4 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 304 



The foregoing descriptions apply to the internal stratigraphy of the Roxana 

 silt only where it is thick, near the bluffs of the Illinois, Sangamon, and Missis- 

 sippi River valleys. East of these valleys it thins rapidly (much more rapidly than 

 the overlying Peoria loess), becomes leached throughout, and its internal subdivi- 

 sions are not recognizable. 



The geographic distribution of thick, fossiliferous Roxana silt is significant. 

 It occurs from the region just above the mouth of the Sangamon River southward along 

 the Illinois River valley and the Mississippi River valley below the mouth of the Il- 

 linois. It is absent farther north along the Illinois River valley, but is exposed 

 along the Sangamon River valley eastward to the mouth of Salt Creek and for a con- 

 siderable distance up Salt Creek valley. This distribution pattern suggests that out- 

 wash in the pre-Shelbyville course of the Illinois River was the source of the loess 

 that constitutes the Roxana silt. 



Farmdalian Substage 



The Farmdale silts are the deposits next younger than the Roxana. The orig- 

 inal type area (Leighton, 1948) is retained. The unit is well exposed in the Farm 

 Creek railroad cut section (Frye and Willman, 1960). The Farmdale generally consists 

 of noncalcareous silt, massive to laminated, pinkish brown to pale purple; it char- 

 acteristically contains humic or woody material (Banner section). In some places 

 it is very peaty and in some places is a compacted mass of twigs and limbs of trees. 

 Although locally it may be in part or entirely ("The Rocks" section) eolian loess, it 

 is thought to consist largely of silts derived by wash from the older Roxana and 

 earlier deposits. In some places (Wedron section) the Farmdale silts were deposited 

 in ponded water and are fossiliferous and somewhat calcareous. However, even in 

 such places they retain the characteristic plant material and predominant composition 

 of silt. 



Woodfordian Substage 



The loesses of the Woodfordian Substage that occur above the Farmdale silts 

 have been classed in three rock-stratigraphic units; the Morton loess, the Peoria 

 loess, and the Richland loess (Frye and Willman, 1960). Beyond the Shelbyville 

 moraine, which marks the outermost extent of Woodfordian glaciers, the loess of 

 this substage is not readily subdivisible on lithologic evidence and therefore is 

 all classed together as Peoria loess. Peoria loess is described in 9 of the 11 

 measured geologic sections included with this report. It consists of tan to yellow- 

 tan, massive, calcareous, fossiliferous silt. In some places along the major valley 

 bluffs (Browns Mound section) it contains lenticular masses of sand and is indis- 

 tinctly bedded, generally with the bedding inclined away from the valley bluff line 

 (Cottonwood School section; Bluffdale section). Where the Peoria loess is quite 

 thick and well exposed it commonly contains incipient soils, or humic streaks, in 

 the mid-portion and higher (Frederick South section; Cottonwood School section and 

 exposures one mile east; exposures at Peters). One of the incipient soils included 

 within the Peoria loess was described by de Heinzelin (1959, p. 271) . Although the 

 color contrast, partial leaching, and small secondary nodules of CaC03 make these 

 minor soil zones conspicuous in some exposures, a recognizable B-zone has not so 

 far been found associated with them. In a few exposures as many as half a dozen 

 humic streaks have been observed in the upper part of the Peoria loess. 



