TITLES AND ABSTRACTS OF PAPERS 89 



DATING OF PENEPLAINS: AN OLD EROSION SURFACE IN IDAHO, MONTANA, 

 AND WASHINGTON— IS IT EOCENE? 



BY JOHN L. RICH 



(Abstract) 



The correct dating of wide-spread erosion surfaces, or peneplains, is a matter 

 of the greatest importance because of their value as datum planes for the 

 interpretation of recent earth history. 



A consideration of the probable fate in store for the elaborate superstruc- 

 ture reared on the determination of the origin and dating of the so-called 

 Cretaceous peneplain of eastern United States, which seems about to crumble 

 as a result of the recent work of Barrell and Shaw, should cause physiogra- 

 phers and geologists to pause and weigh well the evidence before accepting 

 the fact or the dating of similar erosion surfaces elsewhere. 



An old erosion surface, or peneplain, which seems to have wide development 

 in Idaho, Montana, and Washington, has been definitely dated as Eocene by 

 Umpleby and Atwood. Inasmuch as this dating has been questioned, a de- 

 tailed examination of the published evidence has been undertaken. 



The correctness of the determination of the Eocene age of the erosion sur- 

 face hinges mainly on whether certain broad, intramontane troughs, in which 

 lie Oligocene and Miocene sediments, were developed before or, as maintained 

 by Umpleby and Atwood. after the peneplain was cut. Examination of the 

 published evidence fails to reveal any convincing proof that the basins are 

 3'ounger than the peneplain. On the contrary, it brings out many features of 

 the geology and physiography of the region which do not harmonize with this 

 explanation. 



Oligocene and Miocene sediments in the basins are commonly highly de- 

 formed ; the contacts of the sediments with the basin walls, in some places at 

 least, are faults ; the basin floors are far from being parts of a graded system, 

 and to restore the graded condition by uplifting, dropping, or tilting faulted 

 blocks would throw the peneplain remnants far out of harmony; and, finally, 

 the broad basins are physiographically entirely out of accord with the valleys 

 of the' present drainage systems, which are prevailingly narrow, V-shaped 

 canyons, whose derivation in a single cycle from the former system by a 

 process of headward erosion or stream piracy, as has been suggested by 

 Umbleby and Atwood, is a physiographic impossibility. 



Far from proving an Eocene age for the peneplain, the field relations de- 

 scribed by the authors of that dating seem to prove that the basins in which 

 the Miocene sediments lie were blocked out, filled, and suffered most of their 

 deformation before the peneplain was cut. 



Eead in full from manuscript. 



Discussion 



Dr. Bruce L. Clark : Recent vertebrate collections made by Prof. J. C. 

 Merriam in the so-called lake beds of Doctor Umpleby are much later in age 

 than Umpleby supposed, being, if I remember correctly, Upper Miocene or 

 Lower Pliocene in age. 



