CHAP. VII.] 
THE GLACIAL EPOCH. 
109 
for this statement, and as it is most important for our purpose 
to understand the amount of the climatal changes the northern 
hemisphere has undergone, I will endeavour to make the 
evidence intelligible, referring my readers for full details to 
Dr. James Geikie’s descriptions and illustrations . 1 
Glacial Deposits of Scotland: the “ Till .” — Over almost all 
the lowlands and in most of the highland valleys of Scotland 
there are immense superficial deposits of clay, sand, gravel, 
or drift, which can be traced more or less directly to glacial 
action. Some of these are moraine matter, others are lacus- 
trine deposits, while others again have been formed or modified 
by the sea during periods of submergence. But below them 
all, and often resting directly on the rock-surface, there are 
extensive layers of a very tough clayey deposit known as “ till.” 
The till is very fine in texture, very tenacious, and often of a 
rock -like hardness. It is always full of stones, all of wdiich are 
of rude form, but with the angles rubbed off, and almost always 
covered with scratches and striae often crossing each other in 
various directions. Sometimes the stones are so numerous that 
there seems to be only just enough clay to unite them into a 
solid mass, and they are of all sizes, from mere grit up to rocks 
many feet in diameter. The “ till ” is found chiefly in the low- 
lying districts, where it covers extensive areas sometimes to a 
depth of a hundred feet ; while in the highlands it occurs in 
much smaller patches, but in some of the broader valleys forms 
terraces which have been cut through by the streams. Occa- 
sionally it is found as high as two thousand feet above the sea, 
in hollows or hill- sides, where it seems to have been protected 
from denudation. 
The “till” is totally unstratified, and the rock-surfaces on 
which it almost always rests are invariably worn smooth, and 
much grooved and striated when the rock is hard ; but when 
it is soft or jointed, it frequently shows a greatly broken surface. 
Its colour and texture, and the nature of the stones it contains, 
all correspond to the character of the rock of the district where 
it occurs, so that it is clearly a local formation. It is often 
1 The Great Ice Age and its Relation to the Antiquity of Man. By James 
Geikie, F.B.S. (Isbister and Co., 1874.) 
