GLACIAL LAKES TAYLOR. 293 



There is evidence, also, that there were some earlier movements of 

 less extent affecting the region of Lake Superior and the northern 

 part of Lake Huron especially, which probably made land surfaces 

 of limited extent before the great movement which elevated the 

 whole region. 



THE MAKING OF THE GREAT LAKE VALLEYS BY STREAM EROSION. 



The forces of subaerial and stream erosion attacked the surface of 

 the land as fast as it was raised above the level of the sea, and the sea 

 itself, with its waves and tides and currents, attacked the new land 

 all around its shores. Rain and frost, wind and sunshine and the 

 various agents of chemical decomposition attacked every part of the 

 new land surface. Most effective of all was the water that gathered 

 into flov/ing streams. All of these, great and small, did their share 

 in tearmg down and sculpturing the new land — in carving valleys, 

 lulls, and mountams out of the elevated mass. Each one worked 

 with an efficiency dependent upon its volume, the rate of its descent, 

 the character and quantity of sediment carried and upon other 

 factors. The fu'st shapes of the newly emerged land determined the 

 first drainage systems, but as the work of erosion went on the effects 

 produced were greatly influenced by the variously resistent character 

 of the rocks and then* relative position and arrangement. 



In the buildmg of the strata out of which the lake bashis have been 

 excavated, it happened that the region now occupied by the greater 

 part of the basins was for the most of the time not adjacent to the 

 shores of the ancient seas so as to receive coarse sediments, but was 

 offshore some distance from the land, so that the sediments received 

 were mainl}/" of fine texture, mud which afterv,^ards became shale, 

 and limey ooze which afterwards became limestone. Conglomerates 

 and sandstones indicating shore conditions or shallow water near 

 shore occur, but are not common in the lake basins. Limestone 

 does not rank as a hard substance in the scale of mineral hardness, 

 but compared to the shales it is sometimes a hard, resistant rock, 

 especially v/here it occurs m massive form and m great thickness. 

 In the building of the strata it happened that, stretching westward 

 and northwestward from central New York to northern Michigan, 

 there was a group of beds whose arrangement and relative hardness 

 predisposed them to unequal erosion and the formation of valleys 

 bounded by great escarpments. In New York the Lockj)ort lime- 

 stone of the Niagara group is a massive bed of the hardest quality, 

 150 to 250 feet thick, while below it are shales and sandstones — 

 chiefly shales— much softer, but containing two relatively thin, hard 

 layers of Clinton limestone. These lower beds are several hundred 

 feet thick. Then, again, above the Lockport limestone, are the 

 85360°— SM 1912 20 



