20 ON THE FRESH-WATER GLACIAL DRIFT 



pied by Lake Simcoe, reported to be four hundred and fifty feet above Lake Erie, 

 or one thousand and fifteen feet above the ocean. 



- Mr. Lyell states that there are from Lake Ontario up to this summit well-defined 

 " lake ridges," eleven in number. On the south there is m the present conformation 

 of the country a broad outlet in the Mississippi valley reaching from the west^n 

 spurs of the Cumberland mountains to the eastern outline of the Ozarks. In this 

 depression the tertiary beds are deposited, rising only two hundred and fifty to five 

 hundred feet above tide water. The great cretaceous and tertiary formation lying 

 to the east of the Rocky mountains is much more elevated. 



Over a territory embracing the States of Ohio, Indiana, Illinois, Iowa, and parts 

 of Kentucky, Tennessee, Missouri, Wisconsin, Michigan, and Canada West there 

 are no mountains. This space is a flat basin with a rolling surface, the highest 

 parts of which rise only seven hundred to twelve hundred feet above the ocean. 

 A horizontal plane at one thousand feet elevation would cut off very few of the 

 summits, and those beneath would be but little below it. There are valleys, of ero- 

 sion, cutting all the strata, from the vaUey drift to the Potsdam sandstone, but no 

 upheavals. 



The accumulation of ice over so large -a space on the earth's surface could have 

 taken place only from the atmosphere, at the slow rate of a few feet in a year. Its 

 disappearance upon a change of temperature should be equally slow. In addition 

 to the flat surface of the covuitry, this mixed mass of ice, sand, and gravel ob- 

 structed the flow of the retiring waters. 



In this way, a long time must have intervened between the glacier period and 

 the reappearance of the soil, during which changes of a mixed character were 

 going on. In some parts of the field tjiere were glacier.movements, while in others 

 there were aqueous movements only. The tertiary of the Mississippi valley ex- 

 tends northerly to Cairo, at the mouth of the Ohio. 



On account of a similarity in external appearance between the northern edges 

 of the tertiary and the more recent beds of clay, sand, and gravel, it is not practi- 

 cable to fix their limits or superposition without further examination. Between 

 the northern limits of the Mississippi tertiary and the southern edge of the glacial 

 drift, there is in Ohio, Kentucky, Indiana, and Illinois, a belt of debatable ground, 

 the outlines of which are not easily defined. 



Before the glacial period, the general configuration of the surface of the North- 

 western States must have been essentially what it is now. The position of the 

 valleys of the great rivers and lakes were about the same as at present. In the 

 general elevation of the entire region there may have been changes, but these were 

 of so extensive a range as not to disturb the local relations of the surface. 



As the rocks had not then been subject to the grinding process of the drift 

 forces, the superficial materials must have been much less in quantity, and of a 

 much finer quality than they are now. The rocks were decomposed and disinte- 

 grated solely by atmospheric and chemical agencies. 



The mechanical power of immense fields of ice, in places several thousand feet 

 thick, moving slowly over the surface of the land, from about the latitude of 40° 

 north to the Arctic circle, had' not then crushed and pulverized the exposed parts 



