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.^ 



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 which 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 



^ The Great Ice Age and its Relation to the Antiquity of Man. By James 

 Geikie, F.R.S. (Isbister and Co., 1874.) 



