A lookback
at the Yellowstone Ice Cap
By Jessianne Castle EBS ENVIRONMENTAL & OUTDOORS EDITOR
GARDINER –
Ken Pierce pulled his hat low as a katabatic wind swept fiercely down the
valley. The retired geologist moved briskly, despite walking head-on into the
October gale. After collecting himself at the top of a small rise, he stopped
and cast his eyes to the ground.
We were
standing upon an overwhelmingly barren slope, and the surface of the land was punctuated
by a series of ripples as the hill descended toward the road. Amid non-native
cheatgrass and sparse clumpings of rabbitbrush, large rocks protruded from the
soil in long, narrow rows.
The rocks
were deposited by water some 15,000 years ago as a series of glacial
floodwaters 200-feet deep poured from Lamar Valley out into the Yellowstone
River drainage. The volume and force of the water had its way with the land,
and much like the ocean tide creates a kind of washboard in sand on a beach, the
glacial water formed a series of megaripples roughly 60 feet apart.
“This is an
amazing geologic feature. I call this a geewizz stop,” Pierce said. He spoke
from years of research, explaining the process of glaciation in the Yellowstone
range.
We were just
south of Gardiner, and with Sepulcher Mountain gazing sternly upon us, we
climbed from our bus parked on the side of Old Yellowstone Trail, a dirt road
that meanders through a strip of Yellowstone National Park. Pierce, who retired
in 2003 but continues to work as a geologist emeritus with the United States
Geological Survey’s Northern Rocky Mountain Science Center, was leading a class
of about 20 as a part of the Montana State University Alumni Association’s
Wonderlust Lifelong Learning continued education program.
Entitled
“Ice Age Yellowstone,” the day-long course was an immersive look at the remains
of an ancient glacier that once capped a large swath of the Greater Yellowstone
region.
In Gardiner,
before floodwaters rushed over the land, an estimated 3,000 feet of ice claimed
the drainage. Only the very top of Sepulcher Mountain in the Gallatin Range to
the west and a select few pinnacles in the Beartooths to the east were free of
the ice cap’s hold.
Researchers
believe Yellowstone has experienced a number of glacial periods over the last
2.6 million years, with the last major glaciation occurring about 20,000 years
ago. Known as the Pinedale Glaciation, ice and water poured from the Washburn
Range, Beartooth Plateau and over the Gallatins, ultimately forming today’s
landscape around the North Entrance of the park, as well as Paradise Valley.
Indeed, the
unique landscape known for its thermal hot springs and geysers—not to mention, one
that hosts four million visitors each year—is as much thermal as it is glacial.
Yellowstone’s
high-elevation volcanic plateau was an ideal site of glaciation as snow readily
accumulated across the region, and the park’s glaciers formed over an extended period
of time as more snow fell than melted. As the snow built up, it turned to an
ice cap over a large extent of Yellowstone National Park. Along its edges, the
ice began to slowly move under the force of gravity and from its own weight, forming
glaciers that picked up rocks, eroded the earth in some places and deposited
sediment in others.
As the
climate warmed and the earth transitioned into what is known today as the
Holocene geological epoch, the northern outlet glacier in Paradise Valley began
to recede south, leaving behind wide swaths of fine-grain sedimentation known
as outwash plains, and accumulations of rock and mixed sediment that formed
hill-like moraines.
From the
pull off at Mallard’s Rest Campground 14 miles south of Livingston, we observed
the remains of Pinedale’s first recession. Highway 89 juts south from
Livingston across the glacier’s outwash plains, dramatically visible at
Mallard’s Rest. With cars on the highway whizzing past, one need only look
west, down off the 150-foot cut bank and into the Yellowstone River.
“We’re quite
a bit above the Yellowstone River,” Pierce said, gesturing to the cut bank and river
below. “The reason that the outwash is so thick here is that the weight of the
ice cap pushed the crust of the earth down. Normally in Rocky Mountain
glaciers, the outwash is about 30 feet thick.”
As the ice
melted, the resulting water slowly established a route down Paradise Valley,
and huge volumes of water eroded the outwash sediment, forming what we see
today as a large cut bank.
The historic
Chico Hot Springs are nestled within the remains of a second recession period
for the glacier, as the irregular, hilly landscape, characterized by mismatched
smooth and angled rocks of various sizes marks the location of the Chico
moraines, where the edge of the glacial ice sat for a period of time.
Over the
course of several thousand years, the Yellowstone glacier slowly diminished in
size and melted ice accumulated as a lake within Lamar Valley, dammed by
gigantic chunks of ice. But once enough water collected to float the ice, the
lake waters were released, and a deluge swept into the Yellowstone drainage.
With ice
melted and amid a warming climate, the landscape transitioned from a barren
tundra to a semi-forested complex of lodgepole and Douglas fir parklands that
have evolved into today’s dense forests and hot and dry sagebrush desert
steppe.
