PEALE.1 GEOLOGY POST-TERTIARY AND RECENT. 161 



that altliough synclironous with the Green Eiver Basin, that the lakes 

 were connected only as our great lakes are at the present day, by nar- 

 row straits. 



POST-TERTIARY AND RECENT. 



Glacial. 



In the Sawatch range, as was fully detailed in the reports for 1873, 

 there is abundant evidence of glacial action. The moraines in the 

 caGons at the head of Eagle Eiver have been already described. It is 

 impossible, with the present limited amount of data, to define what 

 were the limits of the glaciers. It is probable they were more widely 

 distributed than has been generally supposed. It is possible thftt a 

 glacier once covered the plateau of station 48, and the one to the south. 

 The erosion on these plateaus could sct^cely have been effected by any 

 other agency. There are several lakes on the surface which may have 

 had their origin in glacial action. 



Farther north Mr. JMarvine found evidences of glaciation on similar 

 plateaus, capped with basaltic lava. They were, he thinks, at a higher 

 level, and when he descended, the traces ceased. The Eoches-Mou- 

 tonnees forms were very prominent. 



Terraces, etc. 



Eagle Eiver. — In the valley of the Eagle, above the second canon, ex- 

 tending almost to the moiith of the Piney, are terraces cut in drift. 

 "Whether this drift is stratified or unstratified I am unable to say. It 

 is, I think, in* part at least, of glacial origin. The terraces here are 

 about 100 feet high. Below the canon there are beautiful terraces, as 

 shown in section D, Plate III. These are comparatively recent. The 

 soft character of the strata in the valley renders them easily eroded, 

 and even at the present time an immense amount of material is carried, 

 down the river every spring. Alluvial material occurs at various points 

 along the course of the river, especially above the second canon, where 

 the river has some lake-like expansions surrounded by beautiful 

 meadows. 



Grand Eiver. — A great portion of the Grand Eiver is a caiion, but 

 below the mouth of Eoaring Fork, especially near the plateau of station 

 48, there are terraces. We did not have time to visit this part of the 

 river, and simply noted them from a distance. The alluvial bottoms 

 are very limited in extent. 



Gunnisoji Eiver. — The valleys of the Gunnison, its Il!J"orth Fork, and 

 their tributaries are terraced in a beautiful manner. All th£ drift is 

 probably of local origin. The terraces are cut mainly in the soft shales 

 of Upper Cretaceous age, which have been treated of in the previous 

 chapter. There are scattered patches of alluvial material. These areas 

 have already been described, and description here would be a rej)etition. 



Erosion. 



The valleys of nearly all the streams in the district are simply erosive, 

 although a number were perhaps determined in their present course by 

 breaks in the strata, the result of folding. It would be impossible even to 

 estimate the enormous amount of erosion to which the strata west of the 

 continental divide, in our district, have been subjected since the begin- 

 ning of Tertiary time. The amount of erosion during the Tertiary time 

 was enormous. It was sufficient to form beds thousands of feet in thick- 

 11 H 



