202 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



diameter, beds composed of well-worn and rounded material are frequently 

 observed. It is evident that this latter material is water-laid, and in great 

 part assorted by the action of running streams. Much of this material is 

 sandy, consisting of volcanic gravel deposited under water. It is also 

 evident that all this material is volcanic in nature and derived from still 

 higher sources. These water-laid beds are covered by flows of agglomerate 

 and coarse unaltered breccias derived from explosive action. No dikes 

 were observed cutting the Two Ocean Plateau breccias. 



The mass of Two Ocean Plateau, while apparently horizontal as seen 

 almost anywhere from a distance, has a gentle inclination to the north, aver- 

 aging, however, not more than 3°. It seems probable that the entire 

 plateau has suffered from a slight tilting of the mass. Evidence of such 

 movement may be seen throughout the Absarokas. It would seem that 

 the flow of the coarse brecciated mass at the time of its ejection was from 

 the eastward, but that a subsequent orographic movement was at right 

 angles to it, the mass being tilted northward, due to the intrusion of masses 

 of granite-porphyry to the southwest. 



The east and west sides of Two Ocean Plateau are sharply contrasted 

 in the lateral trenches which cut the plateau body. Along the Yellow- 

 stone Valley they expose masses of breccia 2,000 feet in thickness without 

 cutting through the upper basic breccias, while on the west side, only a few 

 miles across, the walls of the plateau show but a thin covering of breccia, 

 with the Paleozoic and Mesozoic strata lying beneath them. The border 

 line between the sedimentary beds and the breccias follows a sinuous course, 

 with tongues of breccia penetrating into the underlying bodies. One of 

 these tongues is well shown near the junction of Mink and Pacific creeks, 

 where a fine exposure of breccia occurs on the north side of Pacific Creek, 

 while higher up Mink Creek the blue limestones of the Madison formation 

 stand out, capped by the more somber breccias. 



Between the brecciated mass of Two Ocean Plateau and that lying to 

 the westward at the base of Mount Hancock and along Coulter Creek, there 

 is one marked difference in mode of occurrence. The former shows evi- 

 dences of material having been transported for a considerable distance, 

 while at the latter locality there is every indication that the source of 

 material thrown out was near its present position, with every evidence of 

 explosive action along a line of profound faulting having an approximately 

 north-south trend. 



