A PARTICULAR CASE OF GLACIAL EROSION. 1 



FREDERICK W. SARDESON 



Minneapolis, Minn. 



It is the purpose of this paper to describe a peculiar phase of 

 glacial work, where a layer of stratified rock of considerable area 

 was shoved forward bodily on its bed, and then thrust up out of 

 horizontality. Use is made of this case to explain the manner in 

 which a number of other known dislocations of bed-rock in the 

 vicinity might have been made, as well as to explain how glaciers in 

 general may affect the subjacent rocks under certain circumstances. 

 The present illustration is found near the University of Minnesota, 

 in Minneapolis, about a mile below the Falls of St. Anthony, on the 

 east side of the gorge of the Mississippi. The conspicuous feature 

 of the displacement is represented in the right (southeast) half of 

 the accompanying profile (Fig. i). The left (northwest) half of the 

 profile is drawn from notes, aided by photographs taken from time 

 to time as ground was excavated in a stone quarry, which has been 

 working obliquely across the line of the profile. The line of the 

 profile corresponds with the direction of glacial movement. There 

 is also, at the time of writing, an exposed section parallel to and 

 corresponding with that of the left half of the figure. 2 



The section which is represented by the profile (Fig. i) lies near 

 the 800-foot contour line as established by the topographic map. 3 

 As shown on the map, a terrace lies between the 780 and 800-foot 

 contour. This terrace belonged to the Mississippi River at a stage 

 when the falls were below this point, instead of above it, as they now 

 are. Besides the terrace contour, the occurrence of the river shingle 

 and well-preserved shells of species of Unio shown at R (Fig. 1), 

 under a layer of peat (P), proves that the river once flowed over the 



1 This paper was read before the Minnesota Academy of Sciences, January 3, 1905. 



2 Observations on this exposure have extended over several years. Negatives 

 taken are on file in the Department of Geology, University of Minnesota. 



3 St. Paul quadrangle, U. S. Geological Survey. 



351 



