NE-SAW-JE-WON 



sill holding the waters of the lake in the basin. Where the 

 shales which underlie it are cut away, the Niagaran rampart 

 stands in places like a cornice, rather than with the fortress- 

 like character of the Cambrian sandstones ; but it is sufficiently 

 impregnable to have played a most important part in the 

 development of the lakes. The close of Silurian time was 

 marked by dry climates and by a shrinking sea from which 

 life almost departed and which left enormously thick beds of 

 salt and thinner beds of gypsum. 



Then came a return of more genial climates, and a warm 

 sea — the Devonian — entered the lake region and left thick 

 sediments in the deep basin of the Michigan area. A part of 

 the Devonian record is in the limestone which makes the 

 rampart along the northern rim of the Southern Peninsula 

 of Michigan from the Traverse Bay area to Alpena, which is 

 submerged under Lake Huron, crosses Ontario and swings 

 back again into southeastern Michigan, northern Ohio and 

 Indiana, and which forms the western bowl in which the 

 thick soft shales of the late Devonian and early Mississippian 

 were deposited. These soft shales, together with the shales and 

 salt beds of the late Silurian, in time were carved out to form 

 the moat back of the Niagaran rampart, and in them the 

 channels of great rivers were cut — rivers which carved valleys 

 now filled by Lakes Michigan, Huron and Erie. 



Above the thick shales of the lower Mississippian is a hard 

 sandstone overlain by limestone and gypsum beds. Parts of 

 the sandstone and limestone rims come to the surface near 

 Holland, Michigan, and also along the shore of Lake Huron 

 — around the "Thumb" of Michigan and along the opposite 

 shore of Saginaw Bay. The southern rims of these Mississip- 

 pian sandstone and limestone bowls are near the surface along 

 an arc extending from the Thumb through south-central 

 Michigan to Holland, and although inland, they played their 

 parts in the lake history. 



Within the Mississippian bowl in Michigan is the set of 



