504 THE GEOLOGY OF MINNESOTA. 



[Glacial itria-. 



age surface to the south and southwest is slight, probably not exceeding 50 feet. Its length is 

 about twenty-five miles, extending from east to west; and this locality is near the middle of its 

 extent. Very distinct glacial markings occur here promiscuously crossing each other in all di- 

 rections between north to south and S. 60" E., and, very rarely, S. 80 E.; but a great majority 

 are between S. 25" E. and S. 40" E. Many are from ten to thirty feet or more in length, and 

 from an eighth to a half of an inch deep; others are very delicate lilies. Curved strise were ob- 

 served at one place; two or three parallel furrows (fig. 36), covering a width of several inches and 

 extending about ten feet to the southeast, were gradually deflected nine inches southerly from 

 their direct course in the last four feet. All the other very abundant intercrossed striae observed 

 here are straight, or deviate only slightly from straight courses. The outcrop containing pipe- 

 stone in section 2, Amboy, furnished the only similar instance seen in these counties. Here sev- 

 eral parallel glacial scratches bend twenty or thirty degrees in a length of about eight inches (fig. 

 37). The curvature of these ice-marks, where no obstacle existed to cause deflection, indicate 

 that they were engraved during the final melting and recession of the ice-sheet, when it had be- 

 come thin, and that its margin at the date of this curved striation was near, perhaps within a few 

 rods. In such a situation the unequal melting of the edge of the ice must produce changes, such 

 as are thus recorded, in the direction of its motion.* The prominence of the quartzyte ridge 

 doubtless gave unusual irregularity to the outlines of the retreating ice-border in northern Cot- 

 tonwood county, which, by the resulting deflections of the glacial current, appears to have been 

 the cause of the singularly varying and intercrossed striation of this region. 



During the greater part of the last glacial epoch the ice-fields here 

 appear to have flowed in a nearly south-southeast course; but when they 

 were being melted away, the direction of movement close to the ice-border 

 would be often deflected because it must flow toward the nearest part of 

 this irregular and changing boundary, which here and there became in- 

 dented by bays of small or large extent. The intersecting striae on the 

 ledge in section 9, Delton, record very changeable glacial currents, now 

 deflected to a due south course, twenty degrees to the right from the direc- 

 tion which they had previously held through this glacial epoch, but presently 

 diverging as much or twice or three times as much to the left, attaining 

 a southeast or even a nearly east course. The medial moraine directly 

 south of this locality, in Carson and Lakeside, suggests that, when the ice 

 retreated, probably two glacial currents converged here, pushing against 

 each other, and that the striae bearing south were made by the current on 

 the east, and those bearing S. 60 to 80 E. by the current on the west. 



Divergences to the east from the prevailing direction of glaciation 

 were noted also four miles farther northwest, in Amboy and Germantown, 

 upon the northern slope and at the north base of this massive ridge. In 

 Germantown a surface about a yard square was observed, on half of which 

 the striae bear uniformly S. 30 E., and on the other half S. 70 E., as shown 

 in fig. 35, these portions meeting at a slightly beveled angle from which 



Similar curved strise are recorded and figured by Desor ( Foster and Whitney's Report on the take Superior land 

 district, part I., p. 206), and by Andrews (Am. Jour. Sc.(.3), vol. xxvi, p. 100, Aug., 1883). 



