EROSIONAL HISTORY OF DRIFTLESS AREA 47 



been disturbed, the surface is older than tliose deposits. 

 (13) A less accurate method has been used in determining 

 the ages of old erosion surfaces. It has been concluded 

 that a given peneplain is Cretaceous because it is known to 

 be post-Triassic. and because its formation is assumed to 

 have required all the Jurassic. Comanchean. and Cretaceous 

 periods. Or it might be stated that a peneplain is of Eocene 

 age because Pliocene deposits lie in valleys below it and it 

 would have taken the Oligocene and Miocene periods to cut 

 the valleys. The inaccuracy in such criteria is due to the 

 \'arjing rates of degradation by streams under varying 

 conditions and to a general lack of knowledge of the dura- 

 tion of the various geologic periods. 



Xot all of the above-mentioned means of determining the 

 ages of raised peneplains are likely to be applicable in any 

 one region, but it seems that among so large a number of 

 possible criteria, enough would be usable to lead to con- 

 clusions giving at least the approximate age of an old ero- 

 sion surface. 



Once the age of an upland plain is established it may be- 

 come a valuable horizon marker by which the ages of 

 associated topographies and deposits and structures may 

 be determined. If a peneplain known to be of mid-Eocene 

 age is uplifted uniformly and partly dissected, all topogra- 

 phies and deposits which lie above it are early Eocene or 

 pre-Eocene and all topographies and deposits lying strati- 

 graphically below it are late Eocene or post-Eocene. Simi- 

 larly, structures which the plain bevels are pre-Eocene and 

 structures in which the surface of the plain itself is in- 

 volved are late Eocene or post-Eocene. 



The extreme care with which all these points should be 

 used and the difficulties in the way of accurate interpreta- 

 tion are emphasized in the discussion among Umpleby. At- 

 ^^ood. Blackwelder and Rich, references to which were giv- 

 en on page 8. 



The Dates of Movement 

 The dates of diastrophic movements in the erosional his- 

 tories of surfaces can be determined in a general way at 



