Landscape History - Mountains and Plains: The Ecology of Wyoming Landscapes - page 17

As Lake Gosiute dried, continued erosion deposited
thick sediments over the salts and shales that had accu-
mulated. the topography and climate continued to
change, until today the chain Lakes wetland is located
in the bottom of the Great Divide Basin (see fig. 1.2),
surrounded by sagebrush steppe and desert shrubland—
much different than the tropical life that existed there
in the past. the lakes occupy a small portion of the
depression where Lake Gosiute once existed.
Later, in the Miocene (about 10 million years ago),
regional uplifting became more pronounced, eventu-
ally bringing the landscape to near its present elevation
and initiating a new cycle of erosion. this uplifting was
accompanied by localized faulting and folding during
the Pliocene (5.0 to 2.6 million years ago) that led to the
formation of the teton Range—the youngest mountain
range in the region. 21
With uplifting, the climate cooled, and some riv-
ers began to flow more rapidly, increasing the rate of
erosion and eventually cutting canyons through the
igneous rocks of buried mountain ranges (see fig. 2.3).
examples include the canyons of the Bighorn, Green,
Laramie, Platte, Powder, Snake, Sweetwater, and Wind
rivers. Most easily seen are the Snake River canyon
south of Jackson and the Wind River canyon south of
thermopolis. in addition to canyons, the accelerated
erosion of claystones, siltstones, and other nonresistant
strata created badlands, a process that continues to this
day, carving escarpments in the elevated land.
As the mountains rose, the climate became more
arid on the eastern—leeward—side, caused by the rain-
shadow effect. this happens because the prevailing
winds are from the west, and moisture is deposited on
the western slopes as the rising air cools and the mois-
ture condenses as it passes over the mountains. Along
with cooling, the drying climate caused the demise of
some forests and favored the establishment or expan-
sion of shrublands and grasslands (beginning in the

Fig. 2.4. Fossil of a 16-inch fish ( Diplomystus dentatus ) that
lived about 50 million years ago in freshwater Fossil Lake, lo-
cated near Kemmerer in present-day southwestern Wyoming.
Some fossil beds in this area are protected in Fossil Butte
national Monument. the fossils of palms and other tropical
plants and animals are found in the same area.

Pliocene, about 5 million years ago). 22 Drought-tolerant
plant species persisted or immigrated from nearby.
Some, such as sagebrush, needle-and-thread grass, and
bluegrass, migrated from the old World across the Ber-
ing Land Bridge that connected Siberia and Alaska at the
time. Halophytic (salt-tolerant) plants found in modern
inland salt marshes evolved from plants that are char-
acteristic of marine coastal environments, such as salt-
grass, alkaligrass, and cordgrass. Fires killed many of the
trees that still grew on the upland, hastening the spread
of grassland and restricting woodlands to ravines, valley
bottoms, or ridges that burned less often. the grasslands
created new ecological niches for mammals, and the pre-
decessors of all modern Wyoming animals appeared in
the fossil record, including several kinds of pronghorn
that are known only from north America. only one
species of pronghorn survives today. Also present were
the camel, horse, mammoth, mastodon, rhinoceros,
sabertooth cat, giant beaver, giant ground sloth, and
short-faced bear—all of which would become extinct in
north America about 10,000 years ago, a few thousand
years after the first humans arrived from eurasia.

Fig. 2.3. (left) intermountain basin development during the
past 70 million years. ten to five million years ago the basins
were filled with sediments eroded from adjacent mountain
ranges. Since that time, rivers have cut canyons through
mountain ranges because of regional uplifting. Some basin
sediments have now eroded away, and the ancient mountain
ranges are again more fully exposed. See also chapter 17. From
Knight (1974, 1990). Drawings by Samuel H. Knight.

The Past 2 Million Years

Glaciers, warm interglacial periods, and erupting volca-
noes characterized the Quaternary Period —the past 2.6
million years. the first part of the Quaternary is known
as the Pleistocene Epoch or ice Age. Snow accumulated in