PHYSICAL FEATURES OF THE REGION. 17 



sandstone. This was partially covered by the ice sheet. The highest 

 points within the glaciated portion seldom, if ever, exceed 900 feet, but 

 with this altitude they stand 200 to 300 feet or more above the general 

 level of the rock surface near the Wabash, only a few miles to the west. 

 This sandstone forms the western border of a broad tract of elevated land, 

 which is greatly eroded and hence is called the "hill country" of Indiana. 

 Its northern portion has had the irregularities greatly softened by glaciation. 

 The southern portion was less heavily glaciated and is nearly-as rough as 

 the unglaciated tracts. 



The district occupied by the Coal Measures, both in Illinois and in 

 Indiana, has a general altitude somewhat lower than that of the bordering 

 limestone or sandstone. Prior to the ice invasion its surface had been 

 greatly eroded, leaving- narrow divides at the water partings more or less 

 dissected into low hills, while the streams occupied broad shallow troughs. 

 These features are only partially concealed in southern Illinois and south- 

 western Indiana. As already noted, streams which flow from the Coal 

 Measures into the lower Carboniferous limestone show a marked reduction 

 in the size of the channel upon entering the limestone. This is true not 

 only of small rivers like the Kaskaskia, but also of large streams. The 

 valley of the Illinois is reduced in breadth from 8 or 10 miles to scarcely 3 

 miles on entering the limestone on the border of Pike and Greene counties. 

 The preglacial Mississippi, now partly filled with drift, is narrowed in 

 breadth from 10 or 12 miles, or possibly 15 miles, in the Coal Measures of 

 Louisa County, Iowa, and Mercer County, Illinois, to only 5 or 6 miles on 

 entering the limestone a few miles above Burlington, Iowa. Where streams 

 do not enter the limestone a gradual widening occurs toward the mouth. 

 The Wabash occupies a preglacial valley in which the breadth increases 

 gradually southward and reaches 10 or 15 miles in the vicinity of its mouth. 

 Below Terre Haute the breadth varies from 5 or 6 miles to 15 miles or 

 more, and in places the bluffs are very poorly defined, owing to the fact 

 that the strata of the bordering districts have broken down nearly as 

 rapidly as the valley was deepened. The several tributaries of the Wabash 

 in southern Illinois — the Embarras, Bon Pas, and Little Wabash — have 

 broad trough-like valleys 2 to 4 miles in width in their lower courses, and 

 their bluffs are often vaguely defined, like those of the main stream. 

 3ION xxxvin 2 



