THE BLOOMINGTON MOBAINIC SYSTEM. 273 



the level of the gravel surface, showing that the deposit is thin. The 

 western tributary also has gravelly deposits above the point of its emergence 

 from the moraine, but these were not traced to their head. The distance to 

 which the gravel deposits have been carried down the two branches of 

 Sugar Creek beyond the border of the moraine is not ascertained, but it is 

 known to be at least 10 miles. 



Mackinaw River has its source on the inner border of the main ridges 

 of the Bloomington system, but Cropsey Ridge, one of the weaker mem- 

 bers of the system, lies north of the headwater portion of the stream. 

 Gravelly deposits have been noted at a few points on the borders of the 

 headwater portion. They have not, however, been traced into definite 

 connection with Cropsey Ridge. There is not a continuous belt of gravel 

 leading down the valley from this headwater portion. A section several 

 miles in length was examined just below the crossing of the Lake Erie 

 and Western Railway, in which no gravel filling appears to have been 

 made. That portion of the valley was found to contain deposits of silt 

 of considerable thickness which reach a level 20 or 30 feet above the 

 present stream. Upon continuing down the valley to the inner border of 

 the outer morainic ridge a gravel terrace is found to set in abruptly at an 

 altitude about 50 feet above the stream. This terrace merges into low 

 gravelly knolls at its head and on its border, and thus makes a definite con- 

 nection with the moraine. As it stands somewhat higher than the silt-filled 

 portion of the valley just above its head, there was probably a pool in that 

 portion of the valley prior to the excavation of the gravel which forms the 

 moraine-headed terrace, and the silt deposits just noted were probably laid 

 down in this pool. 



Mackinaw Valley appears to have been excavated nearly to the level 

 of the present stream prior to the formation of the Bloomington system and 

 to have had a width of nearly a mile. The terrace has been traced down 

 the valle3'' continuously from the moraine to the point where the Mackinaw 

 enters the Illinois Valley in the eastern part of Sand Prairie Township, a 

 distance by direct line of about 17 miles from the outer border of the 

 moraine. The altitude of the ten-ace decreases about 100 feet in this dis- 

 tance, being 640 feet at the outer border of the moraine and 550 feet at the 

 point where it joins the Illinois Valley. The stream falls 83 feet in the 



MON XXXVIII 18 



