THE BLOOMINGTON MOttAINIC SYSTEM. 289 



suggest the interpretation that it is a beach line. However, this interpreta- 

 tion scarcely affords a satisfactory explanation of the mode of deposition of 

 the silty mantle; hence the question of its mode of formation is left open. 

 There is a lower ridge following the brow of the Illinois bluff, which 

 is well displayed in South Ottawa, and eastward from there to the border 

 of the Marseilles moraine. Its altitude, as shown by the Ottawa topo- 

 graphic sheet, is very nearly 610 feet above tide. Along the north bluff 

 of the Illinois, about midway between Ottawa and Marseilles, there is 

 another beach-like ridge leading- from the outer- border of the Marseilles 

 moraine westward to the border of Fox River Valley, which is represented 

 to stand a few feet lower than the ridge just noted, its altitude being 600 

 feet or less. 



ONARGA RIDGE. 



In the Iroquois Basin there are several small ridges of sand presenting 

 the appearance of -beach lines, some being found in the northwest part of 

 Iroquois County, a short distance south of the Marseilles moraine, and 

 others on portions of the plain farther south. The best-defined and most 

 elevated ridge noted is one leading from Onarga westward through Ridge- 

 ville to one of the headwater forks of Vermilion River, the west end being 

 near the line of Iroquois and Ford counties, about 4 miles southeast of 

 Piper. Throughout this distance of. about 8 miles it has a width of only 

 40 to 80 rods and a height of 15 or 20 feet or less. As far as could be 

 ascertained from the slight exposures and records of occasional wells it is 

 composed entirely of sand. It is underlain at the level of its base by a 

 bluish-yellow, silty, pebbly clay, which grades below into blue till. In 

 some places the sand is immediately underlain by blue till. As a rule the 

 ridge has a smooth surface and gentle slopes, but in places the sand is 

 drifted into low dunes 5 or 10 feet in height. The sand is of a brown color 

 and so far as tested shows no effervescence with acid-. A short distance east 

 from the eastern edge of this ridge a belt of low sand dunes sets in, which 

 has a width of 1 to 2 miles. This belt leads eastward to the vicinity of 

 Watseka, and thence northward and eastward to the Kankakee Basin, 

 occupying much of northeastern Iroquois County. 



The elevation of the Onarga Ridge is about 675 feet above tide, but the 

 dunes to the east seldom reach this elevation, the district covered by them 



3ION XXXVIII 19 



