ILLINOIS RIVER DRAINAGE BASIN. 509 



70 square miles. Like the headwaters of Mazon Creek, they flow north- 

 eastward down the slope from the moraine to the plain. 



It will be observed that Mazon Greek and these streams drain the south 

 part of the Morris Basin. A portion of this' area, appears on the Morris 

 topographic sheet. 



FOX RIVER. 



For about 20 miles in the vicinity of the Marseilles moraine the Illinois 

 receives no tributary worthy of mention, as may be seen by reference to 

 the Marseilles topographic sheet. But just west of the Marseilles moraine, 

 at a distance of 33 miles from the head of the Illinois, Fox River is received. 

 Its drainage area is about 2,500 square miles, or more than one-third as 

 great as the portion of the watershed of the present Illinois above the 

 mouth of this tributary. It lies mainly in Illinois, but the source of the 

 stream is in southern Wisconsin. 



For a distance of nearly 75 miles from its source Fox River drains 

 only a narrow strip among the morainic ridges of the composite belt pre- 

 viously described. In this portion of its course its fall amounts to but a 

 few inches to the mile, and it expands at frequent intervals into lakes and 

 marshes, between which are short spaces having a narrow and well-defined 

 channel. Near the point of divergence of the Marseilles moraine from the 

 Bloomington morainic system above Elgin, Illinois, the river begins a rapid 

 descent to the low plain that lies on the outer border of the Marseilles 

 moraine, and follows this plain to its mouth. 



The stream has no valley until it begins the descent to this plain. It 

 there for a few miles has cut to a depth of nearly 100 feet, but in the 

 passage through the plain its bed is sunk to a depth of only 40 or 50 feet, 

 except for a few miles near its mouth, where it cuts deeper to enter the 

 Illinois. The valley is also narrow throughout its entire length, and pre- 

 sents a conspicuous contrast to the broad valley of the upper Illinois. Its 

 channel even in the lower 75 miles has a breadth of only about one-eighth 

 mile and a depth scarcely half as great as that of the neighboring portion 

 of the Illinois, yet it is better favored than the Illinois for the development 

 of a drainage line, there being no morainic ridge to cross and a proportion- 

 ately small amount of rock to excavate. Instead of an excavation one-third 

 as great as that of the upper Illinois, this stream has accomplished scarcely 

 one-sixteenth as much work. These contrasts are well shown in the Ottawa 



