East of Yellowstone
Geology of Clarks Fork Valley and the Nearby Beartooth and Absaroka Mountains

INTRODUCTION

Clarks Fork of the Yellowstone River originates high in the Beartooth Mountains of south-central Montana. The sources are in the Absaroka Beartooth Wilderness, crowned by 12,799-foot Granite Peak, the highest point in Montana. Even in a dry summer, the river is fed by countless permanent snowfields and a few small glaciers in the Beartooth and Absaroka Mountains, including Sky Top Glacier on the southwest side of Granite Peak. The route that the water of Clarks Fork presently takes from the Beartooth Mountains to the Bighorn Basin is identical to the path ice took 20,000 years ago. In this introduction we will follow the stream and the glacier from their cold, high source to relatively warm, dry Bighorn Basin.

Most of the Beartooth Range is underlain by 2.7 billion-year-old granites and gneisses. The area is one of the best places on Earth to study these Archean – the oldest unit of time on the geologic time scale – crystalline rocks in three dimensions. First, this part of the Earth’s crust has been uplifted many miles. Second, glaciers and rivers have eroded deeply into the Beartooth Plateau, so that the crystalline rocks are exposed in cliffs thousands of feet high. Third, vast areas are above timberline, with little in the way of glacial deposits, soil or vegetation to hide the bedrock. Finally, the Beartooth Highway provides access to these 2.7 billion-year-old rocks and, in one place, to metamorphic rocks 3.4 billion years old.

The Beartooth Mountains are blessed with hundreds of water bodies, many above the 9,000-foot timberline. The names are magical: Leaky Raft Lake, Astral Lake (at the headwaters of Star Creek), Lady of the Lake Creek, Sodalite (a mineral) Lake and Creek. Some lakes are paired: Lone and Companion lakes, Bob and Dick lakes, Round and Corner lakes, Swamp and Marsh lakes, Upper and Lower Aero lakes. The tributary Broadwater River is wider than the main stem Clarks Fork.

Fisher Creek, the westernmost tributary of Clarks Fork, rises near the triple divide of Henderson Mountain. The northwest end of this gold-bearing mountain drains from an abandoned open pit mine into the Stillwater River, which flows north across the Absaroka Beartooth Wilderness to join the Yellowstone River near Columbus, Montana. The southwest flank of Henderson Mountain is drained by Miller Creek, which joins Soda Butte Creek at Cooke City, Montana; Soda Butte Creek flows past tailings left from decades of mining and into Yellowstone National Park where it joins the Lamar River and passes thousands of large mammals in the Lamar Valley. The Lamar River empties into the Yellowstone River at the north end of the Grand Canyon of the Yellowstone.

Fisher Creek lies on the northeast side of Henderson Mountain. Its valley was to be the site of an engineered tailings pile for a proposed underground mine. During all the mining in the New World District from the late 1800s to the mid 1900s, the “mother lode” was never found. Exploratory drilling on Henderson Mountain in the 1980s pierced a deposit of gold, silver and copper worth billions of dollars. For political and ecological reasons, principally because the proposed mine was less than three miles from Yellowstone National Park, the project was killed in the 1990s.

Fisher Creek enters Clarks Fork of the Yellowstone River near Colter Pass, with an elevation above 8,000 feet. To the west of Colter Pass lie Cooke City and the Northeast Entrance of Yellowstone National Park. To the south of Colter Pass are Pilot and Index peaks, part of the Absaroka Mountains, a range much different from the Beartooth Mountains. Whereas the Beartooth rocks are ancient, light-colored and plutonic (formed at great depths in the Earth’s crust), the Absaroka rocks are young (55 million to 38 million years old), dark-colored, and volcanic (chiefly andesites, named for the Andes Mountains).

From the vicinity of Colter Pass, Clarks Fork flows southeast into Clarks Fork Valley. At its northwest end Clarks Fork Valley is quite asymmetric because the top of the resistant granite slopes gently to the southwest. To the northeast the country rises gradually toward the high Beartooths; the forest is interrupted by lakes (e.g., Lake of the Winds, Lake of the Clouds) and granitic hills (e.g., Rock Island Butte). The relief is due to differential erosion by the glacier that repeatedly covered and receded from this terrain. Where the fractures in the granitic rock are more closely spaced, thousands of feet of ice excavated depressions that now hold lakes; where the fractures are more widely spaced, the rock is more resistant, so a hill may have survived abrasion and quarrying by the ice.

Although Clarks Fork crosses the ancient granite from its headwaters all the way to the mouth of Clarks Fork Canyon, just to the southwest of the river are beds of sandstone, shale and limestone deposited in a shallow Paleozoic (540 million to 248 million years ago) sea. Atop these generally light-colored sedimentary rocks are the dark rocks called the Absaroka volcanics. The southwest side of upper Clarks Fork Valley is steep because the sandstones, limestones and volcanics there are resistant cliff formers.

Three glacial horns – pyramid-shaped mountains like the Matterhorn – crown the southwest side of upper Clarks Fork Valley; from northwest to southeast rise Index Peak, Pilot Peak (the highest at 11,699 feet) and Jim Smith Peak. The summit of each was a nunatak – Inuit term meaning “lonely peak” – rising above the glacier that buried Clarks Fork Valley with more than 3,000 feet of ice. At Colter Pass and elsewhere the Clarks Fork Glacier was attached to the larger Yellowstone Ice Sheet, which stretched as far south as Jackson Hole in Grand Teton National Park. Joining Clarks Fork across the valley from Jim Smith Peak, Crazy Creek is born in the snowfields of the high Beartooths; it connects many lakes before plunging over Crazy Creek Falls near its mouth.

Between Crazy Creek and Lake Creek, much of the granitic bedrock is hidden beneath giant left lateral moraines built by the retreating Clarks Fork Glacier (left and right are determined looking down a glacier). Moraines are ridges of till, a poorly sorted mixture of sediment ranging in size from mud to boulders, deposited at the margins of glaciers. Both Crazy Creek and Lake Creek originate above timberline and end in long, steep rapids over granitic rock.

After Lake Creek joins Clarks Fork, the river’s channel changes from braiding over alluvium (river-deposited gravel) to a bedrock channel with many small rapids. Granite underlies the rapids and nearby small, glacially eroded hills. Near Hunter Peak Campground, the Lewis and Clark Trail heads east along the north side of Clarks Fork; this trail goes all the way downriver to the mouth of Clarks Fork Canyon.

Hunter Peak rises above the right bank of Clarks Fork to an elevation of 9,034 feet. This peak’s summit was near the top of the glacier that filled Clarks Fork Valley 20,000 years ago. Hunter Peak is composed of horizontal sedimentary rocks from top to bottom, but near the summit is a boulder of gneiss carried there by ice from the Beartooth Mountains to the north.

Looking at Hunter Peak and nearby mountains, one would assume that all of their rocks are in place, that they have all been here since originally deposited in a Paleozoic sea hundreds of millions of years ago. But the upper half of the mountain slid here 50 million years ago during the volcanism that produced the dark Absaroka volcanic rocks. The giant mass originated near the Northeast Entrance of Yellowstone National Park and slid southeast far out into the Bighorn Basin. The feature is called the Heart Mountain detachment, named for Heart Mountain, where some of the same old rocks found in Hunter Peak lie on much younger sedimentary rocks.

East of Hunter Peak, Clarks Fork is joined by a major stream from the north, Beartooth Creek. This creek’s headwaters lie east of Beartooth Butte; one of its tributaries, Bear Creek, is adjacent to the Beartooth Highway near Beartooth Lake and Beartooth Falls. Ice from the area east of Beartooth Butte flowed south along Beartooth Creek and other routes to join Clarks Fork Glacier.

Where Clarks Fork changes direction from southeast to east, the river’s discharge is increased by the flow from one of its largest tributaries, Crandall Creek. The two forks of Crandall Creek originate on Yellowstone National Park’s east boundary, the drainage divide between Clarks Fork and the west-flowing Lamar River. The glacial history here is unusual in that at times the large Clarks Fork Glacier moved east up Crandall Creek, whereas later the ice along Crandall Creek flowed west down into Clarks Fork Valley.

Just after Clarks Fork is joined by Crandall Creek, it flows south of Table Mountain, a sloping mesa (elevation 8,761 feet) capped by resistant limestone. Here the river lies north of Swamp Lake, a wetland in a shallow basin resulting from differential glacial erosion of the granitic bedrock. South of Swamp Lake, Cathedral Cliffs, which rise 3,000 feet above Clarks Fork, are composed of sedimentary and volcanic rocks subject to landslides and mudflows.

Between Table Mesa and Swamp Lake, Clark Fork Canyon abruptly begins. At first the canyon is only hundreds of feet deep, but as the river plunges over rapids and waterfalls, the canyon deepens so that its walls tower thousands of feet high. Glacial striations near the bottom of the gorge prove that it was carved not only by water but also by ice.

Wyoming is graced with only one nationally designated wild and scenic river; it is Clarks Fork of the Yellowstone River from Crandall Creek downstream 20.5 miles to the mouth of Clarks Fork Canyon. The gorge is deep and quite inaccessible in most places except to brave kayakers.

A prominent landmark, Windy Mountain crowns the mountains just south of the start of Clarks Fork Canyon. With an elevation of 10,262 feet, this peak is the highest between Clarks Fork Valley and its southern neighbor, Sunlight Basin. The elevation of this area was too low for its own glaciers to form, but thick Clarks Fork ice pushed up Lodgepole and Reef Creeks, crossed the drainage divide, and descended Trail Creek and Painter Gulch toward Sunlight Basin.

Near the mouth of Canyon Creek, which originates near Frozen Lake and Beartooth Pass, Clarks Fork bends to the southeast. The bottom of Clark Fork Canyon is 3,000 feet below the summit of Sugarloaf Mountain, which was surrounded but not overtopped by the Clarks Fork Glacier. Pushing against Dead Indian Hill, the ice divided. Part of the glacier advanced southwesterly up Sunlight, Elk and Dead Indian creeks, damming lakes in each valley. The rest of the ice descended northerly in the deepest part of Clarks Fork Canyon.

Sunlight Creek winds through Sunlight Basin before cutting a canyon through limestone and then granite. Located high in the steep Absaroka Mountains, the upper part of the drainage basin had its own ice and contains small glaciers to this day. Twenty-thousand years ago, icebergs carrying granitic boulders calved into 600-foot-deep Glacial Lake Sunlight, which was dammed by Clarks Fork ice.

Flowing north and then east in the last six miles of its canyon, Clarks Fork is far below the Beartooth Plateau (elevation greater than 10,000 feet) to the west and Bald Ridge (elevation more than 8,000 feet) to the east. At the mouth of Clarks Fork Canyon, at an elevation of only 4,400 feet above sea level, the river leaves its granitic gorge to enter the Bighorn Basin. Tens of thousands of years ago, ice originating high in the Beartooth and Absaroka mountains deposited ridges of sediment called moraines a few miles east of the mouth of the canyon. Clarks Fork flows east through the moraines, then swings north past the tiny town of Clark. Having left the rapids and waterfalls of the canyon, Clarks Fork meanders across its floodplain on the way to Montana and the Yellowstone River.

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