160 GEOLOGICAL SUEVEY OF THE TERRITORIES. 



and rather uniform rocky slopes rise on the north to the rugged mount- 

 ain mass of Long's Peak, and are cut by several deep canons, while on 

 the south they emerge in great curves with the smoother and more pla- 

 teau-like granite mass lying on that side. From the end of the massive 

 spur, which comes down over this carLon valley between two and three 

 miles from its mouth, commences a great glacial moraine, as indicated 

 at {i Jc) in figure. It is at first several hundred feet high, and presents 

 a most rugged and uneven surface, with great irregular depressions and 

 a tendency to a parallel ridge structure. Enormous boulders, many 20 

 feet ui)on a side, are found scattered upon its surface, while here, again, a 

 maze of fallen timber lies around. It is rather singular to see the East 

 Fork cutting a rather abrupt passage through this mass at a point where 

 it rises perhaps nearly a hundred feet above the stream on either side, 

 while to the west the continuation of the main valley on to the Grand 

 is perfectly free from obstructions, the soft surface soil rising but little 

 above the stream. This fact would seem to have its explanation in the 

 explanation of another singular fact, which is, that the main Grand, 

 rather than follow the main open valley west of the granite mass I, has 

 cut a rugged caiion through the latter as indicated in the figure. It 

 would seem as if originally it had followed the former course, passing 

 north and west of Z, while the glacier, reaching to I, and being deflected 

 northward by it, had a terminal moraine, which was but the extension 

 of the lateral, the glacier stream having its natural outlet northward 

 through it at some point k. Certainly the outlet of the East Fork could 

 never be at Tc, if the original course of the Grand was through I, as it is 

 now, while it is inconceivable that the Grand should have selected the 

 hard and higher spur Z, in which to cut a channel, rather than the low 

 depression occupied by soft sedimentary beds, except influenced by some 

 abnormal cause. The cause that would seem to have deflected the Grand 

 from the main valley to its jDresent course would appear to be the flood 

 of lava which has poured down from the region between Willow and 

 Stillwater Creeks ; (see map at head of chapter. Fig. 8.) As will be seen 

 later, many of the hills are capped with remnants of this lava, while at 

 many points it occurs low down, as if occupying former stream beds. 

 These remnants, as shown in the figure (9) prove that this lava stream 

 reached to the granite I, and occupied as high a level as it, as indicated 

 by the table-topped hill near by. The small mass of lava on I appears 

 to occupy a little caiion, probably that of a stream running from the 

 granite mass on the south across I, northwestward to join the old course of 

 the Grand. 



Thus dammed off by the flow of lava the accumulating waters rose in 

 a lake, and escaping at the lowest point around the end of the lava, 

 gradually wore out the present channel through the granite. At that 

 time, the waters must have passed through the channel Jc, but there was 

 probably sufficient water present to also wear the outlet between the 

 end of the moraine Jc and the rock I, as the lower portion of the caiion 

 through the latter was slowly cut. If this explanation is the true one, 

 and the facts, so far as known, admit of no other, this lava is more 

 recent than the older glacial phenomena. Some of the lava near by 

 seems even still younger than that here shown. A few miles farther 

 down the Grand a small sheet rests on the gentle surface-slopes of lake- 

 beds, apparently so recent that erosion has not yet had time to throw 

 it into prominence by the wearing away of the softer beds on which it 

 rests. 



Another point seems explained by the action above described. The 

 small area m, lying between the Grand and Stillwater, west of the roll- 



