THE LOESS. 413 



integration. But the amount of kaolinized products is nearly 

 four times as great in the residuary clays as in glacial clays or 

 loess. 



The altitude of the loess deposits in the Northwest is by 

 no means uniform, and its variations present great theoretic 

 difficulties. In the lower part of the Mississippi Valley it is 

 limited to a height of about two hundred and fifty feet. In 

 the upper Mississippi Valley, however, it rises to a height of 

 seven hundred feet above the bed of the river. To account 

 for this, the first impulse would be to suppose a general de- 

 pression of the northern region. But this theory is excluded 

 by various irregularities in this region itself. For example, 

 the loess rises much higher upon the west side of the upper 

 Mississippi than upon the east side, especially in the vicinity 

 of the driftless area of Wisconsin. 



To explain this it is necessary to resort to a rather com- 

 plex theory. In part, perhaps, the peculiar distribution of 

 loess is attributable to a period of general northerly depres- 

 sion during the Glacial epoch. This apparent depression, 

 however, was probably not caused wholly by an oscillation 

 of the crust of the earth itself, since it is shown that it may 

 be due in some degree to the attraction of the ice which had 

 accumulated to the north. Mr. R. S. Woodward, of the 

 United States Geological Survey, has worked out the prob- 

 lem of attraction and its influence in producing a higher level 

 of water at the north, on the supposition that the ice-sheet 

 was ten thousand feet thick, and covered an area to the north 

 2,600 miles in diameter, and finds that the possible influence 

 of such attraction might change the water level at the ice- 

 margin thirty-eight degrees from its center as much as 573 

 feet. But the elevation of the loess attains a height in Iowa 

 and Wisconsin of 1,285 feet. This theory of change of the 

 water-level by glacial attraction, therefore, though not ade- 

 quate, very evidently comes in for a part of the credit of pro- 

 ducing the puzzling facts relating to the deposition of loess.* 



* "Driftless Area of the Upper Mississippi Valley," pp. 291-301. 



