374 
MORTIMER : SECTIONS OF DRIFT. 
A glance at the sections shows the frequent interbedding of 
thin patches of a sandy nature ; and the peculiarly complicated 
manner in which masses of almost pure chalk-gravel of every form 
and size, are pushed into and deposited on the clays. No two 
parallel sections, if even but a few yards apart, would show the 
same appearance in the arrangement of the beds. 
The clays consist of the feather edges of what are known as 
the " purple clay," and the brown " Hessle clay," of Messrs. Wood 
and Rome, which are frequently so intricately clove-tailed and 
squeezed together as to have obliterated every well marked line of 
separation. On the other hand, on the Nafferton road and in 
a few other well marked places, hardly anything but brown "Hessle 
clay" was exposed, which in places seemed to overlap the edge 
of the "purple clay." 
The two clays contained boulders which are, in the main, more 
or less rounded, very variable in size and derived from many differ- 
ent kinds of rock. They are frequently polished and finely striated. 
Yet not infrequently they are found with their angles and edges as 
sharp as if quite recently detached from their parent beds. In a 
few instances unbroken pieces of rock, showing lines of bedding 
and cross fractures, but still held together in a connected mass, 
have been safely carried and dropped in the clays and gravels, as at 
" A," in Westgate. 
The gravels consist almost entirely of more or less water-worn 
pieces of chalk of small size, with occasional boulders of travelled 
rock, chiefly water-worn and very variable in size. Many of the 
small patches of pure chalk gravel enclosed in the clays, are true 
boulders, having been conveyed in a frozen condition, or in pock- 
ets in the ice, which carried and dropped them into the clays. This 
view is supported by the fact that much of the gravel is found 
standing on end. Large foreign boulders are not infrequently 
found in the gravel, as well as in the clays, standing on, or nearly 
on their ends, and sometimes on their small ends. 
