Activity along the Teton Fault raised the modern Tetons 9 to 12 million years ago, making the range one of the youngest in the Rocky Mountain cordillera to which it belongs. Jackson Hole, the valley just east of the Teton scarp, lies on the other side of the fault.
While erosion has reduced the local relief, the sheer ascent of the peaks from the Snake River floodplain is a major contributor to their beauty. The highest and steepest face of the Tetons faces the fault and Jackson Hole; their western flanks have a more gradual rise.
The rocks uplifted by this faulting are mainly metamorphic and igneous in nature, some billions of years old.
The Black Dike & the Skillet
The east face of Mount Moran (12,605 feet) possesses two features that reflect different aspects of Teton geology.
Arcing downward from the upper summit is a severe dark hash, like a charcoal scar: This is the Black Dike, a 150-foot-wide intrusion of igneous rock (diabase) dating from the late Precambrian period about 1.3 billion years ago. Such dikes are scattered throughout the Tetons.
Immediately north of the Black Dike is the Skillet Glacier, one of a dozen small glaciers located in cirques in the high Tetons. Their presence is a reminder of the corrosive power of ice, which, during glacial periods, helped fashion the jagged shoulders of the range.
The contours of the Tetons stem in large part from the erosive action of alpine glaciers, which have been at work in the high country for millions of years. Though much reduced from Ice Age zeniths, the Teton glaciers persist because their winter accumulations exceed losses to summer melting.
Horns, like the Grand Teton summit, were created by glaciers as were gouge cirques, natural amphitheaters, slice arêtes, sharp ridges between glaciers, and erect moraines, which are sediment piles that mark the lateral and terminal edges of the ice sheets.
Grand Teton, at 13,770 feet, is the highest peak in the range; it also supports the largest remaining glacier, Teton Glacier, which flows 3,500 feet from its head below the northeastern summit. Grand Teton and the other highest peaks in the range constitute the Cathedral Group.
The name "Teton" derives from Les Trois Tetons---"the three breasts," a name applied by French trappers to South, Middle, and Grand Teton.