SURFACE DEPOSITS. 93 



rated, the scratches and other traces of glacial action that were 

 once donbtless abundant npon them, have to a great extent 

 gradually become obliterated. For this reason, together with 

 the fact that the drift so generally and deeply covers those 

 surfaces which may be supposed to yet contain such traces, 

 we seldom have an opportunity to observe them in our State. 

 The only places where such scratches have been observed upon 

 ledges of rock in Iowa, thus far, are upon the Burlington 

 limestone — sub-Carboniferous — near Burlington; upon the 

 Upper coal-measure limestone, in Mills and Pottawattamie 

 counties, and upon the Sioux quartzite in Lyon county. 

 Boulders, having their sides flattened and striated are how- 

 ever found in various parts of the State, and at great distances 

 from the localities just mentioned. 



We observed a very interesting fact in southwestern Minne- 

 sota in connection with the glacial-scored surface of the Sioux 

 quartzite. This rock, as before said, is intensely hard, almost 

 glassy in fracture, and has the peculiarity of containing 

 numerous vertical fissures at right angles to the plane of 

 stratification. These fissures or cracks were observed to have 

 their angles splintered in a peculiar manner when their direc- 

 tion was eastward and westward. If in other directions, 

 particularly northward and southward, they were compara- 

 tively intact. These fissures are really but mere cracks, the 

 vertical faces being only very slightly separated, so that they 

 form sharp right angles with the upper surface. These angles 

 were found to be unbroken upon the distal or southern side of 

 the cracks, but on the northern or proximal side the angle was 

 alwas chipped off, as if by the presence of some ponderous, 

 southerly -moving, unyielding body, the chips remaining in 

 the fissure. It is only in such hard and glassy rock as this 

 that such effects could be produced, or that would admit of 

 such distinct preservation after they were produced. The 

 instruments which produced this smoothing, scoring, and 

 chipping of the quartzite, were the granite boulders which were 

 frozen firmly into the bottom of the glacier that once moved over 

 it, and the power was the moving glacier itself. These granite 



