548 THE GEOLOGY OF MINNESOTA. 



[Glacier-marks. 



It seems almost impossible that in so level and open a country, and 

 on the same rocks, without apparent cause, the glacier which must have 

 been hundreds of miles wide,lf it existed here at all, could have taken such 

 diverse directions in so short distances. It cannot be doubted, however, 

 that this marking was done by a force that exerted a great pressure at the 

 same time that the marks were made. This pressure is evinced not only 

 in the marking itself, which is on the hardest formation found in the state, 

 but in the minute cross-fractures that cover the surface where this rasping 

 has taken place, and yet leave it in the main a smoothed and moutonned 

 surface. These cross-fractures run curvingly downward at varying angles 

 with the surface, and to all depths less than an inch, but usually less 

 than one-sixteenth of an inch, and indicate perhaps an incipient crushing 

 to the depth of at least an inch. They show in what manner the rasp- 

 ing reduced the original projecting knobs. Where the natural seams or 

 planes of jointage cross the rock, causing the quartzyte to chip off sooner 

 and deeper with a curving and conchoidal fracture, these little checks are 

 larger. Their prevailing direction is transverse to the rasping force, so 

 that the rock, along some grooves, has a short conchoidally fractured 

 structure transverse to the grooves, penetrating it to the depth of a quarter 

 to half an inch, exhibited now in a series of little curving furrows where 

 the laminae broke off successively, the convexities of the laminae being to- 

 ward the north.* 



NORTH. 



SOUTH 

 FIG. 43. GLACIAL MARKINGS ON THE BED QUARTZYTE. 



This marking is represented in fig. 43, but the figure does not show a 

 great many fine checks with which the surface of the rock is nearly cov- 

 ered. It shows correctly the prevailing direction of the curvature, and its 

 relation to the moving force. This manner of glaciated marking is visible 



"Compare the sixth annual report, p 107. Dr. E. Andrews has recently described a similar cross- fracture strintion 

 seen on the northeast shore of lake Huron on a aimi'ar rock; his observation makes the convexities of the lamina- 

 turned toward the moving force. Bulletin of the Chicayo Acad. of Set., vol. i, No. 1. Am. Jour. Sci., (3), xxvi, 101. 



