SIGNS OF FORMER GLACIATION. 



123 



of a glacier as readily and unmistakably as we would the 

 familiar foot-prints of an animal. The indications upon which 

 glacialists have depended for their information as to the ex- 

 tent of the glaciated region during the great Ice age are of 

 three kinds : 1. Grooves and scratches preserved upon the 

 rocks in place and upon the bowlders and pebbles shoved 

 along under the ice. 2. The extensive unstratifled deposits 

 called " till," which are traceable to glacial action. 3. Trans- 

 ported material found in such positions that it must have 

 been left by glacial ice rather than floating ice. 



In respect of the nature of ice we are compelled to ad- 

 mit that it is capable of motion like such semi-fluids as cool- 



Fig. 40.— Scratched stone from the till of Boston. Natural size about one foot and a half 

 long by ten inches wide. (From photograph.) 



ing pitch or lava. But, though it does move, it is not capa- 

 ble of adapting itself so perfectly as a real fluid to the 



