15(5 THE ILLINOIS GLACIAL LOBE. 



decreases in thickness, especially on the Iowa side of the river. From a 

 thickness of 40 feet at Muscatine it decreases to but 10 or 12 feet at 

 Burlington and to about 6 feet at Fort Madison. On the east side of the 

 river, however, the loess maintains a thickness somewhat greater than on 

 the west, there being immediately opposite Fort Madison twice as thick a 

 deposit as on the Iowa side. This extra thickness is perhaps attributable 

 to whid, for along the eastern bluff of the river there are dunes composed 

 of fine sand drifted by the wind from the broad bottoms. The prevailing 

 wind being from the southwest, the dunes are found only on this bluff. 



The Illinois Valley is bordered both on the east and west below the 

 mouth of the Sangamon River by heavy accumulations of loess, 40 to 50 

 feet or more. But upon passing back a distance of 5 or 10 miles from the 

 stream, on either side, the thickness becomes reduced to 8 or 10 feet. The 

 heavy loess of the Illinois and Mississippi valleys, together with a belt 

 along the Missouri valley, continues down the Mississippi to the Gulf of 

 Mexico. On the borders of southern Illinois the thick loess is confined to 

 a belt but 5 or 10 miles in width, the thickness farther east being scarcely 

 one-fourth as great as on the immediate borders of the stream. In central 

 and southern Illinois the thickness of the loess seldom exceeds 10 feet and 

 probably averages not more than 6 feet. The Kaskaskia, Embarras, and 

 other streams of south-central and southern Illinois do not have such heavy 

 belts of loess on their borders as characterize the Illinois, Mississippi, and 

 Missouri valleys. There is but little thickening upon approaching the 

 Wabash River, the average t thickness along the west bluff of the stream 

 being less than 10 feet. 



On the lower portion of the Ohio from near Rockport, Indiana, to its 

 mouth, loess deposits are 15 to 25 feet in thickness, but above Rockport 

 the thickness is seldom more than 5 or 6 feet, and this thickness prevails 

 over southeastern Indiana and southern Ohio. 



STRUCTURE. 



The loess and associated silts are not so uniform in structure as might 

 be inferred from some of the published descriptions. The earlier descrip- 

 tions apply to a very porous deposit found on the borders of the large 

 valleys which was the first to attract the notice of geologists and which 

 mav he designated bluff loess. The great extent of the surface silt over 



