Geoscience Reference
In-Depth Information
in the northern Great Basin—Lahontan and Bonneville lakes—were about
ten times larger during the late Pleistocene than today. Searles and Russell
lakes in the southern Great Basin, in contrast, expanded by a factor of only
four to six. Benson hypothesized that, during the late Pleistocene, the position
of the jet stream may have been predominantly situated over the northern
Great Basin, bringing many more rain-bearing storms to that region.
the thawing of the west
h e world that the earliest human immigrants would have found was
quite dif erent from the world we know today. Large regions of North
America were still shrouded by ice, and glaciers blanketed the mountain
ranges, reaching down to much lower elevations than during the warmer
Holocene.
As the mountain glaciers l owed downslope, they carved out valleys.
Later, when the ice had retreated, these valleys would be let with a char-
acteristic “U shape,” with steep sides and wide, l at valley l oors. When the
i rst human immigrants were crossing the land bridge (over what is called
the Bering Strait today), the l oors of these glacial valleys, like Yosemite in
California's Sierra Nevada, were still buried under hundreds of feet of snow
and ice, with only the highest peaks standing above the ice. Cold tempera-
tures during this glacial period displaced the forests in the mountains to
lower elevations and lower latitudes. h e plant assemblages in the moun-
tains dif ered as well: i rs and certain pines were the dominant trees, and
sagebrush communities tolerant of cold, dry conditions were more common
at the lower elevations.
At er the peak of the last ice age, when the world began to thaw, condi-
tions in the western mountain ranges changed rapidly as the alpine glaciers
melted, leaving behind accumulations of rock, gravel, and sand that had
been scraped of the canyon bottoms and walls. h ese deposits were massive
enough to block streams swollen with glacial meltwater, forming vast glacial
lakes, hundreds of feet deep. Yosemite Lake, for instance, was formed when
the ancient Merced River was dammed, forming a lake that covered a region
over i ve miles long. Half Dome and El Capitan towering above the lake
would have been rel ected in its surface. h e lake bottom now forms the l at
valley l oor of Yosemite Valley. In the higher Sierra Nevada, lakes formed by
glacial dams still exist today.
