PLATTE AMPHITHEATER, 95 



due the semicircular form and remarkable verticality of the upper walls of 

 glacial amphitheaters or cirques. 



The main area of the Platte amphitheater lies directly west of Mount 

 Lincoln, but a smaller northwest branch extends back of North Peak, hold- 

 ing on its basin-shaped floor, which is about six hundred feet higher than 

 the other, several pretty glacial lakes with characteristically emerald-tinted 

 waters. The glacier formed by the confluence of the two immense neVe" 

 masses that once filled these amphitheaters, which must have been about two 

 thousand feet thick, flowed directly east, carving out a straight U-shaped 

 valley in the crystalline rocks, whose general form remains essentially 

 unchanged to the present day. 



On the upturned sedimentary beds which rest upon the Archean, how- 

 ever, later erosion has acted more rapidly and irregularly, and at the little 

 town of Montgomery the valley suddenly widens out into a broad, grassy 

 bottom-land, with forest-covered hills sloping away more gently on either 

 side. Immediately above Montgomery, as shown in Plate IX, 1 the present 

 stream bends a little southward around a boss of Archean, composed chiefly 

 of gneiss and amphibolite, penetrated by a fine-grained white granite, in 

 which reticulated veins of white pegmatite stand out prominently. In the 

 bottom of the valley, above this boss for a mile or more, extend glacier- 

 worn hillocks (roches moutonne"es) of typical form, evenly rounded and 

 scored by very distinctly-marked grooves and striae on the upper side, but 

 breaking off unevenly on the lower side toward the stream. On either 

 side of the gorge, above the talus slopes of broken rock masses at their foot, 

 steep walls of Archean rocks rise about two thousand feet, with a thin 

 capping of nearly horizontal Paleozoic strata at the very summit. The 

 structure planes of the Archean, which are unusually distinct in the Platte 

 gorge, stand nearly vertical, with a strike south- southeast. 



The eastern portion of the Archean mass seems mainly composed of 

 gneiss and crystalline schists, granite occurring only in subordinate masses. 

 The granite near Montgomery is of the gray, fine-grained type, suggestive 



'In this and the succeeding diagrammatic sketches, which are intended mainly to illustrate the 

 geology of the various exposures shown, the letters on the outcrops are the same that are used on the 

 geological maps to designate the different rock formations, i. e., a = Archean, 1> = Cambrian, c = Silu- 

 rian, etc. 



