54 



MAN AND THE GLACIAL PERIOD. 



creased sixty-four fold.* So frequent are the changes in 

 the velocity of running water, that the stratification of 

 its deposits is almost necessary and universal. If large 



Fig. 20.— Scratched stone from the till of Boston. Natural size about one foot 

 and a half long by ten inches wide. (From photograph.) 



fragments of rocks or boulders are found embedded in 

 stratified clay, it is pretty surely a sign that they have 

 been carried to their position by floating ice. A small 

 mountain stream with great velocity may move a good- 

 sized boulder, while the Amazon, with its mighty but 

 slow-moving current, would pass by it forever without 



Le Conte's Geology, p. 19. 



