GLACIAL AND POSTGLACIAL HISTOKY OF GKEAT LAKES EEGION. 317 



to a large degree on the character of the strata out of which they have been excavated — on 

 then relative hardness, thickness, and arrangement. 



This was the constructional period in which nature was getting ready for the subsequent 

 making of the lake basins. 



STAGE OF EMERGENCE. 



The lake basins did not begin to be made until another great event in geologic history, 

 involving a great change in the relative altitude of the land and sea, had taken place. Begin- 

 ning at the close of the Paleozoic era great earth movements affected all of the eastern part of 

 North America, including the whole of the Great Lakes region, lifting the land now occupied 

 by the lakes to an altitude estimated by some to be 2,000 or 3,000 feet higher than its present 

 altitude. This was the time of the uplifting and folding of the Appalachian Mountains. The 

 process probably occupied some thousands of years, but in a geologic sense its duration was 

 short. 



There is evidence also of some earlier movements of less extent that affected the region of 

 Lake Superior and especially the northern part of Lake Huron, and that probably produced 

 small land surfaces. 



STAGE OF DESTRUCTION. 



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 shores. Rain and frost, wind and sunshine, and the agents of chemical 

 decomposition attacked every part of the land surface. Most effective of all was the water 

 that gathered into flowing streams. All of these, great and small, did their share of work in 

 tearing down and sculpturing the new land, earring valleys, hills, and mountains out of the 

 elevated mass. Each worked with an efficiency dependent upon its volume, the rate of its 

 descent, the character and quantity of the sediment it carried, and on other factors. The 

 first 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 resistant 

 characters of the rocks and their relative position and arrangement. 



The strata out of which the lake basins were later excavated were laid down for the most 

 part not along the shores of that ancient time but at some distance offshore, so that the sedi- 

 ments received were mainly of fine texture, mud which afterward became shale and limy 

 ooze which afterward became limestone. Conglomerates and sandstones indicating shore or 

 shallow near-shore conditions occur, but are not common. Limestone does not rank as hard 

 in the mineral scale, but relatively to the shales some of it is hard and resistant, especially 

 where it occurs in massive form and in great thickness. In the building of the strata it hap- 

 pened that a group (Niagara) of beds whose arrangement and relative hardness predisposed 

 them to unequal erosion and to the formation of valleys bounded by great escarpments was 

 laid down from central New York to northern Michigan. The limestone of this group, a massive 

 bed of the hardest quality, 150 to 200 feet thick, is underlain by several hundred feet of the 

 Clinton and Medina formations of shales and sandstones — chiefly shales — and thin layers of 

 limestone much softer than the overlying limestone. It is overlain by 200 to 300 feet of the very 

 soft, marly, salt-bearing beds composing the Salina formation. The selective processes of 

 erosion led the streams to attack the softer strata and to wear them away, leaving the harder 

 limestone of the Niagara group to form the great escarpment which now stretches from New 

 York to Wisconsin. Extensive valleys were eroded in the soft underlying rocks, undermining 

 the limestone and driving it back. Other valleys were excavated in the soft overlying Salina 

 formation. 



Thus Lake Ontario, Georgian Bay, the northern channel of Lake Huron, and Green Bay 

 were excavated out of the soft rock below the limestone of the Niagara group; and Lake Erie, 

 the main body of Lake Huron, and all of Lake Michigan were excavated out of the soft strata 

 above the hmestone. Lake Superior appears to be somewhat exceptional. It is thought to be 



