In the Beginning
Wyoming -- huge empty basins with wraparound horizons, followed by massive
mountain ranges that give vertical expression to breathtaking beauty.
What geologic excesses created this pattern of high plains and great
mountains? For Wyoming, to start at the beginning means to start at
the top. The schists and gneisses now in the highest peaks of
Wyoming's mountain ranges are the tattered relics of the Precambrian, the
first and longest chapter of Wyoming's history, extending 90% of the way
along earth's time line. Geologists believe that during this time,
processions of mountain ranges were lifted up, then eroded to nothing, one
after another. Their remnants have been "folded, metamorphosed, and
intruded upon by other rock types,” and show up in the basement level rocks
that are exposed, in a classic twist of story line, on Wyoming’s mountain
peaks.
The next two chapters in our history were long, wet, and dreary -- 375
million years as an ocean floor during the Paleozoic and 160 million years
of transition in the Mesozoic when the environment fluctuated between wet
and dry. One result of all this water was a layer of sedimentary rock
several miles deep covering all of Wyoming. Within this rock lay
buried treasure: a spectacular fossil record of the fish that swam the
Paleozoic seas; Wyoming's oil reserves from the masses of dead marine life
left behind when the seas retreated; and dinosaur fossils and coal from the
Mesozoic.
The Birth of the Rockies
Then came the Laramide Orogeny, a musical name for a spectacular event --
the birth of the Rocky Mountains. Why the mountains rose is uncertain,
but it may have had something to do with the movement of various plates
within the earth’s crust. The Laramide Orogeny occurred about the same
time that the North American plate moved west to accommodate the growth of
the Atlantic Ocean plate, and collided with the Pacific Ocean plate,
crumpling the continent’s western edge much like a car's hood might crumple
when it hits a wall. Highly respected Wyoming geologist David Love,
however, is not sure that a collision 1,000 miles away on the coast can
totally explain what happened here.
However murky its causes, the effects of the Laramide Orogeny are very
clear. In Wyoming, the orogeny began relatively slowly at the end of
the Mesozoic, with the raising of the Wind River, Granite, and Medicine Bow
ranges into high gentle arches like gigantic loaves of French bread.
Next came the ancestral Teton and Gros Ventre range, then the Beartooth, the
Owl Creek, the western Gros Ventre, the Washakie and the Bighorn ranges, and
the Black Hills. By this time dinosaurs were facing extinction and
mammals were soon to dominate the earth.
As the Cenozoic opened, the Laramide Orogeny got really exciting.
Changes in the earth's core, like a monstrous gastric attack, created
intense pressures that churned upwards. Earthquakes and volcanoes
broke what had been a relatively intact sheet of rock into a patchwork of
pieces. Each rode the currents of the disturbance below, bumping,
sliding and grinding into each other like contestants in an overcrowded
demolition derby. When the dust settled some 55 million years ago,
Wyoming was no longer a place of sedimentary rock draped smoothly over
French bread mountain ranges.
Some mountains had been pushed up 14,000
feet; basins had dropped 30,000 feet or more. The Wind River Mountains
had moved to the southwest; one part of the Bighorns had moved east while
the other part shifted south. The Beartooths had done something
similar, moving east and southwest. The Medicine Bows had gone east,
the Washakie Range west, while the Sierra Madres stood still. The ranges
themselves ran every which way, and their sedimentary covering was in sad
shape -- cracked, twisted, bent, tilted and faulted. The underlying
rock of the mountain tops (those Precambrian schists and gneisses we
mentioned earlier) punched through their sedimentary crust, sloughing it off
into the basins below. Pieces of Utah's sedimentary covering broke
loose from their underlying rock and skidded 50 to 75 miles east over the
top of younger rock, to pile up against the Gros Ventre and ancestral Teton
ranges, each section overlapping the next like shingles.
When the Laramide Orogeny was over, Wyoming's mountain landscape was complete except
for three sections: the Tetons, the eastern Gros Ventre range, and the
Absaroka range.
Monster Volcanoes
Mother Nature, however, still had a few surprises left. Wyoming spent the
rest of the Cenozoic under the reign of monster volcanoes run amok.
Thousands of volcanic vents in the Absarokas and Yellowstone Park (with
minor assistance from vents in the Rattlesnake and Black Hills), spewed
volcanic debris nonstop for millions of years.
In the end, the
mountainous landscapes of the Laramide Orogeny were virtually gone, buried
under thousands of feet of lava and ash. Only the tallest peaks poked
through as small hills. Because the protruding tops aged differently
than the buried mountain slopes, a volcanic "high water mark" can still be
found on Wyoming's highest ranges at 11,000 to 12,000 feet above current sea
level.
At the close of the Cenozoic, a new volcanic area developed
along the northwest side of the Tetons, dumping yet more debris. As if
this last explosion had unbalanced the earth itself, a 50-mile crack opened
in the Teton landscape. The western side of the fault rose rapidly,
even as the eastern side sank; creating Wyoming's youngest mountains, the
Tetons, and the complementing basin, Jackson Hole. Across this grim
volcanic landscape a few lazy rivers wandered toward the Gulf of Mexico.
Mighty Rivers
Then, in the Quaternary Period, an epeirogeny (in which the continent rose
as a whole) boosted Wyoming to its current elevation and breathed new life
into the lethargic rivers. As their gradient steepened, they
quickened, straightened, and began to cut and erode, hauling the volcanic
blanket out to sea. In a mere three million years they removed what it
took the volcanoes some 40 million years to deposit. These mighty
rivers, which had established their courses on the flat landscape of the
volcanic blanket, paid scant attention to the sensibilities of the mountain
ranges beneath them. If a mountain happened to surface, they simply
cut through it from the top down.
As a result, Wyoming's major rivers
follow courses that seem to defy logic. The Laramie River, flowing
north along the west side of the Laramie Mountains, suddenly makes a sharp
bend to the east, cutting directly across the range. Similarly, the
North Platte River cuts through the Seminoe and Granite mountains and the
Sweetwater slices the end off a granite ridge, creating Devil's Gate, when
it would surely have been simpler to have gone around.
High above these rivers a final force was also at work -- glaciation.
Northwestern Wyoming was a center for a huge ice mass during the
Pleistocene. Jackson Hole and the Tetons were scraped clean by at
least three successive rivers of ice, and many small glaciers still remain
in the Absarokas and Teton ranges. Erosion from the rivers, the
glaciers, and the wind have left us with the Wyoming you see today.
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