Malott: The ''American Bottoms" Region 



?A 



terial which had been brought from the land mainly by the 

 master stream. The present delta head is some distance in 

 the interior, but, assuming that at one stage in the history 

 of the embayment area the mouth of the Mississippi River 

 was near this position, the Gulf waters once extended north- 

 ward in a broad shallow embayment to this position. With 

 the southward extension of the delta into this embayment, 

 the graded portion of the stream had to adjust itself to the 

 new condition. This was done by building up the stream 

 bed, which in the course of time resulted in filling up the 

 entire valley. Since the delta continually extended the river 

 southward, the valley at the delta head was continually being 

 built up. Today it stands 50 feet above sea level. Like- 

 wise the valley was filled up to a corresponding depth as far 

 back up stream as its grade had previously been perfected. 

 The graded condition of the streams extended well into south- 

 western Indiana. Thus the extension of the present delta 

 of the Mississippi River in itself will account for a filling 

 amounting to something like 50 feet. The vital question here 

 is : Just where is the real head of the Mississippi River delta ; 

 that is, just where Avas it originally in the Gulf Embayment 

 area? 



An important feature to be considered is the date of the 

 valley filling. This refers to the time when the chief delta 

 extension took place. i\Iississippi valley physiography is in- 

 timately associated with the Pleistocene glaciation. The re- 

 peated extensive glaciation of the upper portion of the valley 

 where the major tributaries have their origin has determined 

 almost wholly the present features. Bordering regions give 

 a time check on the Pleistocene epochs. The lower Missis- 

 sippi valley was greatly affected by glaciation. One of these 

 eft'ects is the associated loessial deposits. A chief effect must 

 have been the bringing diOvai of great quantities of debris 

 by the glacial waters. The coming of the glacial ice from 

 the north with its successive loads of debris and its attendant 

 fluviatile waters obliterated to a large extent the pre-glacial 

 drainage systems, and gave rise to new ones, or greatly modi- 

 fied or enlarged the old ones, with waters centered in the 

 master stream toward the head of the Gulf Emba^mient. 

 The result was the extension of the Mississippi delta into 

 the Gulf Embayment by enormously rapid delta building, 



