92 PHYSICAL GEOGRAPHY. 



centre of the State, and there it then swept more directly to the 

 southeastward. 



Again, occasionally in the central and eastern parts of the 

 State we find loose masses of buff colored magnesian lime- 

 stone in the drift, which, in addition to its lithological 

 identification with strata in Minnesota and northeastern 

 Iowa, we have the evidence also of the contained fossils that 

 they are identical with and derived from those ledges. 



Evidences of Glacial Origin. Drift scratches and other 

 phenomena observed in relation to that deposit are regarded 

 as quite unmistakable evidences of its glacial origin since we 

 have become better acquainted with the character of the 

 glaciers of Greenland and the Alps ; but they are in reality 

 no more so than the great fact of the existence of the drift 

 itself, but which loses its force as an illustration on account 

 of our familiarity with it, now that it is no longer associated 

 with ice, and of our unfamiliarity with glaciers in action. 

 The drift scratches are found both upon the upper surfaces of 

 the rocks in their natural position and upon the flattened 

 surfaces of the boulders themselves. The flattening of the 

 surfaces, together with their scratches, were produced upon 

 each respectively by rubbing upon each other, the boulders 

 having been frozen firmly into the bottom of the glacier while 

 it was moving over the ledges, 



The scratches vary in distinctness with the difference in the 

 character of the rock acted upon. Upon limestone, the scored 

 surfaces are usually quite flat and the scratches distinct. 

 The scratches vary from mere lines to grooves of such a depth 

 that one might lay his finger within them, all being as straight 

 and parallel as the joints in a floor. Upon granite, the rock 

 being harder, they are less distinct and deep, but still, they 

 have the same general character. Those upon the Sioux 

 quartzite are less distinct than any others on account of the 

 extreme hardness of the rock, but yet they are quite unmis- 

 takably of the same origin. 



On account of the friable character of a large part of the 

 rocks of Iowa, or the facility with which they become disinteg- 



