CHAP. xn. GEOLOGICAL PRINCIPLES. 121 



various official geological surveys of the chief civilized 

 countries; and as they constitute one of the most re- 

 markable chapters in the past history of the globe, and 

 especially as this great change of climate occurred dur- 

 ing the period of man's existence on the earth, a brief 

 sketch of the facts must be here given. 



There are four main groups of phenomena which 

 demonstrate the former existence of glaciers in areas 

 where they are now absent: (1) Moraines, and glacial 

 drifts or gravels; (2) Smoothed, rounded, or planed 

 rocks; (3) Striae, grooves, and furrows on rock-surfaces; 

 (4) Erratics and perched blocks. 



(1) Moraines are formed by all existing glaciers, con- 

 sisting of the earth and rocks which fall upon the ice- 

 rivers from the sides of the valleys through which they 

 flow. The slow motion of the glacier carries these down 

 with it, and they are deposited in great heaps where it 

 melts. In some glaciers where the tributary valleys are 

 numerous and the debris that falls upon the ice is abun- 

 dant, the whole of the lower part of the glacier for many 

 miles is so buried in it that the surface of the ice cannot 

 be seen, and in these cases there will be a continuous 

 moraine formed across the valley where the glacier ter- 

 minates. The characteristics of moraines are, that they 

 consist of varied materials, earth, gravel, and rocks of 

 various sizes intermingled confusedly; and they often 

 form mounds or ridges completely across a valley, except 

 where the stream passes through it, while in other cases 

 they extend laterally along the slopes of the hillsides, 

 where, owing to the form of the valley, the glacier has 

 shrunk laterally and left its lateral moraine behind it. 

 In many cases huge blocks of rock rest on the very sum- 



