Mountain Building

Compression of the earth's crust 80 million to 40 million years ago caused uplift of the Rocky Mountain chain, from what is now Mexico to Canada. While the mountains on the south and east formed during this period, the rise of the Teton Range as we now see it had not yet begun. Stretching and thinning of the earth's crust caused movement along the Teton fault to begin about 6-9 million years ago.

Every few thousand years, when the elasticity of the crust stretches to its limit, a fault (or break) of about 10 feet occurs, relieving stress in the earth's crust. The blocks on either side of the fault moved, with the west block swinging skyward to form the Teton Range, the youngest and most spectacular range in the Rocky Mountain chain. The east block dropped downward, forming the valley called Jackson Hole. The valley block has actually dropped down four times more than the mountain block has uplifted. Total vertical movement along the Teton fault approaches 30,000 feet. Evidence for the amount of movement comes from the present location of the Flathead Sandstone. Activity along the Teton fault separated this formation on the opposing blocks.

On the summit of Mt. Moran 6,000 feet above the valley floor, lies a pink cap of Flathead Sandstone, visible when the snow has melted. On the valley side of the fault, this formation lies buried at least 24,000 feet below the surface.

Early nineteenth century fur trappers referred to high mountain valleys as "holes". When they named this valley Jackson Hole, they were geologically correct! Today the sheer east face of the Teton Range, rising abruptly more than a mile above the valley, captures our attention more than the valley does. Rocks and soil, thousands of feet thick, transported into the valley over the past several million years, mask the subsidence of the valley. Some of the deposits filling Jackson Hole contain innumerable rounded rocks varying in color from white to pink and purple. These quartzite rocks eroded from an ancestral mountain range probably located 20 to 70 miles northwest of the Teton Range. Rivers rounded the quartzite into cobblestones as they carried the rocks into this area.

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