The Range That Is Still Rising

Stand on the eastern shore of Jenny Lake and look west. What you see is not a mountain range with foothills and a gradual approach. What you see is a vertical wall. The Teton Range rises seven thousand feet in less than seven miles of horizontal distance, beginning almost at road level and ending at the summit of Grand Teton, thirteen thousand seven hundred seventy-five feet above sea level. There is no transition. The mountains do not ease you in.

The Teton Range is geologically young, having been rising for less than ten million years, which by the standards of North American ranges makes it a newcomer. It is actively rising along the Teton Fault, a rupture zone that runs along the base of the range on the Jackson Hole side. The fault works by asymmetry: the block of rock west of the fault rises; the Jackson Hole valley floor drops. The vertical relief you see is the product of both movements happening simultaneously, the range going up while the valley goes down.

The Teton Fault last ruptured somewhere between four thousand and seven thousand years ago. The magnitude of that rupture is estimated at between six point five and seven point five on the modern scale. The fault will rupture again; the question is when, not whether. USGS monitors it continuously.

The glaciers that carved Jenny Lake retreated approximately fifteen thousand years ago. Smaller glaciers still exist in the high couloirs of the Cathedral Group, Grand Teton, Mount Owen, Teewinot. They are retreating. The range itself continues rising at roughly a millimeter per year. The landscape here is not stable. It is moving in four directions at once, and has been for nine million years.