JOWETT AND MUFF I GLACIATION OF BRADFORD, ETC. 199 
Boulders of Millstone Grit become abundant after entering 
upon the Millstone Grit outcrop to the south of Skipton, and 
they frequently attain a large size (seven feet or more in length). 
Large boulders of Grit are scattered about the Coal Measure 
hills to the north and east of Bradford. A peculiar compact 
siliceous rock, locally known as " blue-stone,'' crops out beneath 
the Rough Rock around the head of the Worth Valley, to which 
district it appears to be limited. Pebbles of this rock have 
been found in the drift in one or two places on the south side 
of Airedale and south-east of the outcrop of the stratum. 
Scratches are more often noticed on gannister boulders than 
on boulders of grit and sandstone, which do not appear to have 
received or retained them readily. 
Pieces of coal and ironstone nodules, which are chiefly derived 
from the Coal Measures, are found in the till on the south and 
east sides of the Bradford basin. 
Fragments of cleaved green slate and pebbles of greywache 
grit have occasionally been met with. Two boulders of cleaved 
slate were found in the boulder-clay on the northern slope of 
Combe Hill at an altitude of 1,200 feet above O.D., and several 
other occurrences are noted in Section V. These rocks can be 
matched in the Silurian grits and slates outcropping on the 
Malham Moors at the head of Airedale and around Horton- 
in-Ribblesdale. Similar grits and slates were pointed out to 
us in the drift mounds at Bingley by Mr. E. E. Gregory, and 
Dr. Monckman has recently recorded " Silurian grits " from 
the boulder-clay behind Grange Road, Bradford.* 
The above-mentioned facts serve to confirm the conclusion 
arrived at by previous observers, that the distribution of boulders 
indicates a general and regular south-easterly movement of the 
transporting agent in Airedale. 
Boulders of igneous rocks have only rarely been found. 
Russell recorded " taking a few pebbles of trap and ash rocks 
as far up towards the watershed between the Aire and Calder 
[south of Bradford] as Rooley and Great Horton, and [took] 
* " The Glacial Geology of Bradford and the Evidence obtained 
from Recent Excavations of a Limestone Track on the South Side of the 
Valley." By James Monckman, D.Sc, Proc. Yorks. Geol. and Polyt. 
Soc, vol. xiv., p. 157, 1901. 
