SIGXS OF FORMER GLACIATION. 129 



movements, also, where the crust of the earth is broken and 

 the edges of successive strata are shoved over each other, a 

 species of striation is produced which in technical terms is 

 called a sliekenside. Occasionally this deceives the inex- 

 perienced or incautious observer. But by due pains all these 

 semblances may be detected and eliminated from the prob- 

 lem, leaving a sufficient number of unquestionable phenom- 

 ena due to true glacial action. 



A second indubitable mark of glacial action is found in 

 the character of the deposit left after the retreat of the ice. 

 Ice and water differ so much from each other in the extent of 

 their fluidity, that ordinarily there is little danger of confus- 

 ing the deposits made by them. A simple water deposit is 

 inevitably stratified. The coarse and line material can not be 

 deposited by water alone, simultaneously in the same place. 

 Along the shores of large bodies of water the deposits of 

 solid material are arranged in successive parallel lines, the 

 material growing finer and finer as the lines recede from the 

 shore. The force of the waves is such in shallow water that 

 they move pebbles of considerable size. Indeed, where the 

 waves strike against the shore itself, vast masses of rock are 

 oftentimes moved by the surf. But, as deeper water is 

 reached, the force of the waves becomes less and less at the 

 bottom, and so the transported material is correspondingly 

 fine, until, at the depth of about seventy feet, the force of 

 the waves is entirely lost ; and beyond that line nothing will 

 be deposited but fine mnd. the particles of which are for a 

 long while held in suspension before they settle. 



In the deltas of rivers, also, the sifting power of water 

 may be observed. Where a mountain-stream first debouches 

 upon a plain, the force of its current is such as to move large 

 pebbles, or bowlders even, two or three feet in diameter. 

 But, as the current is checked, the particles moved by it be- 

 come smaller and smaller until in the head of the bay. or in 

 the broad current of the river which it enters, only the finest 

 sediment is transported. The difference between the size of 

 material transported by the same stream when in flood and 



