184 GEOLOGICAL SURVEY OF THE TERRITORIES. 

 THE VALLEY OF THE BLUE RIVER. 



The sources of the Blue Eiver are in the area of arcpean, with some 

 small patches of older sediiaentary rocks lying between Gray's Peak on 

 the east and Mount Lincoln on the southwest. From their mountain- 

 surrounded valleys, the collected waters, at about thirty miles from the 

 river's mouth, pass into an area of Cretaceous rocks, and it is this lower 

 portion only which will be regarded at present. For this thirty miles 

 up from its mouth, the Blue, in several respects, is the direct comple- 

 ment of the Muddy, and especially as it occui)ies a monoclinal valley in 

 Cretaceous rocks, the valley bottom being eroded in the soft, eastward- 

 dipping, mid-cretaceous shales, with the hard lower sandstones lying up 

 on the archEean rocks of the Park range on the west, and the sandstones 

 of No. 5, with rocks above, forming the eastern side of the valley. But 

 it is a far better-defined valley than that of the Muddy, being both 

 straighter, narrower, and deeper, though there is yet room nearly all 

 along mid-valley for a zone of recent terraced beds, which, as in the 

 Muddy, conceal most of the Cretaceous shales from view, and take away 

 any idea of ruggedness of bottom, which might attach itself to tlie term 

 narrow, as applied to the valley. 



From its monoclinal nature, the river natutally lies nearer the eastern 

 side of the valley, or the base of the Williams River range. The first 

 steep slopes of these mountains rise along a very straight line, and while 

 the Blue is for a short distance close to their base, a zone of terraces 

 from one to two miles wide usually sweep from the mountain-base 

 westward to the river. The range is exceedingly symmetrical in its 

 outline, both in cross-section and longitudinally, the highest point, which, 

 rises some 3,600 feet above the river, being nearly in the center. From 

 this iDoint the summit-line, which varies in distance only from two to 

 four miles from the river, descends in uniform wavy slopes both north- 

 west and southeast, while in cross-section the eastern slope is compara- 

 tively gentle, the one facing the Blue being very steep. This west front 

 is made up of the edges of the strata, which show upon it as horizontal 

 bands. There are two bands which are more prominent than the rest, 

 forming escarpments, one about half way up, where the range is higliest, 

 the other near the base. Toward the northwest, where' the range falls, 

 the upper horizons are eroded away, and the lower bed forms the general 

 summit of this lower portion of the range, reaching to within five miles 

 of the Grand. Both of these more prominent beds appear as bands of 

 ashen-gray color, and are apparently sandstones, separated by a series 

 of shales, weathering gray, with many white bands. 



The summit of the Park range varies from six to eight miles from the 

 river on its opposite side. For fifteen miles south from the Grand, 

 except being somewhat narrower, this ridge has much the same char- 

 acter as north of that river, a massive rolling ridge of the archsean rocks 

 with the Lower Cretaceous sandstones lying up against it. South of 

 this point, however, it rises in one steep slope to the jagged crest of 

 the main Blue Eiver range, the principal characteristics of which will 

 be described a little later. 



Proceeding up the valley, the long middle slopes of the Park range, 

 capped with the Lower Cretaceous sandstones, lie on the right; the upper 

 portions of the ravines cutting through these to the metamorijhic 

 rocks below. Lapping up on these slopes are the terraced beds which 

 run out toward the river in long graceful slopes. The stream usually 

 has a much narrower bottom or flood-plain than the Muddy, and it is there- 

 fore much less serpentine in its course, tbe lower terrace generally rising 



