The youngest division of the _Mesozoic_ Era is the Cretaceous Period.
Near the beginning of this period, brightly colored rocks continued to
be deposited. Then, the Teton region, as well as most of Wyoming, was
partly, and at times completely, submerged by shallow muddy seas. As a
result, the brightly variegated strata were covered by 10,000 feet of
generally drab-colored sand, silt, and clay containing some coal beds,
volcanic ash layers, and minor amounts of gravel.
The Cretaceous sea finally retreated eastward from the Teton region
about 85 million years ago, following the deposition of the Bacon Ridge
Sandstone (fig. 40). As it withdrew, extensive coal swamps developed
along the sea coast. The record of these swamps is preserved in coal
beds 5 to 10 feet thick in the Upper Cretaceous deposits. The coal beds
are now visible in abandoned mines along the east margin of the park.
Coal is formed from compacted plant debris; about 5 feet of this
material is needed to form 1 inch of coal. Thus, lush vegetation must
have flourished for long periods of time, probably in a hot wet climate
similar to that now prevailing in the Florida Everglades.
Sporadically throughout Cretaceous time fine-grained ash was blown out
of volcanoes to the west and northwest and deposited in quiet shallow
water. Subsequently the ash was altered to a type of clay called
_bentonite_ that is used in the foundry industry and in oil well
drilling muds. In Jackson Hole, the elk and deer lick bentonite
exposures to get a bitter salt and, where the beds are water-saturated,
enjoy “stomping” on them. Bentonite swells when wet and causes many
landslides along access roads into Jackson Hole (fig. 17).
The Cretaceous rocks in the Teton region are part of an enormous
east-thinning wedge that here is nearly 2 miles thick. Most of the
debris was derived from slowly rising mountains to the west.
Cretaceous sedimentary rocks are much more than of just scientific
interest; they contain mineral deposits important to the economy of
Wyoming and of the nation. Wyoming leads the States in production of
bentonite, all of it from Cretaceous rocks. These strata have yielded
far more oil and gas than any other geologic system in the State and the
production is geographically widespread. They also contain enormous coal
reserves, some in beds between 50 and 100 feet thick. The energy
resources alone of the Cretaceous System in Wyoming make it invaluable
to our industrialized society.
[Illustration: Figure 40. _The yardstick and the sea. The shaded
part of the yardstick shows the 500-million-year interval during
which Paleozoic and Mesozoic seas swept intermittently across the
future site of the Tetons. When they finally withdrew about 85
million years ago, a little more than 5/8 of an inch of the
yardstick remained to be accounted for._]