EROSIONAL HISTORY OF DRIFTLESS AREA 9 



tirely in field work, while those of the other group believe 

 that raised peneplains exist, but are doing little construct- 

 ive thinking or writing in substantiation of the theory itself. 



It now seems appropriate to bring together all the 

 methods which have been used in the interpretation of ero- 

 sional histories, to analyze each method, to discuss its uses 

 and abuses, and to attempt to assign to each its proper 

 value. These are the purposes of this paper. 



Both in the analysis of the principles and in the con- 

 struction of the paper, the writer has been greatly assisted 

 by Professors R. D. Salisbury, M. M. Leighton, and Leroy 

 Patton, of whom all VN-ere so kind as to read the first draft 

 and to make helpful suggestions for incorporation in the 

 final paper. 



MORE THAN ONE CYCLE OF EROSION 

 Th eoretic Considera t io us 



The rate of land degradation by streams has been esti- 

 mated at 1 foot in 9000 years, under conditions which ex- 

 ist in the United States\ If the average altitude of the 

 land today be taken as 2300 feet it would take more than 

 20,000,000 years for streams to reduce the land to sea level. 

 But the process of degradation becomes slower as the lands 

 are reduced. This progressively decreasing rate of re- 

 duction carried through from youth to the ideal base- 

 levelled condition would involve an amount of time 

 approaching infinity. Indeed, it seems doubtful if geologic 

 time has been as long as a complete cycle of erosion would 

 be. But, though it be uncertain that lands were ever re- 

 duced to base level, they have been reduced to low levels; 

 that is, perfect baselevel plains are probably not formed, 

 but peneplains may be. There is no theoretic reason for 

 believing that extensive areas have not been peneplain ed 

 again and again. 



If the history of land surfaces were merely a matter of 

 formation and subsequent degradation, most lands should 

 to-dav be in the condition of peneplains. The fact that 



1. Water Supply Paper No. 234, U. S. Geological Survey, pp. 78-83. 



