82 THE GLAQIAL PHENOMENA OF THE COUNTRY 



are .five: in number, viz.: — Boulder Clay or Till, Prismatic 

 Clay, Later Gravels, Erratic Blocks, and Glacial Sands. 



The Boulder Clay is always tlie lowest of these, and is 

 found, of varying thickness, resting upon the original {i.e., 

 pre-glacial) rock-surface, accommodating itself to it in a 

 manner unknown among sedimentary deposits. Thus in the 

 broad and deep pre-glacial valley of the Ouseburn the boulder 

 clay not only lines the bottom of the valley, but also covers 

 the sides and even caps the topmost ridges. In character it 

 does not differ materially from the type so often described as 

 occurring elsewhere. It is a stiff blue clay in which are em- 

 bedded grooved and polished boulders of all sizes ; the larger 

 ones are chiefly of local origin, sandstone, limestone, and 

 whinstone (basalt and dolerite) predominating; the smaller 

 boulders comprise in addition to these ironstone, shale, 

 claystone (very frequently with cone-in-cone structure), and 

 occasionally coarse granite. The smaller boulders are often 

 well rounded, and appear to have been transported part of 

 their journey at least by the agency of running water, 

 Porphyritic rocks, and others far-travelled, are comparatively 

 scarce, a feature which distinguishes the till of this region 

 from that of the neighbouring Tyne Valley, and possibly also 

 from that nearer the coast. 



The Prismatic Clay, when its base is visible, generally rests 

 upon the boulder clay, and differs from it in being less plastic 

 and relatively free from boulders, and in having apparently a 

 larger proportion of far-travelled rocks amongst these than the 

 boulder clay. On drying it shrinks into prismatic forms, from 

 which peculiarity it derives its name. The mode of origin of 

 this clay is uncertain ; its formation has been ascribed to the 

 action of the sea upon the boulder clay proper, and it is quite 

 possible that in some places this in the case. An alternative 

 hypothesis will be given later. 



The Later Gravels. — These are fairly extensively distributed 

 in the country under the 400-foot contour, and include kaims 

 and other pebbly deposits, all of which are alike in one 



