within the basin, at an equal 
altitude with Thornbury, would seem to indicate that the sec- 
tion of the ice-sheet under which this till accumulated had 
not traversed the district west of Cononley. Is it possible 
that the ice- stream, already described as flowing down Aire- 
dale, was joined by an overflow from the Wharf edale stream, 
