When did the Tetons rise?
A study of the youngest sedimentary rocks on the floor of Jackson Hole
shows that the Teton Range began to rise rapidly and take its present
shape less than 9 million years ago. The towering peaks themselves are
direct evidence that the rate of uplift far exceeded the rate at which
the rising block was worn away by erosion. The mountains are still
rising, and comparatively rapidly, as is indicated by small faults
cutting the youngest deposits (fig. 15).
How rapidly? Can the rate be measured?
We know that in less than 9 million years (and probably in less than 7
million years) there has been 25,000 to 30,000 feet of displacement on
the Teton fault. This is an average of about 1 foot in 300-400 years.
The movement probably was not continuous but came as a series of jerks
accompanied by violent earthquakes. One fault on the floor of Jackson
Hole near the southern boundary of the park moved 150 feet in the last
15,000 years, an average of 1 foot per 100 years.
In view of this evidence of recent crustal unrest, it is not surprising
that small earthquakes are frequent in the Teton region. More violent
ones can probably be expected from time to time.