well county and extends across nearly the whole of Mason 

 county to the Sangamon, with a niaximuni width of eighteen 

 miles near the upper end, narrowing southward to al)Out ten 

 miles. The lower part — that below the Sangamon — forms a 

 minor expansion of about half this width in western Cass 

 county and the northwestern coi-ner of Morgan county. 



The total area is approximately seven hundred s(juare 

 miles. Except for light superficial deposits its substance is 

 largely sand, which usually reaches to considerable depths. 

 The surface exhibits broad level areas capped by a shallow but 

 rich soil, many of which were originally wet or swampy, but 

 are now drained and cultivated. These alternate with lai'ge 

 areas of surface sand — the great sand-bars of the glacial river — 

 drifted by long-continued wind action into irregular, undulat- 

 ing dunes, often barren and de.solate, which have traveled 

 northeastward with the prevailing winds. Freciuently these 

 dunes form long ridges parallel to the general direction of the 

 valley. Even the l»ordering uplands are frequently capped with 

 small sand-dunes and ridges which presumably came from the 

 adjacent valley-margin. In drifting with the wind these sand 

 masses have of course overridden to some extent the soil of the 

 level areas, but were probably originally continuous with the 

 underlying .sands. 



THE SURFACE SANDS. 



The surface sand begins at the north with a few narrow 

 strips along the river near Pekin, al)ove the mouth of the 

 Mackinaw. Below this river the sand-covered area is (juite 

 large. Only an estimate can be given of its actual extent. 

 The governmental Soil Survey (Bonsteel, '03 ) reports 22,976 

 acres of it in southwestern Tazewell county, within a tlood- 

 plain area of about eighty square miles, or 51,200 acres. The 

 sand therefore occupies rather more than forty percent of the 

 total flood-plain area in this county. This ratio is doubtless 

 larger in Mason county, which has not yet been examined by 

 the Soil Survey, but accepting it for the entire bat^in of seven 



