56 GEOLOGICAL SUEVEY OF THE TERRITORIES. 



An important fact should be continually borne in mind, that a critical 

 examination of this detritus reveals no evidence of the existence of 

 rocks from any distant point outside of the river drainage in which 

 they are found ; in other words, these superficial deposits are entirely 

 made up of the materials disintegrated from the rocks in the vicinity. 

 The examination of this detritus is also important to determine the 

 formations that may be sought for within the limits of that drainage. 

 Underlying all this detritus, in this valley, is a greater or less thickness 

 of the Pliocene deposits, and the little streams on their way to the main 

 river show very distinctly where these sediments have been cast by the 

 waters of the lake against the mountain sides. Not unfrequently some 

 of the older unchanged rocks, or even the metamorphic strata, are ex- ' 

 posed — ^remnants left after the great erosion which preceded the present 

 jjeriod. The degradation of all kinds of rocks has been going on continu- 

 ally through all geological times, and the most important geological 

 changes have thus been wrought. We may date back, first, to the time 

 when all the formations known in the West, from the metamorphic rocks 

 to the Eocene coal group, inclusive, extended uninterruptedly over the 

 valley of the Yellowstone; and now only a few patches remain, here 

 and there, of from 5,000 to 10,000 feet of sedimentary strata. Then, too^^ 

 the mountain ranges have been pared down, we know not how much, 

 since they began their upward movements to the present time. At any 

 rate, we know that the erosion of the mountains has been immense; that, 

 in many cases, entire ranges have been degraded, so that only fragments 

 remain. Again, since this valley was a lake-basin, extensive degradation 

 has taken place, removing a considerable thickness of the Pliocene 

 deposits. It is only when they have been protected by a sheet of basalt, 

 that we can form any correct idea of their original thickness. We may 

 suppose this to be a good proof, from the fact that the basalts seem, in 

 almost all cases, to have coolecl under water at some depth, probably 

 not great. At the upper portion of this valley, just below the second 

 canon, there are quite large areas covered Avith the Pliocene marls and 

 sands, several hundred feet in thickness, overlaid with a chick floor of 

 basalt. These Pliocene beds present the evidence of having been de- 

 posited in moderately quiet waters, so that we may suppose that they 

 once extended all over the valley with a pretty uniform thickness. Since 

 these valleys have been drained, or, perhaps, during the process of 

 drainage, the surface has been worn into its present form, and the irre- 

 gularities have been filled up with a greater or less thickness of local de- 

 tritus. 



It was doubtless during the slow process of drainage that the terraces, 

 which constitute so conspicuous a feature of all these mountain valleys, 

 were formed ; these, also, were carved out of the Pliocene deposits. Some- 

 times these modern Tertiary beds are quite conspicuous, forming high 

 vertical bluff walls along the valley. Again, they are removed, so that, 

 with the exception of a narrow belt along the immediate base of the 

 mountains on either side, the valley has been shaped into a low grass- 

 covered lawn, but little raised above the bed of the stream. In many 

 instances, as along the base of Emigrant Peak, the line of junction of 

 the valley deposits with the sides of the mountain is indicated by the 

 vegetation, and the descent, from that line down to the river bottom, is 

 very gentle and smooth as a lawn, and covered with a thick growth of 

 grass and other vegetation. This complete and gentle transition from 

 mountain to valley forms one of the most striking and beautiful features 

 in the landscape. 



We will now proceed up the valley of the Yellowstone toward the 



