198 JOWETT AND MUFF : GLACIATION OF BRADFORD, ETC. 
i. — THE BOULDER-CLAY OR TILL. 
The boulder-clay is a tough, bluish, unstratified clay 
containing numerous striated and moulded stones lying in 
all positions, and scattered irregularly through the clay. 
Its surface is smooth or slightly undulating, and the minor 
rock features are more or less hidden and levelled over 
by it. Though it occasionally forms a rude terrace-like 
feature, boulder-clay is rarely arranged into drumlins in 
this district, as it is in the open country to the north-west 
of Skipton. A few examples, however, have been noted, 
the best of which occur in the transverse valley east of 
Rumbles Moor, between Burley Wood Head and Guiseley. 
The boulder-clay usually forms a thick deposit, covering 
the floors of the valleys and thinning out up the hill 
slopes. As pointed out by previous observers, it occurs 
most abundantly on those hill-slopes which face south or 
south-east, i.e., it occurs on the lee sides of hills with 
reference to the direction of the flow of the ice.* 
The stones in the clay are chiefly of Carboniferous 
rocks, and include limestone, chert, grit, sandstone, gannister, 
shale, coal, and ironstone nodules. The stones are generally 
more rounded the further they are found from their parent 
outcrop. Amongst the boulders of Carboniferous Limestone 
are found all the varieties which occur in the vicinity of 
Upper Airedale, viz., the white encrinital and compact blue 
varieties, the fossiliferous limestone of the " reef -knolls " of 
Craven, and the black Pendleside limestone. The shale, which 
occurs in the Carboniferous Limestone series has also been 
recognised in striated fragments in the boulder-clay. The 
limestone boulders may have their angles and edges rounded 
off, presenting a bruised and battered appearance, or they may 
be rounded and striated in a direction parallel to their long 
axes. They are most abundant in the till near Skipton, and 
become fewer in number and generally smaller in size as we 
proceed down the main valley, and as we approach the periphery 
of the drift-covered area from the main valley. Locally, 
however, the distribution of limestone boulders is very irregular. 
* Dakyns, Quart. Journ. Geol. Soc, vol. xxviii., p. 382, 1872. 
