Figure 21. Folds in layered gneisses.
A. North face of the ridge west of Eagles Rest Peak. The face is about 700 feet high. Notice the extreme contortion of the gneiss layers.
B. Closeup view of some of the folds near the bottom of the face in figure A. The light-colored layers are composed principally of quartz and feldspar. The darker layers are rich in hornblende.
Irregular bodies of granite gneiss are interleaved with the layered gneisses in the northern part of the Teton Range. The granite gneiss is relatively coarse grained, streaky gray or pink, and composed principally of quartz, feldspar, biotite, and hornblende. It differs from enclosing layered gneisses in its coarser texture, lack of layering, and more uniform appearance. The dark minerals (biotite and hornblende) are concentrated in thin discontinuous wisps that give the rock its streaky appearance.
The largest body of granite gneiss is exposed in a belt 1 to 2 miles wide and 10 miles long extending northeastward from near the head of Moran Canyon, across the upper part of Moose Basin, and into the lower reaches of Webb Canyon. This gneiss may have been formed from granite that invaded the ancient sedimentary and volcanic rocks before they were metamorphosed, or it may have been formed during metamorphism from some of the sediments and volcanics themselves.
At several places in Snowshoe, Waterfalls, and Colter Canyons the layered gneisses contain discontinuous masses a few tens or hundreds of feet in diameter of heavy dark-green or black serpentine. This rock is frequently called “soapstone” because the surface feels smooth and soapy to the touch. Indians carved bowls ([fig. 22]) from similar material obtained from the west side of the Tetons and from the Gros Ventre Mountains to the southeast. Pebbles of serpentine along streams draining the west side of the Tetons have been cut and polished for jewelry and sold as “Teton jade”; it is much softer and less lustrous than real jade. The serpentine was formed by metamorphism of dark-colored igneous rocks lacking quartz and feldspar.
Granite and pegmatite
Contrary to popular belief, granite (crystalline igneous rock composed principally of quartz and feldspar) forms only a part of the Teton Range. The Grand Teton ([fig. 6]) and most surrounding subsidiary peaks are sculptured from an irregular mass of granite exposed continuously along the backbone of the range from Buck Mountain northward toward upper Leigh Canyon. The rock is commonly fine grained, white or light-gray, and is largely composed of crystals of gray quartz and white feldspar about the size and texture of the grains in very coarse lump sugar. Flakes of black or dark-brown mica (biotite) and silvery white mica (muscovite) about the size of grains of pepper are scattered through the rock.