130 THE ICE AGE IN NORTH AMERICA. 



when at low water is very great, and is the main agent in pro- 

 ducing the familiar phenomena of stratification. During the 

 time of a flood vast bodies of pebbles, gravel, and sand are 

 pushed out by the torrent over the head of the bay or delta 

 into which it pours ; while during the lower stages of water 

 only fine material is transported to the same distance ; and 

 this is deposited as a thin film over the previous coarse deposit. 

 Upon the repetition of the flood another layer of coarser 

 material is spread over the surface ; and so, in successive 

 stages, is built up in all the deltas of our great rivers a series 

 of stratified deposits. In ordinary circumstances it is impos- 

 sible that coarse and fine material should be intermingled in 

 a water deposit without stratification. Water moving with 

 various degrees of velocity is the most perfect sieve imagi- 

 nable ; so that a water deposit is of necessity stratified. 



To this general principle, however, exception must be 

 made in the case of accumulations taking place slowly in 

 deep water containing icebergs from which bowlders and 

 pebbles may be dropped. These, of course, will be found 

 distributed through the fine deposit without stratification. 

 It is thought by President Chamberlm,* however, that in 

 cases where the bowlders dropped from the icebergs are of 

 marked irregularity in their configuration, their position in 

 the ooze at the bottom would betray their origin. Flattish 

 stones, in falling through the water, would often descend 

 edgewise, and would not uniformly lie in the mud upon 

 their flat surface. I have myself observed numerous places 

 in southern Ohio where the arrangement of the limestone 

 fragments abundantly illustrates and confirms this theory. 

 The horizontal position of such fragments in the clay of that 

 region seems to show that they were arranged in the deposit 

 by a moving ice-sheet, and were not dropped from floating ice. 



It is evident that ice is so nearly a solid that the earthy 

 material deposited by it must be unassorted. The mud, sand, 

 gravel, pebbles, and bowlders, dragged along underneath a 



* " Terminal Moraine of the Second Glacial Epoch," p. 297. 



