188 GEOLOGICAL SURVEY OF THE TEREITOEIES. 



the greater mountains, is a glacier-cut gorge, widibning and narrowing, 

 with many glacial lakes scattered here and there, while its whole bot- 

 tein is a maze of uneven roche-moutonee, which frost and vegetation are 

 now fast breaking down, and gradually obliterating. It has cut deep 

 into the granites, apparently leaving the Lower Cretaceous sandstones 

 bordering the northern edge, and, farther to the east, where the main 

 ridge falls to a lower and iiat-topped spur, the southern edge also. 

 Lower down, where these flat-topped spurs fall off rather abruptly, mo- 

 rainal masses run out from their ends on either side, and, first running 

 down the valley, finally cross it and join in a bulky terminal mass be- 

 low, which covers the granite, and then hides the upturned edges of 

 the lower sedimentary rocks. One or two of the valleys draining off the 

 rolling Park ridge north of this one present some similar features, 

 being cut through the sandstones and exposing much metamorphic 

 rock. About midway of this ridge, toward the Grand, a higher point 

 presents some of the characteristics of a lava mass. It is probably 

 basaltic. 



THE BLUE EIVER OR MOUNT POWELL GROUP. 



The Park range, after its abrupt rise from the broad rolling ridge at 

 the north, entirely changes in its characters. It appears to be a rectan- 

 gular-shaped mountain mass cut into the most profound amphitheatral 

 headed gorges, which are separated by the most rugged and sharp saw- 

 like ridges of rock imaginable. The main ridge lies along the south- 

 western side of the mass, and from it the valleys and their sharp sepa- 

 rating ridges trend in a general northeast direction. The northernmost 

 spur was composed of a very distinctly and evenly bedded series of 

 schists, gneisses, and granites which had a strike nearly with the ridge, 

 and a dip of 40° or 50° to the southward. Looked at from the east, the 

 general impression is received that all of the large ridges of the range 

 have a similar structure. These rugged ridges, in their easternmost 

 portions, (see Plate III, section 6, west end,) present a pretty uniform 

 general elevation, [a &,) and as the northern ridge expands at its end into 

 an even-surfaced table-like mass of rock, the impression is given that all 

 of these sharp ridges are but the remnants left from the catting away of 

 a plateau-like step which once followed along the mountain-face. These 

 ridges also end quite similarly along a pretty straight line, and descend 

 to rather a uniform level. Eegarding now more particularly the north- 

 ern ten or fifteen miles of the high range, which includes but four or five 

 of the ridges, it is observed that at the base of each steep end^ the low- 

 ered spur does not continue on as a sharp ridge, but slopes off, a flat-sur- 

 faced, plateau-like area, descending gently eastward, (c d, section 6.) 

 Since upon the corresponding area at the base of the northernmost 

 ridge great quantities of debris of the Lower Cretacerous sandstones 

 were found, abundantly proving that they covered the area, it appears 

 that all of these flattish areas either are now, or have comparatively 

 recently been, covered with the same sandstones. Such features would 

 seem to indicate that the Cretaceous had once extended high up, or 

 quite over the whole range, and that the latter, in its upfolding, had 

 received the most pronounced uplifts along certain well-defined lines, 

 the intervening i^ortions not being tilted up at high angles. It is by 

 such a process that the front range, at least from the Big Thompson 

 to the South Platte, has received much of its uplift. Major Powell and 

 Mr. Gilbert have noticed similar folds in the Kaibab plateau and adja- 

 cent regions on the great Colorado plateau of Northern Arizona, through 



