Remnants of two sets of lake deposits in Jackson Hole record preglacial
events in Quaternary time. Downdropping of southern Jackson Hole along
the Hoback and Teton faults blocked the southwestward drainage of the
Snake River, and a new lake formed overlapping and extending south of
the site of the long-vanished Teewinot Lake. Incorporated in the lake
sediments are fragments of lava like that in nearby Quaternary flows.
From this we know that the lake formed after at least some of the lava
was emplaced. Apparently subsidence was more rapid than filling, for a
time, at least, because this new lake was deep. Fossil snails preserved
in olive-drab to gray fine-grained claystone overlying lava flows at the
north end of East Gros Ventre Butte are the kind now living at depths of
120 to 300 feet in Lake Tahoe, California-Nevada. Near the margins of
the lake, pink and green claystone and soft sandstone were deposited.
The duration of this lake is not known but it lasted long enough for 200
feet of beds to accumulate. Subsequent faulting and warping destroyed
the lake, left tilted remnants of the beds perched 1,000 feet up on the
east side of Jackson Hole, and permitted the Snake River to reestablish
its course across the mountains to the southwest.
Later downdropping of Jackson Hole impounded a second preglacial lake.
Little is known about its extent because nearly everywhere the soft
brown and gray shale, claystone, and sandstone deposited in it were
scooped out and washed away during subsequent glaciations. A few
remnants of the lake deposits are preserved in protected places,
however; two are within the Gros Ventre River Valley—one downstream from
Lower Slide Lake about a mile east of the park and the other 4 miles
farther east. The latter remnant is nearly 500 feet thick and the upper
half is largely very fine grained shale and claystone. This fine texture
suggests that the lake existed for a good many thousand years, for such
deposits commonly accumulate more slowly than coarser grained debris.
[Illustration: Figure 57. _Map showing extent and direction of
movement of first and largest ice sheet. See figure 41 for State
lines and location map._]