PIPESTONE AND ROCK COUNTIES. 549 



Glacier-marks. J 



on section 18, Eden, and also on "the mound," near Luverne; also at Sioux 

 Falls in Dakota. It can be compared to a cross-drained planed board, where 

 the plane has been drawn against the grain, except that the cut edges are 

 curved so as to present their convexity toward the cutting or planing force. 

 Thickness of the i/lacier in Rock county. This incipient crushing of the 

 red quartzyte under the ice may be taken as a datum on which to com- 

 pute the thickness of the ice at the time of its production. According to 

 the table of the qualities of the building stones of Minnesota, already given 

 (page 195), the red quartzyte from Pipestone City has a crushing resistance 

 equal to 27,000 pounds per square inch. The specific gravity of ice is 0.92. 

 Hence a column of ice nearly eleven miles in hight would be required to 

 produce the pressure of 27.000 pounds. This calculation is subject to cor- 

 rection for the following sources of error: 1st. The ice may have carried 

 a large amount of drift, rendering the comparative weight much greater, 

 and requiring less perpendicular hight. 2d. The crushing produced is su- 

 perficial, and is not the same as the breaking down of a cube of stone 

 placed between steel plates. 3d. The fractures were formed by a rasping 

 or scratching force and would be more easily produced than a total crush- 

 ing down of the strata. 4th. The stones which were agents in the grasp of 

 the ice in thus marking the quartzyte, presented only their tangential 

 points, but must have supported a column of ice equal to the area of their 

 horizontal periphery. 5th. The strength of the quartzyte may be over- 

 stated.* The import of this calculation therefore cannot be much more 

 than to warrant the statement that the ice was very thick, perhaps several 



miles. 



Mr. Upliam gives the following further observations on glacial striae in Rock 

 county: "Very interesting glacial striae were seen on the quartzyte, one rod east 

 of the road about a mile north of where the east road from Luverne to Pipestone 

 City rises upon the quartzyte of the Mound, probably in the southwest quarter 

 of section 23, Mound. At its west edge a width of two feet, as shown in the 

 FIG 44 annexed sketch (fig. 44 >, is striated from north to south, while the rest is striated 



striated surface in S. 35 W. The line dividing these areas, marks a definite change of plane in 

 the rock surface, which is inclined downward at the west four or five degrees, 

 and at the east about half as much; making a beveled angle of 5 or perhaps T . It seems to me 

 that these striae were probably engraved at different dates by one ice-sheet which had con- 

 stantly covered this district. When the ice attained its maximum area, the current of this por- 

 tion would be nearly from north to south; but during the final melting, as its retreating western 

 border came nearer and nearer to this place, the current must have been deflected south west ward, 



*Compjirative strength of Minnesota and New England granites, Proceedings of the American Association for the 

 Advancement ol 1 Science, Minneapolis meeting, 1RH1 Jiiincs I'roll has estimated the presei t thickness ol the ice on ihe 

 anlarclic continent ut twelve miles, with :i tu| ei final slope <if one half a degree.- -I'limaie and lime, p. 375. Prof. J. D. 

 Dunn has estimated il at 5000 feet in New England during Ihe glacial epoch. Am. Jour. Hci. 



