6o 



A NATURALIST IN THE GREAT LAKES REGION 



bed rock with little wear or tear, rock fragments held by the ice 

 make deep grooves, imbedded sand grains striate the rock sur- 

 face in the direction of glacial motion, while finer particles like 

 clay polish it. Such work necessarily wears away the graving 

 tools. Rock fragments held by the ice and pressed against the 

 rock over which it moves are themselves planed off, polished, 

 and scratched, first on this side, then on that, as they turn in 





Fig, 40. — A good-sized glacial bowlder on Lake JNIichigan shore near Glencoe 



the somewhat plastic ice. Ultimately they are ground to powder 

 unless they are dropped in the morainal material, in which case 

 they are subangular stones more or less marked by their use 

 (Fig. 40) . The bed rock where freshly uncovered in the Chicago 

 region often shows a polished surface, sometimes striated, occa- 

 sionally deeply grooved, mute evidence of events that were 

 transpiring here during the great ice age. In general these 

 markings are parallel and have a northeast-southwest direction, 



