EROSION AL HISTORY OF DRIFTLESS AREA 45 



other. In a region in which there is a summit peneplain 

 at 2,000 feet, an intermediate peneplain at 1,000 feet and 

 g-raded flats at 500 feet, it could be inferred that there had 

 been an uplift of approximately 1,000 feet, and a second 

 one of about 500 feet. 



This method of interpretation seems simple enough but 

 its application to field conditions involves possibilities of 

 error. Differences in altitude between remnants of con- 

 secutive graded plains vary from point to point in any 

 region, (1) if uplift was accompanied by tilting, warping, 

 or faulting, (2) if either surface was irregular, (3) if final 

 grades differed because of changes in volume or load of the 

 streams. These being common conditions, it seems pos- 

 sible to get accurate figures on the amount of uplift for 

 individual districts only, and even this is subject to error. 

 For whole regions, only approximate averages are possible. 



THE DETERMINATION OF DATES 



The complete history of a surface involves dates as well 

 as events and sequences. Various criteria have been used 

 for the determination of the geologic dates of the various 

 events in the histories of land surfaces. Some of these 

 criteria are readily applicable and accurate if properly ap- 

 plied. Others are not so valuable. The problem involves 

 the ages of old erosion surfaces, the dates of diastrophic 

 movements, the duration of time involved in erosion cycles, 

 etc. 



The Age of Old Erosion Surfaces 



It has been customary in designating the ages of raised 

 peneplains to refer to the date at which the plain was com- 

 pleted and still intact rather than the whole time during 

 which it was in process of formation. For instance, the 

 Kittatinney peneplain in the northern Appalachians is re- 

 ferred to as the Cretaceous plain, not because its formation 

 was accomplished during the Cretaceous period only, but 

 because it was believed to have been completed during that 

 period. The cycle during which it was formed was prob- 

 ably inaugurated long before the Cretaceous. Although 

 this departs in a way from the usage of time terms in rela- 



