
GEOLOGICAL SURVEYING 315 
blocks are erratics which have been carried by the ancient 
glaciers, and successively stranded as the great ice-flows 
melted away. 
Sheets and Mounds of Gravel and Sand.—Throughout 
wide areas, boulder-clay is often more or less deeply buried 
under gravel and sand. These deposits may assume the 
form of extensive sheets with a gently undulating surface ; 
or they may occur as long curving and irregularly winding 
ridges ; or as tumultuous groups of closely associated mounds, 
hummocks, and ridges, known as kames. Sometimes the ac- 
cumulations consist principally of fine sand—often diagonally 
bedded—or of interbedded sand and laminated clay. In 
other places, sand and gravel are equally prominent, while 
elsewhere coarse shingle and boulders are most abundant. 
At rarer intervals a mound or ridge may be composed of rude 
morainic débris—a rubble of angular blocks and rock-rubbish. 




9; 
ee 
FIG 121.—COARSE GRAVEL AND SHINGLE, SHOWING IMBRICATED 
STRUCTURE. 
The arrow indicates the direction of the current. 
It is obvious that deposits so heterogeneous could not 
all have been formed in quite the same way; and it would 
be out of place here to discuss the various views which have 
been entertained as to their origin. The general belief, 
however, is that all the deposits in question were accumulated 
while the old icy covering of the land was gradually melting 
away. ‘The observer will note that the long winding ridges 
(known as eskers) are composed chiefly of gravel—often 
very coarse—with more or less numerous boulders. They 
have obviously been laid down by torrential water—and 
when good sections across an esker are exposed, the stones 
sometimes show that imbricated arrangement which one may 
often observe amongst the stones and coarse shingle of 
streams and rivers (Fig. 121). Many geologists incline to the 
belief that these eskers mark the sites of subglacial torrents 

