PHYSICAL FEATURES OF THE REGION. 15 



5 to 10 miles wide and stands 650 to 750 feet above tide, while the Coal 

 Measures district bordering it stands only 450 to 600 feet above tide. It is 

 interrupted by gaps 2 miles or less in width where the Big Muddy and the 

 Kaskaskia pass through it to the Mississippi; otherwise it forms a continuous 

 belt. Its altitudes are no greater than those of the district across the river 

 in Missouri. On the contrary, there is a rise in that direction to the Iron 

 Mountain district of southeastern Missouri. It seems remarkable that the 

 Mississippi should have taken a course across this limestone belt, and as yet 

 no satisfactory explanation for this feature has been found. The stream is 

 apparently in a preglacial valley. Its course seems, therefore, to be inde- 

 pendent of glaciation. This belt, like the ridge of southern Illinois, seems 

 to mark the limit of the ice sheet. Drift in small amount occurs over most 

 of its surface, but, so far as known to the writer, does not extend beyond 

 the Mississippi. Near St. Louis, however, where the limestone border passes 

 to the west side of the river, drift is found in small amount. 



Passing up the Mississippi to the mouth of the Illinois, a narrow axis 

 of upheaval is found, trending nearly east and west, along which the altitude 

 is somewhat greater than on bordering districts. Just east of the mouth of 

 the Illinois there are a few points where the rock surface rises to about 800 

 feet, while on the plain north of this ridge the rock scarcely exceeds 650 

 feet. West from the Illinois the altitude is not markedly greater at this axis 

 than to the northward, there being a narrow limestone ridge between the 

 Mississippi and the Illinois through the entire length of Calhoun County, 

 whose crest is generally 700 to 750 feet above tide. The altitude of the 

 ridge west of the Illinois is fully 100 feet greater than that of the bluffs 

 immediately east of that stream. These elevated limestone ridges, and 

 similar ridges on the Missouri side of the Mississippi, near the town of 

 Louisiana and southward, carry very little drift and, as noted some years 

 since by Salisbury, may not have been completely covered by the ice 

 sheet. 1 



Of the three belts just described two have been sculptured very deeply 

 by surface erosion, viz, the ridge crossing southern Illinois and the ridge 

 near the mouth of the Illinois. The remaining belt (along the Mississippi 

 below St. Louis) is less deeply sculptured because in much of its extent 

 underground drainage through sink holes and caves has been established. 



i Proc. Am. Assoc. Adv. Sci., Vol. XL, 1891, pp. 251-253. 



