304 



EARTH FEATURES AND THEIR MEANING 



for another. In the gravings upon the glacier pavement, earlier 

 records have been likewise in large part effaced by later, though in 

 favorable localities the two may be read together. Thus, as an 

 example, at the great limestone quarries of Sibley, in south- 

 eastern Michigan, the glaciated rock surface wherever stripped of 

 its drift cover is a smoothly polished and relatively level floor 

 with striae which are directed west-northwest. Beneath this gen- 

 eral surface there are, however, a number of elliptical depres- 

 sions which have their longer axes directed south-southwest, one 

 being from twenty-five to thirty feet long and some ten feet in 

 depth (Fig. 330). These boat-shaped depressions are clearly the 



remnants of an earlier 

 more undulating sur- 

 face which the latest 

 glacier has in large 

 part planed away, 

 since the bottoms of 

 the depressions are no 

 less perfectly glaciated 

 but have their striae 

 directed in general 

 near the longer axis of 

 the troughs. Palimp- 

 sest-like there are 

 here also the records 

 of more than one 

 graving. 



The dispersion of the drift. Long before the " ice age " had 

 been conceived in the minds of Agassiz and his contemporaries, 

 it had been remarked that scattered over the North German plain 

 were rounded fragments of rock which could not possibly have been 

 derived from their own neighborhood but which could be matched 

 with the great masses of red granite in Sweden well known as the 

 " Swedish granite." Buckland, an English geologist, had in 1815 

 accounted for such " erratic " blocks of his own country, here of 

 Scotch granite, by calling in the deluge of Noah ; but in the late 

 thirties of the nineteenth century, Sir Charles Lyell, with the results 

 of English Arctic explorers in mind, claimed that such traveled 

 blocks had been transported by icebergs emanating from the polar 



FIG. 330. Limestone surface at Sibley, Michigan. 



