20S MAN AND THE GLACIAL PERIOD. 



the torrent went rushing down through the narrow val- 

 ley, sweeping everything before it ; and nothing but the 

 unsettled condition of the country prevented a disaster 

 like that which occurred in 1889 at Johnstown, Pa. 

 Doubtless there are many other lakes held in position 

 by equally slender natural embankments. Artificial res- 

 ervoirs are by no means the only sources of such dan- 

 ger. 



The buried channel of the old Mississippi River in 

 the vicinity of Minneapolis is another instructive example 

 of the instability of many of the present lines of drainage. 

 The gorge of the Mississippi River extending from Fort 

 Snelling to the Falls of St. Anthony at Minneapolis is of 

 post-glacial origin. One evidence of this is its narrow- 

 ness when contrasted with the breadth of the valley below 

 Fort Snelling. Below this point the main trough of the 

 Mississippi has a width of from two to eight miles, and 

 the faces of the bluffs on either side show the marks- of 

 extreme age. The tributary streams also have had time 

 to wear gorges proportionate to that of the main stream, 

 and the agencies which oxidise and discolor the rocks 

 have had time to produce their full effects. But from 

 Fort Snelling up to Minneapolis, a distance of about 

 seven miles, the gorge is scarcely a quarter of a mile in 

 width, and the faces of the high, steep bluffs on either side 

 are remarkably fresh looking by comparison with those 

 below ; while the tributary gorges, of which that of the 

 Minnehaha River is a fair specimen, are very limited in 

 their extent. 



Upon looking for the cause of this condition of things 

 we observe that the broad trough of the Mississippi River, 

 which had characterised it all the way below Fort Snel- 

 ling, continues westward, without interruption, up the val- 

 ley of the present Minnesota River, and, what seems at 

 first most singular, it does not cease at the sources of the 

 Minnesota, but, through Lake Traverse and Big Stone 



