36 W. Upham—Minnesota Valley in the Ice Age 
of drift and reaches the underlying rocks, which have frequent 
exposures along its entire course below Big Stone Lake. This 
excavation shows that the thickness of the general: drift-sheet 
upon this part of Minnesota averages about one hundred and 
fifty feet. The contour of the old rocks thus brought into: 
view is much more: uneven than that of the drift. In the 
hundred miles from Big Stone Lake to Fort Ridgely the strata 
are metamorphic gneisses and granites, which often fill the , 
whole valley, one to two miles wide, rising in a profusion of 
knolls and hills, fifty to one hundred feet above the river.. 
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. 
Ulm to its mouth the river is at many places bordered by 
Cretaceous and Lower Silurian rocks, which are nearly level in 
stratification. These vary in height from a few feet to fifty or 
rarely seventy-five or one hundred 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, fifty to seventy-five feet high. After 
excavating the overlying one hundred and twenty-five to one 
hundred and fifty feet of till, the river here found a former 
valley, eroded by pre-glacial streams. Its bordering walls of 
rock, varying from one-fourth of a mile to at least two 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 enclosed by rock-walls, is 
in its last two miles, where it flows between bluffs of Trenton 
limestone underlain by St. Peter sandstone, one hundred feet 
high and about a mile apart. This also is a pre-glacial channel, 
its farther continuation being occupied by the Mississippi River. 
The only erosion effected by the Minnesota River here 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 bottomland of the Min- 
nesota River at Belle Plaine, where two hundred and two feet 
of stratified gravel, sand and clay were penetrated before reach- 
ing the rock. The bottom of the pre-glacial channel there is: 
thus at least one hundred and sixty-five feet lower than the 
mouth of the Minnesota River. 
Heights of the bluffs, which form the sides of this valley, 
composed of till enclosing layers of gravel and sand in some 
places, and frequently having rock at their base, are as follows, 
stated in feet above the lakes and river: along Lake Traverse, 
100 to 125; at Brown’s Valley and along Big Stone Lake, 
mainly about 125, the highest portions reaching 150; at Or- 
tonville, 180; at Lac qui Parle and Montevideo, 100; at Gran- 
