SALIENT FEATURES OF THE GEOLOGY OF OREGON 99 



apparent folding may be due to pre-existing irregular topography. 

 Along the Columbia River gorge the flows are seen to be thrown into 

 an anticlinorium. 



The age of this series of flows is definitely limited by the middle 

 Miocene Mascall and the John Day Oligocene. 



Beds unconformably overlying the Columbia lava within the 

 John Day Valley have been named the Mascall formation/ These 

 beds, aggregating about one thousand feet, consist of conglomerate, 

 sand, ash, and tuff, and have yielded a rich vertebrate fauna and a 

 well-defined flora,^ of middle Miocene age. 



The Pliocene of eastern Oregon is represented by the Rattle- 

 snake,^ Ironsides, and Idaho beds. The oldest of these, the Rattle- 

 snake, is typically developed on Rattlesnake Creek, near the Mascall 

 Ranch on John Day River. These beds have been recognized 

 by Osmont in the Crooked River Valley. The Rattlesnake con- 

 sists of coarse basal gravels, brown tuff, and a rhyolite flow. 

 This formation has been deformed and in places faulted. Its verte- 

 brate fauna, though meager, is sufficient to indicate an early 

 Pliocene age. 



Sedimentary beds at Ironside,'' Malheur County, have recently 

 been designated the Ironside formation. Buff-colored, sandy shales 

 and shales of possibly two hundred feet in thickness, now deformed, 

 yielded a small vertebrate fauna which Merriam considers as being 

 younger than the Rattlesnake Pliocene. 



Lindgren^ has described certain lacustrine deposits along Snake 

 River as the Idaho formation. This has not been distinctly sepa- 

 rated structurally and lithologically from the earlier beds of the 

 same region known as the Payette, though the presence of equine 

 and proboscidian remains prove the presence of later beds. 



PSYCHOZOIC 



Pleistocene. — The Pleistocene of Oregon includes lake, glacial, 

 and river deposits, marine conglomerates, sands, peat, and extensive 

 igneous formations. It is yet too early to put all these widely 



' J. C. Merriam, op. clL, II, 305. 



^ F. H. Knowlton, U.S. Geol. Surv., Bull. 204, 1902. 



^Op. cit., p. 310. 4 J. c. Merriam, op. ciL, X, 129. 



5 W. Lindgren, U.S. Geol. Surv., Folio 103, p. 2. 



