Quartzite
At about the same time as the dikes were being intruded in the Tetons, many thousands of feet of sedimentary rocks, chiefly sandstone, were deposited in western Montana, 200 miles northwest of Grand Teton National Park. The sandstone was later recrystallized and recemented and became a very dense hard rock called quartzite. Similar quartzite, possibly part of the same deposit, was laid down west of the north end of the Teton Range, within the area now called the Snake River downwarp ([fig. 1]).
The visitor who hikes or camps anywhere on the floor of Jackson Hole becomes painfully aware of the thousands upon thousands of remarkably rounded hard quartzite boulders. He wonders where they came from because nowhere in the adjacent mountains is this rock type exposed. The answer is that the quartzites were derived from a long-vanished uplift (figs. [42] and [46]), carried eastward by powerful rivers past the north end of the Teton Range, and then were deposited in a vast sheet of gravel that covered much of Jackson Hole 60 to 80 million years ago. Since then, these virtually indestructible boulders have been re-worked many times by streams and ice, yet still retain the characteristics of the original ancient sediments.