THE MINNESOTA EIVER VALLEY. 17 



whicli often fill the entire valley, 1 to 2 miles wide, rising- in a profusion of 

 knolls and hills 50 to 100 feet above the riverr- The depth eroded has been 

 limited here by the presence of these rocks, among which the river flows in " 

 a winding course, crossing them at many places in rapids or falls. 



P^'rom New Ulm to its mouth the river is at many ])laces bordered by 

 Cretaceous and Lower Silurian and Cambrian rocks, which are nearly 

 level in stratification. These vary in height from a few feet to 50 or rarely 

 75 or 100 feet above the river. From Mankato to Ottawa the river 

 occupies a valley cut in Shakopee limestone underlain by Jordan sandstone, 

 which form frequent bluffs upon both sides, 50 to 75 feet high. After 

 excavating the overlying 125 to 150 feet of till, the river here found a 

 former valley eroded by preglacial streams. Its bordering walls of rock, 

 varying from one-fourth of a mile to at least 2 miles apart, are' in many 

 portions of this distance concealed by drift, which alone forms one or both 

 sides of the valley. The next point at which the river is seen to be 

 inclosed by rock walls is in its last 2 miles, where it flows between bluffs 

 of Trenton limestone underlain by St. Peter sandstone, 100 feet high and 

 about a mile apart. This also is a preglacial channel, its further continu- 

 ation being- occupied by the Mississippi River. The only erosion effected 

 here by the Minnesota River has been to clear away a part of the drift 

 with which the valley was filled. Its depth at some earlier time was much 

 greater than now, as shown by the salt well on the bottom-land of the 

 Minnesota at Belle Plaine, where 202 feet of stratified gravel, sand, and 

 clay were penetrated before reaching the rock.^ The bottom of the 

 preglacial channel there is thus at least 165 feet lower than the mouth of 

 the Minnesota River. 



The height of Lake Traverse,' in the range between its lowest and 

 highest stages, is 970 to 976 feet above the sea; the lowest point in Browns 

 Valley between this and Big Stone Lake is only 3 feet above the ordinary 

 stage of Lake Traverse; Big- Stone Lake^ ranges from 962 to 967 feet 



'Geology of Min., Vol. II, p. 134. 



'A translation of the Dakota name (Williamson, 1. c, p. 108). "The lake has received its 

 present appellation from the circumstance that it is in a direction nearly transverse to that of the Big 

 Stone and Qui Parle lakes.'' Keating's Narrative, Vol. II, p. 2. 



■'Translated from the Dakota (Williamson, 1. c, p. 105). The name probably alludes to the 

 conspicuous outcrops of granite found in the Minnesota Valley 1 to 3 miles below the foot of the lake. 

 MON XXV 2 



