Vol. 6] Merriarn: Virgin Valley and Thousand Creek. 



229 



between 1,000 and 2,000 feet thick, and are seen in the narrow- 

 trough because of deformation of the underlying lava since 

 their deposition, it is evident that they originally existed out- 

 side this depression, but have been eroded away. The Rattle- 

 snake rests in a very slightly inclined position upon the eroded 

 edges of this steeply tilted Mascall, and it is clear that the 

 time of beginning deposition of the Rattlesnake must have been 

 separated from the closing of deposition of the Mascall by a 

 period in which very marked deformation and extensive erosion 

 of the Mascall occurred. It seems improbable that this deforma- 

 tion and erosion could have taken place in a period shorter than 

 that represented by the Upper Miocene. This l>ein>>- the case 

 the Rattlesnake would not be older than early Pliocene. 



The upper limit of age of the Rattlesnake seems to be fixed 

 by the beginning of the erosion period during which the great 

 canons of this region were cut. Terrace deposits near the floor 

 of the present canon of the John Day River contain undisturbed 

 remains representing a Pleistocene fauna. The canon-cutting 

 period must, therefore, have ended sometime before the close of 

 Pleistocene time. The presumption is that the canon-cutting 

 was accomplished in early Pleistocene time. As the John Day 

 Canon cuts through the typical Rattlesnake section, the upper 

 limit of age of these beds seems determined as not later than the 

 beginning of the Pleistocene. 



The Rattlesnake Beds as we know them in the John Day 

 Valley were evidently laid down in a basin of comparatively 

 limited extent, which was bounded on the north by the Columbia 

 Lava monocline, and reached south to the ridge of the Blue 

 Mountains south of the John Day River. The greatest thickness 

 of the beds known to the writer, including the maximum thick- 

 ness of the various members of the series, would be a little less 

 than 500 feet. A small part of the series consists of beds which 

 have the appearance of old soil mantles, but the greater portion 

 of the whole accumulation is made up of coarse gravel. The 

 time required for the deposition of the whole thickness may, 

 therefore, have been rather short, and presumably does not rep- 

 resent more than one-half of the Pliocene, in which it seems 

 probable that the formation of this series of beds occurred. 



