GRAND TETON NATURE NOTES

TETON GEOLOGICAL NOTES - No . 5
POSTGLACIAL FAULTING IN THE TETON RANGE

by
Dr. F. M. Fryxell

At the east base of the Teton Range, the upper slopes of several small treeless moraines (Pinedale stage) are crossed by straight, east-facing scarps 150 to 200 feet high. Such scarps occur below the two prominent gulches on Teewinot and are especially well displayed near the base of Rockchuck Peak. A lateral moraine immediately north of Laurel Lake is also interrupted by an offset, with apparent upthrow on the west. The several scarps are in line with each other; they strike North 10 to 20 degrees East parallel to the mountain front and dip eastward at 50 to 60 degrees. Beneath one scarp on Teewinot is an accumulation suggestive of slumping, but the other scarps exhibit no such features. The scarps are therefore attributed to post Pinedale movement on the main Teton fault. The interval of time since faulting has sufficed for streams to gully the scarps to a depth of 50 feet and to build subjacent alluvial fans.

Continuation of the scarps becomes uncertain at points where they enter the heavier, wooded moraines outside of Garnet, Cascade and Paintbrush Canyons.

Postglacial fault scarps support the concept of the Tetons as a first-cycle range, suggesting that the precipitous east slope is primarily an eroded fault scarp rather than a fault-line scarp; they indicate that at their mouths the east-slope canyons are hanging because of rejuvenation; and they suggest that local earthquakes may have originated in consequence of renewed movements on a Teton fault.