268 THE ILLINOIS GLACIAL LOBE. 



same direction as now, would have carried much atmospheric dust from 

 them eastward. Tt is quite probable that some of the material was derived 

 from this source, though perhaps only a minor part. It is found that this 

 loess loam, when reaching- a depth of 4 to 6 feet or more, is usually highly 

 calcareous in its lower portion, while the loess of the outer-border districts 

 is thoroughly leached to a depth of several feet. In all probability it had 

 suffered considerable leaching before the ice sheet withdrew from the 

 Bloomington morainic system. It is to be expected, therefore, that a non- 

 calcareous or leached deposit would be made by transportation of dust 

 from these plains. The presence of the calcareous material in the silt 

 which caps the western border of the Bloomington till sheet seems to make 

 it necessary to call in the action of glacial waters charged with fresh cal- 

 careous silt. The manner in which the silt was distributed is an unsettled 

 question and one on which further light seems necessary. It will probably 

 be found in a combination of aqueous and asolian agencies. As shown 

 below, the drainage conditions on the outer border of the Bloomington 

 system were favorable for the transportation of gravel by streams issuing 

 from the ice margin. There would appear, therefore, to have been a rapid 

 descent for these streams to the region south and west from the ice sheet. 

 Such being the case, we can scarcely infer that the loess which covers the 

 elevated parts of the morainic system was deposited by a sheet of water, 

 for this would imply a general submergence. It is suggested that there 

 may have been portions of the border district in which the waters found 

 inadequate drainage. In such places silts may have been spread out which 

 were afterwards transported by wind to the moraine. The matter, however, 

 is one of conjecture rather than of demonstration. 



Surface bowlders are numerous only at a few points on this morainic 

 system, being rarely met where no sand or silt deposits are present to 

 conceal them. But in this respect the Bloomington system is not different 

 from other moraines of the early Wisconsin series. In Kane, Dekalb, 

 and Ogle counties there are occasional bowlders along the crests of the 

 ridges, but they seldom become conspicuous. On the plane tracts in these 

 counties they are quite rare, but this is not a fair field for study because 

 there is usually a sufficient amount of silt to conceal them. Bowlders 

 abound along the outer face of the outer belt for a few miles in south- 

 eastern Lee County and on the crest and outer face at a few points in 



