EXTENT OF IOWA PORTION OF 10 WAN DRIFT. 149 



uncertain, prop to support the hypothesis of an extension of ice from Iowa 

 into Illinois at the Iowan stage of glaciation. 



The elevated tract just noted stands near the south end of a strip which 

 is nearly free from loess. The strip extends northward several miles into 

 Carroll County and has a general width of only about 2 miles. It is in 

 decided contrast with the thick belt of loess on the west which follows the 

 east bluff of the Mississippi and probably averages not less than 30 feet in 

 average depth. It is also in contrast with the district on the east which for 

 a distance of several miles back is covered to a depth of 12 to 15 feet with 

 loess. On the south also there is heavy loess separating this tract from a 

 similar tract south and east of Morrison. The tract with scanty loess is in 

 an exposed situation, but apparently no more so than the bluff of the Mis- 

 sissippi both to the west and south. It, therefore, seems difficult to account 

 for its scant deposition b^y the force of the wind. It stands like the fresh 

 till as a feature out of harmony with the general features of the region. 

 It seems, however, to have a parallel in the region of Iowan drift to the 

 west, where similar areas free from loess are bordered by loess-covered 

 tracts. In this connection it may be remarked that the belt of loess along 

 the Mississippi bluff leads down the river from far to the north and may be 

 somewhat later than the loess to the east, and possibly subsequent to the 

 occupancy of eastern Iowa by the Iowan ice sheet. 



East of this tract characterized by thin loess is a narrow sandy belt in 

 which dunes are common. This belt is best developed in Clyde Township, 

 Whiteside County, where it has a breadth of fully 1 mile. Its southern 

 end is found at Rock Creek Valley about 5 miles above Morrison, as indi- 

 cated on PI. XII. From this point it bears slightly west of north into 

 Carroll County, lying mainly east of the valley of Little Creek. Its general 

 altitude is slightly lower than the tract on the west, but it has about the 

 average elevation of the region, being not less than 200 to 250 feet above 

 the Mississippi River. It is so far removed from the river as to be outside 

 the range of the aeolian deposits which in places accumulate on the east 

 bluff. The best developed portion, as may be seen by reference to PI. XII, 

 stands 6 to 10 miles back from the river bluff and is separated from it bv 

 the still more elevated tract just discussed. 



On the south, east, and north borders of the sandy belt there is a 

 blanket of loess 12 to 15 feet thick near the margin of the sand, but decreasing 



