The episode of mountain building that resulted in formation of the
ancestral Rocky Mountains has long been known as the _Laramide
Revolution_. West and southwest of Wyoming, mountains had already
formed, the older ones as far away as Nevada and as far back in time as
Jurassic, the younger ones rising progressively farther east, like giant
waves moving toward a coast. The first crustal movement in the Teton
area began in latest Cretaceous time when a broad low northwest-trending
arch developed in the approximate area of the present Teton Range and
Gros Ventre Mountains. However, this uplift bore no resemblance to the
Tetons as we know them today for the present range formed 70 million
years later.
[Illustration: Figure 41. _Grand Teton National Park region slightly
more than 80 million years ago, just before onset of Laramide
Revolution. The last Cretaceous sea still lingered in central
Wyoming._]
One bit of evidence (there are others) of the first Laramide mountain
building west of the Tetons is a tremendous deposit of quartzite boulder
debris (several hundred cubic miles in volume) derived from the _Targhee
uplift_ (fig. 42). Nowhere is the uplift now exposed, but from the size,
composition, and distribution of rock fragments that came from it, we
know that it was north and west of the northern end of the present-day
Teton Range. Powerful streams carried boulders, sand, and clay eastward
and southeastward across the future site of Jackson Hole and deposited
them in the Harebell Formation (table 4). Mingled with this sediment
were tiny flakes of gold and a small amount of mercury. Fine-grained
debris was carried still farther east and southeast into two enormous
depositional troughs in central and southern Wyoming. Most of the large
rock fragments were derived from Precambrian and possibly lower
Paleozoic quartzites. This means that at least 15,000 feet of overlying
Paleozoic and Mesozoic strata must first have been stripped away from
the Targhee uplift before the quartzites were exposed to erosion.
[Illustration: Figure 42. _Teton region at the end of Cretaceous
time about 65 million years ago. The ancestral Teton-Gros Ventre
uplift had risen and prominent southeastward drainage from the
Targhee uplift was well established. See figure 41 for State lines
and location map._]
Remains of four-legged horned ceratopsian dinosaurs, possibly
_Triceratops_ (fig. 43), reflecting the last population explosion of
these reptiles, have been found in pebbly sandstone of the Harebell
Formation in highway cuts on the Togwotee Pass road 8 miles east of the
park.
[Illustration: Figure 43. _Triceratops, a horned dinosaur of the
type that inhabited Jackson Hole about 65 million years ago. Sketch
by S. H. Knight._]
Near the end of Cretaceous time, broad gentle uplifts also began to stir
at the sites of future mountain ranges in many parts of Wyoming. The
ancestral Teton-Gros Ventre arch continued to grow. Associated with and
parallel to it was a series of sharp steepsided elongated
northwest-trending upfolds (_anticlines_). One of these can be seen
where it crosses the highway at the Lava Creek Campground near the
eastern margin of Grand Teton National Park.
During these episodes of mountain building, erosion, and deposition, the
dinosaurs became extinct all over the world. The “Age of Mammals” was
about to begin.