526 THE ILLINOIS GLACIAL LOBE. 



about 500 feet above tide, while the stream falls to about 450 feet. In the 

 lower half the watershed is diversified by drift ridges and knolls which rise 

 abruptly in some cases to a height of 75 feet or more above border dis- 

 tricts. These ridges for a few miles in southeastern Madison County con- 

 stitute the east border of the watershed, but just south of the line of 

 Madison and St. Clair counties the stream passes through the main belt of 

 ridges, and has but few prominent ridges and knolls on its east side below 

 that point. At its mouth the stream has an elevation of only 370 feet, and 

 the border districts, aside from knolls, stand scarcely 400 feet above tide. 

 This stream, like Shoal Creek, appears to be largely independent of 

 preglacial drainage lines. It trenches into the rock at numerous points 

 along its course, and its immediate bluffs stand at the general level of bor- 

 dering uplands. There may, however, have been a preglacial divide near 

 the headwaters of the creek. 



BIG MUDDY RIVER DRAINAGE BASIN. 



The onl} r remaining important tributary of the Mississippi is the Big 

 Muddy, a stream draining about 2,400 square miles in the low district lying 

 north of the " Ozark Ridge." It is the line of discharge for the greater 

 part of Williamson, Franklin, Jefferson, Perry, and Jackson counties and 

 the southeastern part of Washington and the southern part of Marion 

 County. The lower 20 miles of its course lies within the Mississippi 

 bottom. 



With the exception of the elevated district on the south border, which 

 stands 600 to 800 feet above tide, this watershed has few points rising 

 above 550 feet. It stands mainly between 400 and 500 feet above tide. 

 The immediate borders of the main valley fall below 400 feet, and the 

 mouth of the stream at low water in the Mississippi is but 320 feet. 



The principal tributaries of Big Muddy River are Beaucoup Creek and 

 Little Muddy River, which drain the western side of its watershed. An 

 eastern tributary — Crab Orchard Creek — drains about 250 square miles of 

 the district bordering the "Ozark Ridge." 



Throughout the greater portion of its course Big Muddy River occu- 

 pies a preglacial line of drainage, and meanders about in the broad bottoms 

 which have been filled with drift and alluvium to an elevation of 50 to 100 

 feet or more above the rock bottom. Just below Murphvsboro, however, 



