376 THE ILLINOIS GLACIAL LOBE. 



of cobble and coarse gravel. The belt occupies a width of a mile or more 

 and the gravel is in places built up 60 to 75 feet. In other places it has a 

 depth of only 10 or 20 feet. The average depth is probably not less than 

 40 feet. 



On the west Dupage River, which throughout much of its course fol- 

 lows the outer border of the Valparaiso morainic system, there are deposits 

 of gravel and cobble occupying a belt a mile or so in width, which connect 

 at the south with a still broader belt of gravel. On the slopes of the 

 moraine immediately east of this stream there are numerous gravelly knolls, 

 which connect with the plain of gravel on the borders of the river in such 

 manner as to indicate that the}' should be included with the glacial drain- 

 age, though the}' suffered so little transportation as to make the term 

 "outwash" seem scarcely appropriate. The lower Dupage River appears 

 to have been an avenue of discharge, not only for glacial waters from the 

 Upper Dupage but also from the neighboring section of the Des Plaines 

 River, as indicated below. 



On the Des Plaines Valley the gravel filling apparently reached such a 

 height that a delta-like system of streams was formed in the vicinity of 

 Joliet, some of which were thrown across the low divide on the west into 

 the Dupage River. Along the courses of the glacial streams, well-defined 

 valleys were formed which are now occupied by marshes and insignificant 

 tributaries of the Des Plaines and Dupage. The position, size, and relation 

 of these abandoned valleys are set forth on the Joliet topographic sheet of 

 this Survey. One valley occupied by the Isle la Cache Creeks has a summit 

 level slightly above the 600-foot contour; another, drained by Mink Creek, 

 has a summit level about 620 feet; a third, now largely unchained but con- 

 necting at the west with Rock Run, was cut down to about 575 feet. The 

 one drained by Mink Creek seems to have been cut no lower than the upper 

 limit of the gravel filling on the neighboring portion of the Des Plaines, but 

 the others were cut a few feet below the level of that filling. 



On the immediate outer border of the main ridge of the Valparaiso 

 morainic system, near Romeo, the gravel filling reaches about 620 feet, and 

 this appears to be the head of the gravel, though the great excavation sub- 

 sequently made by the Chicago Outlet leaves the question of the precise 

 head somewhat uncertain. Upon passing down the Des Plaines the gravel 

 filling shows a marked descent. At Joliet Mound, 2 miles below Joliet, it 



