DRAINAGE OF THE GLACIAL PERIOD. 315 



telegraph to span the distances of time separating ns from 

 those events, we have come into possession of signs as intel- 

 ligible as the lines and dots of the Morse alphabet, and even 

 more trustworthy. These floods along the lines of glacial 

 drainage have left their marks, and their direction and ex- 

 tent can be traced almost as readily as in the case of the 

 present streams. 



Ascending the channel of the Mississippi above its junc- 

 tion with the Ohio, one enters a region where it is bordered 

 on each side by rocky bluffs, and iinds himself in a valley of 

 erosion whose main features were determined in preglacial 

 times. Above Grand Tower, in southern Illinois, and as far 

 north as St. Louis (a distance of about one hundred and fifty 

 miles), the extreme margin of glacial deposits rests upon the 

 east side of the river, and an unglaciated region is upon the 

 west ; the width of the eroded valley being from five to ten 

 miles, and its depth several hundred feet. Above St. Louis 

 the valley gradually narrows, though it is still from two to 

 eight miles in width, and about the same depth as below the 

 city. At various places, along the sides of this eroded val- 

 ley, the observer will find gravel terraces one or two hundred 

 feet above the present flood-plain. These terraces are, in 

 fact, the high- water mark of the closing floods of the Ice age. 



But the culmination of interest is reached on coming to 

 the present junction of the Minnesota and Mississippi Riv- 

 ers near St. Paul. From Fort Snelling, just above St. Paul, 

 northward, the present Mississippi River is a comparatively 

 recent stream, occupying a post-glacial bed. The true exten- 

 sion of the trough of the Mississippi follows up the Minne- 

 sota River. The gorge of the Mississippi, leading from Fort 

 Snelling up to Minneapolis, is scarcely a quarter of a mile in 

 width, and is about two hundred and fifty feet in depth ; 

 while the trough of the Minnesota is from one to four miles 

 in width, and its rocky bottom is more than one hundred and 

 fifty feet lower than the present bed of the stream. In the 

 bottom of this broad valley, for a distance of two hundred 

 and fifty miles, the Minnesota River wanders about from 



