Malott: The ''American Bottoms'' Regiov 



in places over the old valley the Illinois glacial till was de- 

 posited on top of the valley-fill material. Thus, along Eel 

 River the material which fills the valley is at least middle 

 Pleistocene in age. This harmonizes with the idea that most 

 of the delta building of the Mississippi River in the Gulf 

 Embayment occurred in the earlier part of the Pleistocene. 



Should the entire Mississippi valley be depressed 100 feet, 

 the Gulf waters would extend as an embayment up the lower 

 Mississippi valley as far as the north line of Louisiana, about 

 mid-way between Greenville and Vicksburg. The lower 575 

 miles of the present Mississippi River would be submerged. 

 This submergence would allow the Mississippi River to en- 

 trench itself not less than 30 or 35 feet at Cairo and the 

 entrenchment near the mouth of the Wabash would be some- 

 what like 40 feet. This cutting down would take place be- 

 cause of the greater proximity of the interior regions to the 

 sea in comparison to the present distances. It simply means 

 that the low grade of the lower Mississippi would be trans- 

 ferred into the interior regions whereas the present grade 

 is fairly high in comparison. The grade of the lower Missis- 

 sippi would be transferred some 500 miles farther into the 

 interior. Thus, a moderate depression of the Mississippi val- 

 ley would not result in valley filling as is ordinarily postu- 

 lated, but would result in valley cutting, just the reverse of 

 the ordinary interpretation. In order to account for valley 

 filling by regional depression care must be exercised in choos- 

 ing the sort of depression to be postulated, and limits must 

 be set to its application. It is only logical to infer that should 

 the mouth of the Mississippi have been at a position near the 

 north line of Louisiana, the well-graded streams of the middle 

 interior Mississippi valley, like those of southwestern Indiana, 

 must have flowed some 100 or 150 feet low^er than at present, 

 and the extension of the Mississippi River by delta building, 

 or by some other method, must have caused the filling up of 

 the valleys as vv^e see them today. 



The two interpretations of valley filling in southwestern 

 Indiana outlined above have been presented rather as if either 

 one alone could account for the phenomenon. The intention, 

 however, has been to emphasize the latter one, but not to such 

 an extent as to preclude any other interpretation. There is 

 considerable evidence of differential crustal movements of a 



