Malott: The ''American Bottoms'' Region 



27 



the present grade level of the streams. The broad valley flat 

 in the lower course of Beech Creek and the much broader 

 valley of Plummers Creek suggest this. No records of the 

 depth of the alluvium were obtained in either of these val- 

 leys, but it is noticeable that these stream valleys are filled 

 valleys, and that nowhere in their middle and lower reaches 

 do they flow over bed-rock. Their gradients are also low 

 and well adjusted. Cliffy Creek cannot be used as an example 

 since it runs over bed-rock on account of having been deflected 

 from the course it formerly followed. This point will be 

 discussed later. 



When it is realized that all the major valleys of the south- 

 ern part of the state have the characteristics of filled valleys, 

 it will be seen that this question is an important one. Numer- 

 ous records show that the major streams are now flowing 

 from 50 to 150 feet above their bed-rock floors. The fact 

 that the largest of these streams are incapable of bed-rock 

 scour beyond a depth of 40 to 50 feet eliminates any suggestion 

 that these great depths of alluvium are the normal flood-plain 

 material which may be entirely shifted and reworked by the 

 waters of the streams even in the passage of their highest 

 flood waters. There are two explanations that may be ofll'ered 

 to account for valley filling. These will now be discussed. 



Valley Filling hderpreted as a Result of Regional Depres- 

 sion. Valley filling in southwestern Indiana has long been 

 recognized as one of the chief characteristics of its physiog- 

 raphy. Ashley, in his description of the topographic types 

 in southwestern Indiana, reaches some very interesting con- 

 clusions in this regard.' In comparing the characters of the 

 the stream valleys of the middle section with those of the 

 western area, he uses Twin Creek, Washington County, and 

 Big Blue River as types of stream valleys of the former area, 

 and Pigeon Creek as a type of the latter. He says in part: 



, Twin Creek has along its course steep, precipitous banks. Pigeon 

 Creek has well-rounded banks, nearly everyw^here suitable for cultiva- 

 tion. Twin Creek has broad bottoms, but evidently carved out of the 

 rock by erosion, the creek bed being everywhere in rock. Pigeon 

 Creek's broad bottoms are evidently due to the filling of sunken val- 

 leys. ... In brief, the area in which Pigeon Creek lies has evi- 

 dently sunk belov/ drainage level so that all the valleys have been filled 



' G. H. Ashley, The Geolo<>y of the Lower Caibonifei'ous Ai-ea of Southern Indiana. 

 27th Annual Report of the Indiana Department of Geology and Natural Resources. 1902. 



