ROCK FORMATIONS AND PRE-GLACIAL HISTORY 



face. One of these depressions, produced late in this early 

 history, in time became the elongated curved basin of Lake 

 Superior. Another much older depression, centering in the 

 Southern Peninsula of Michigan and there more or less circu- 

 lar in shape, extended eastward in a rising scoop-like trough 

 to east-central New York. This depression became the basin 

 in which later invading seas deposited a great series of layered 

 formations, one above another, made of the sediments weath- 

 ered and eroded from the ancient rocks of the adjacent 

 uplands. Slow earth movements caused sea after sea to enter 

 and withdraw or be spilled from this basin, each sea being 

 smaller than and within the boundaries of its predecessor, and 

 each leaving layers of accumulated sedimentary material to be 

 buried — in the central area of the basin — by the deposits of its 

 successor. These sediments were compressed and hardened into 

 rock — sandstones, limestones, shales, rock salt and gypsum. 

 Coal beds, formed from the abundant vegetation which grew 

 in the marshes associated with the later seas, were inter-bedded 

 with the upper rock formations. 



With the withdrawal of the last sea the land was elevated, 

 and a long period of weathering and erosion followed. The 

 highest portions of the bedded rocks around the rim of the 

 scoop-like basin wore away first, bevelling the edges of the 

 basinward-sloping rocks but not disturbing the lower portions 

 in the interior of the depression. Thus developed a series of 

 gigantic "rock bowls" — one within another — occupying the 

 basin. They are like a great nest of mixing bowls with lips to 

 the east. Some of the rock layers are hard and resistant to 

 weathering, others are soft and easily eroded; hence in time 

 the once-beveled edges of the resistant sandstones, limestones 

 and dolomites came to have the general appearance of a series 

 of parallel, concentric, stone "ramparts" with steep outward- 

 facing exposures and gentle inner slopes toward the basin. 

 These are separated by the depressions worn in the softer, 

 less resistant layers — the shales, salt and gypsum beds. Where 



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