THE BLOOMINGTON MOEAINIC SYSTEM. 279 



Gravel deposits of medium coarseness have been observed at and above 

 Lexington on the headwater portion of Mackinaw 'River near the outer 

 border of the moraine They underlie the plain for only a short distance 

 back from the river valley at these points. The extent of these deposits 

 and their relation to the moraine have not been ascertained. It seems 

 probable, however, that they are a glacial outwash. 



The latest of the moraines in the Bloomington system follows the east 

 border of the Illinois- Vermilion River throughout much of its course, a 

 position that under present conditions would afford fair escape for the 

 o-lacial waters. But at the time the moraine was forming, the channel now 

 occupied by Vermilion River had not been excavated. The broad basin 

 which it traverses has scarcely 20 feet -descent in the 40 miles from Pontiac 

 to the borders of the Illinois River. The conditions were favorable for the 

 ponding or accumulation of water issuing from the ice sheet, as well as for 

 water draining into it from the land areas on the southern and western 

 borders. A belt several miles in width might have thus become submerged 

 and a lake-like river formed. Even though the volume of water were great, 

 the force of the current would be weak until the channel had been cut 

 back several miles into the basin; The opening of this channel has been 

 very slow, for it is now but partly accomplished, the main part of the chan- 

 neling being in the portion below Streator. Evidence of a ponding of 

 waters in this basin is found in deposits of sand and silt which cover it. 

 The sand deposits are most conspicuous in the southern portion of the 

 basin, and are there drifted in places into low dunes and ridges. From 

 Pontiac southward the deposits consist of silt or fine sand. The ponding 

 of waters and deposition of sand and silt probably began with the with- 

 drawal of the ice sheet from the divide on the west border of the basin, 

 and continued until the ice sheet no longer contributed its waters to the 

 basin. This would involve not only the time when the moraine under dis- 

 cussion was forming, but also that embraced in the production of the Mar- 

 seilles moraine. Possibly the ponding continued to much later date, though 

 in less volume than at the morainic substages As shown below (p. 290), 

 there was a discharge into this basin from a small glacial lake held in the 

 basin of the Iroquois River. This line of discharge followed the east fork 

 of Vermilion River, which passes through, the moraine under discussion, 8 

 to 10 miles southwest of Pontiac. The character of the outwash appears 



