Malott: The "American Bottoms'' Region 29 



It is conceded that valley filling would be the result of 

 regional depression, and such an interpretation of valley fill- 

 ing in southwestern Indiana and associated regions is a most 

 natural and simple one, especiallj^ when looked at from the 

 standpoint of any limited region. But when the question is 

 considered from the standpoint of the entire area affected, 

 other causes than regional depression are suggested. These 

 causes call into account processes, which, if not now in opera- 

 tion, have undisputably been in operation in the past, and 

 were in themselves alone capable of accounting for the filled 

 valleys in southwestern Indiana. This alternative explana- 

 tion will now be discussed. 



Valley Filling Interpreted as a Result of the Seaward Ex- 

 tensio7i of the Master Stream. Large streams, like the Mis- 

 sissippi River, which carry enormous quantities of sediment 

 and empty directly into a great embayment of the sea, must 

 in the course of time change their regimens miarkedly. Such 

 streams must always maintain a grade that will permit them 

 to carry their burden to the sea. The Mississippi river is 

 contributing enormous quantities of material to the Gulf, but 

 at present there is little extension of the delta seaward on 

 account of the intensity of wave action on the now altogether 

 peculiar, slender, protruding delta of the main distributaries. 

 This peculiar, protruding, crow-foot delta, perhaps now some- 

 what artificially encouraged in its idiosyncracies, is added to 

 the generally protruding deltaic land. If there should be ever 

 so little extension of this delta seaward, there would neces- 

 sarily be an adjustment of the entire graded portion of the 

 stream to fit the extended stream, since the present load 

 capacity of the stream is delicately adjusted to the grade. 

 In other words, the grade must be maintained, and any ex- 

 tension of the stream must result in the synchronous build- 

 ing up of the stream bed as far back up stream as the graded 

 condition prevails. 



The fact that the master stream of the great interior 

 plains of the United States has always emptied into an em- 

 bayment is of considerable importance in relation to the 

 physiography of the area. It is possible that the Gulf Em- 

 bayment area may have been a deep arm of the Gulf basin 

 extending northward, and that it has gradually been filled by 

 the debris which the Mississippi River system has contributed 



