Fire and Ice - Paradise Valley Geology 101

John Gillespie & Whitney Tilt

First Fire

Strata volcanoes erupted frequently 65-35 million years ago in Paleocene through Eocene times (Figure 1), leaving deposits of andesitic lava, lahar debris flows and petrified forests – forming the mountains of the Absaroka Volcanic Supergroup. These volcanoes, part of a “ring of fire”, were caused by the subduction of the Pacific Plate under the North American Plate and a contemporaneous uplift called the Laramide Orogeny. Two northwest trending belts of eruptive centers, termed the Eastern Absaroka Belt and the Western Absaroka Belt, include well known peaks like Sunlight, Washburn, Electric, Emigrant and Hyalite. The ancestral Yellowstone River drained the mountainous topography. Paradise Valley formed around during this time.

35-25 million years ago during the Oligocene epoch the valley filled with sediments from the eroding mountains and the valley collected the fluvial deposits of the Renova Formation – much of which is now eroded.

From 25 to 8 million years ago during Miocene epoch tuffaceous siltstones like those at Hepburn’s Mesa were deposited in shallow alkaline lakes and mudflats much like those seen at the Salton Sea today.

First Hints of More Fire

Much of the volcanic ash within Hepburn’s Mesa siltstone originates from volcanic eruptions tied to the first arrival of the North American Plate over the mantle hot spot that dominates the active volcano at Yellowstone today. The track of the heat is along the Snake River Plain where the sheet-like edge of the North American Plate moves to the west-southwest at an inch or two a year over the relatively stationary propane torch of the hot spot, allowing super-heated magma to penetrate through the crust along its route. The 16.5-million-year-old white tuffaceous siltstone at Hepburn’s Mesa coincides with air-fall ash from the first eruption of the hot spot – the McDermitt caldera, which occurred soon after the deposition of the Columbia flood basalts in Washington State. The McDermitt ash-fall caused the death and preservation of a rich assemblage of Miocene mammal life at Hepburn Mesa – including Merychippus – the first equine to exhibit the head of today’s horses. Another of these ash-falls, tied to the Bruneau-Jarbridge caldera, ~12 million years ago, resulted in similar death and preservation at Ashfall Fossil Beds State Park in Nebraska where excellently preserved Teleoceras, a hippo-like ancestor of the rhinoceros, are stacked in layers of ash-rich siltstone.

Coming after the Miocene lakes the Yellowstone River dominates the return from lacustrine deposition to fluvial deposition. At Hepburn’s Mesa and at many places in Paradise valley, a coarsely-sorted, mixed-grain, mixed-color conglomerate, deposited in the river bottom, can be observed immediately above white tuffaceous siltstone.

Red Hot Fire: Arrival of Yellowstone over the Mantle Hot Spot

2.2 million years ago, the waters of the Yellowstone River had formidable competition. A lava flow, a precursor to the Huckleberry Ridge super eruption –one of the most cataclysmic volcanic eruptions in history, gravitated to the topographic low occupied by the river. In a boiling battle with sound and light, hiss and sizzle beyond comprehension like can be seen where lava from Pu’u O’o meets the ocean at Kilauea today, the lava bested the water as is documented by the thick black deposition of basalt that lies immediately above the river bed conglomerate. The basalt originates from a local vent and can be traced down flow to a site where it pinches out just north of Emigrant. The 2.2-million-year radiometric age of this basalt matches that of Junction Butte in the national park, predating the 2.1 million year Huckleberry Ridge super eruption by 100,000 years. Incongruently, there are glacial striations scratching the top of the basalt and a thin profile of sediment above the basalt is glacial till with glacial erratic boulders – hints of the ice that followed the fire.

The Snake River Hotspot Track. (from Branney 2007)

A Geological Collision

Paradise Valley lies between the Great Plains and the edge of the Yellowstone volcanic plateau. According to the Montana-Yellowstone Geologic Field Guide Database, the valley shares basement rocks with the continental interior; Paleozoic and Mesozoic lithologies with the western interior, compressive tectonics with the Fold and Thrust Belt to the west; Basin and Range with the west and south; and Cenozoic volcanism with much of the surrounding region. The database describes the area as a “microcosm of the evolution of the geological understanding of the American West.”

While many of us may or may not understand the above, overly-technical description, here is a partial list of some of the geological highlights in the Paradise Valley. All are easily found and observed:

Landforms: Gallatin Range, Beartooth Range, Hepburn Mesa, Yankee Jim Canyon
Metamorphic basement rocks of the North Snowy Block, near Pine Creek
Paleozoic, Mesozoic, and Cenozoic sedimentary rocks
Hepburn’s Mesa Formation basalts
Point of Rocks volcanic/intrusive center (Absaroka Volcanic Field)
Travertine deposits at Gardiner and the Liberty Cap at Mammoth
Glacial deposits (Wisconsinan/Pinedale age) – more below
Devil’s Slide (Figure 2)
Giant Ripples flood deposits at Corwin Springs
Rangefront alluvial fans
Chico and La Duke Hot Springs hydrothermal features
Nelson and Armstrong Spring Creeks

Mountain Formation

Beginning some 65 million years ago, during the early Cenozoic, the Yellowstone region underwent periods of uplift and folding that alternated with periods of quiet and erosion. Volcanic activity, accompanied by uplift and folding, showered the landscape with volcanic ash, dust, silt and sand. Deluges of coarser debris flowed down the volcano flanks in lahars which became the volcanic breccias. Periods of quiet allowed semi-tropical plants and animals to flourish and nearly 200 fossil species have been identified in the Absarokas, perhaps the most diverse petrified forest in the world. These include unexpected species like breadfruit, persimmon, magnolia and mangrove. Resumed volcanism buried or transported entire forests into valleys as witnessed at Spirit Lake below Mount St. Helens and other places around the globe.

The Beartooth Plateau, forming the eastern boundary of Paradise Valley and consisting mainly of Precambrian basement rocks with a local veneer of Eocene Absaroka volcanic rocks, pushed its way upward through sedimentary rock during the Laramide orogeny (~70-45 million years ago). It contains some of the oldest exposed rock on Earth. The Quad Creek Quartzite on Beartooth Pass has been radiometrically dated as 3.95 billion years old. Yankee Jim Canyon provides exposures of foliated crystalline metamorphic rock - granitic gneiss, micaceous schist, and pegmatite dikes formed miles below the surface.

Cinnabar Mountain, home to the Triassic Chugwater red bed of the Devil’s Slide, provides a view of remnants of the hanging wall of the Gardiner thrust fault exposing nearly vertical beds of Paleozoic and Mesozoic rock (Figure 2) and provides “hieroglyph-like” evidence of ~500 million years of Earth history.

The Gallatin Range, forming the western boundary of Paradise Valley, is generally lower in elevation and less vertical as compared to the Beartooths. Now at ~10,000 elevation, many of the peaks are comprised of Paleozoic carbonates that were originally deposited in epieric oceans. Paradise Valley is in a downthrown position relative to the Beartooths as a result of Basin & Range normal faulting that arrived about the same time as the Yellowstone Hot Spot and they were buried by younger volcanic rock after faulting occurred. Paradise Valley is the easternmost expression of the Basin and Range province. The extensional tectonics, often described as a rift, are represented by the presence of scarps along the Deep Creek-Luccock Park-Emigrant fault.

As uplift continued across the region, the mountains and valleys grew in elevation and cooled. Increasing snowfall failed to melt as rapidly as it fell. The Ice Ages arrived.

Devils Slide, Corwin Springs. Vertical bands with older (Paleozoic) rocks on right and younger (Mesozoic) rocks on left. 1- Pinedale glacial till; 2-Devonian-Mississippian undifferentiated limestone; 3-Triassic rock; 4-Cretaceous undifferentiated rock.

Ice Age Cometh

Beginning about two and a half million years ago, yesterday in geologic time, the Pleistocene epoch in the northern hemisphere has been characterized marked by advances and retreats of continental ice sheets. The Yellowstone region underwent a series of glacial stages. The earliest glaciations were largely bulldozed away by subsequent ones, removing evidence of their distribution.

The Bull Lake glaciation, ~200,000 years ago through 130,000 years ago, concurrent with the continental Illinois glaciation, created widespread valley glaciers including one that covered the northwest portion of Yellowstone Park, and flowed northward down the current course of the Yellowstone River. Overtopping lesser peaks like Cinnabar and Dome Mountain as it flowed northward, its width varied from 3-6 miles and it height estimated at 3,000 feet. Most of the landforms created by the Bull Lake glaciation are gouged and overprinted by the subsequent Pindale glaciation, but some good landforms are preserved in the Teton and West Yellowstone areas.

The region’s last glaciation, the Pinedale, occurred concurrent with continental Wisconsin glaciation began approximately 70,000 years ago and reached its maximum ~25,000 years ago. It left the most visible record and its recession has been carefully studied by cosmogenic and other dating methods. The Beartooth Mountains, capturing precipitation from the caldera-rich lowland of the Snake River Plain, provided the incubator for the Yellowstone Outlet Glacier. From its source high in the Beartooths near Granite Peak to its terminus at the Eight Mile Moraine near the junction of the Yellowstone River and Mill Creek, it was nearly 70 miles long. Based on eroded mountain tops and remnant horns and aretes, in some places towards its center the glacier was approximately 4,000’ thick. Tom Miner and Big Creek were both covered by Pinedale ice. Evidence of this glaciation can be seen in moraine remnants around Chico Hot Springs and the ice- margin channels and terraces on the mountain slopes between Big Creek and Fridley Creek on the west side of the valley. As the glacier receded, much like the collapse of Glacial Lake MIssoula, ice dams broke in the Lamar Valley and catastrophically released massive volumes of impounded water. Enormous floods, some with headwalls estimated at 150-300’, ripped down the Black Canyon of the Yellowstone and deposited ripple marked flood bars of boulders as much as 50’ high near Gardiner.

Glacial ice left its signature on the Paradise Valley landscape in the form of moraines, glacial striations, moraines, ice marginal channels, kettles, polished sheepback hills, till and glacial erratics, including many along Big Creek.

In the blink of an eye (geologically speaking), plants and animals recolonized the landscape and humans arrived to hunt and forage the abundance of life at the ice edge. Along with sheep traps and bison jumps, lithic quarries, stone circles, fire pits and other artifacts of Clovis and other Paleo-Indian peoples are found in the area.

Hepburn’s Mesa

A glimpse of the geological history of the valley is on view at Hepburn’s Mesa (Figure 3), the bluffs dominating the east side of the Yellowstone River, opposite Mountain Sky Guest Ranch lands. At the bottom are the oldest rocks-light colored fine-grained sedimentary rocks formed from sediments deposited in an ancient lake that once occupied the site. Atop these rocks are the unconsolidated river bed gravels and conglomerates that are in turn capped by a basaltic lava flow, and lastly by glacial till.

The sedimentary beds contain abundant Miocene fossils, including ancient rodents, moles, and a proto-horse called Merychippus. The basalt lava flow erupted from a volcanic vent some 2.2 million years ago. It is of the same age as the Junction Butte basalt in Yellowstone Park and is a precursor lava flow to the Huckleberry Ridge super eruption at Yellowstone. Its dark coloring is the result of high concentration of iron, including magnetite. On top of the mesa lies the youngest material, glacial till dating to the Pleistocene age. The surface of these deposits is hummocky and is littered with abundant glacial erratic boulders. For generations, Native Americans drove bison off the mesa’s cliffs and captured sheep in rock traps.

Hepburn’s Mesa is named for John Hepburn, a local rancher, rockhound and amateur paleontologist. He arrived in the Emigrant area in 1909 after working in Yellowstone National Park for many years. From 1935 until his death in 1959, he operated a museum displaying many of the geologic specimens and fossils he had found in this area. The museum still stands and is listed on the National Register of Historic Places.

Fire & Ice SPEAK

Fire (Volcanics). Volcanic activity arises from the Earth’s mantle as magma pushes its way up through fissures and other fractures in the overlying rock forming dikes, sills, caps, and flows. Sheep Rock is a volcanic dike – a vertical shaft of igneous rock (dacite) that has become exposed as the softer surrounding rock has weathered away. There are several types of volcanic rock including basalt, andesite, dacite, and rhyolite. Basalt and andesite are dark colored and fine-grained while dacite is quartz-rich and generally lighter in color. Rhyolite is silica rich and light-colored and it the “Yellowstone.”

Glaciers are great rivers of ice formed by the accumulation and compaction of snow on mountains. During their residency in the region they sculpted mountains and carved out valleys. For thousands of years, snowfall accumulated and fed the river of ice as it flowed downslope from the mountain tops. Glaciers are frozen conveyor belts that take part of the surrounding land with it as it moves downslope. Rocks became entrained in its ice. As the ice moved, these rocks scoured away more of the underlying surface. Where the glacier’s forward-most process stopped, it discharged its melt water, dropped its rocks, and staged a series of retreats.

Looking across the Paradise Valley an irregular topography is evident including a series of hummocky hills, some shaped as linear ridges, some as flat-topped headlands above the Yellowstone River, and some as a series of gently rolling hills. These moraines and outwashes are the remnants of long-gone glaciers.

The landform of present-day Paradise Valley is the result of several large glaciers that flowed down the valley, from south to north, leaving a landscape shaped by moraines and outwash. The glaciers may be gone, but their influence is plain to see today.

Moraines are composed of glacial till, an unsorted mixture of silt, sand, pebbles, and boulders dumped directly from the glacial ice. Ground moraines accumulated as irregular blanket of till deposited under the glacier while terminal, recessional, and lateral moraines formed as retreating glaciers left a series of “windrows” at the terminus of the glacier, and alongside the ice mass. Collectively these moraines record the glacial record. In the Paradise Valley, terminal moraines are evident at the outlet of Pine Creek and most of the other creeks flowing from the Absaroka Mountains. Just south of Pine Creek on Highway 89, the road climbs up and over the terminal moraine of the Pinedale Glacial Stage, which reached its maximum northward extent some 16,500 years ago. To the south, the rolling hills area evidence of the ground moraine that covers the entire valley floor. The larger cross-valley ridges formed in front of the retreating glaciers while the lateral moraines formed alongside the ice forms. A definitive characteristic of a moraine is that its till is unsorted – sediment of all different sizes is found jumbled together with no layers evident. This characteristic helps distinguish materials deposited by glaciers from those deposited by running water which tends to deposit different sizes of rocks in different areas. Since the end of the last ice age, some 10,000 years ago, soil has developed on the moraines and trees have taken root.

Outwash is well sorted sand and gravel deposits delivered by the meltwater flowing from the end of the glacier. Outwash deposits are flat on top as compared to the hummocks of the moraines. Imagine 3,000 feet of ice on top of you. As the glacier flowed, its sheer weight ground the underlying rock surface and carried the debris along. The meltwater emerging from the foot of the glacier deposited its load of sediment over the outwash plain, with larger boulders being deposited near the moraines, and smaller particles travelling further downstream before being deposited. On either side of the Yellowstone there are prominent, tall, flat-topped benches formed by the outwash from the last glaciation. Over more recent time, the river has cut its course down through these sediments.

Pine Creek Drainage in background; Pine Creek terminal moraine in mid-ground, and Yellowstone River in foreground. Picture taken looking ENE from Mallard’s Rest Fish Access Site.

Sources and Resources

Berg, Richard B., Jeffrey D Lonn, and William W. Locke. 1999. Geological Map of the Gardiner 30’ x 60’ Quadrangle, South-Central Montana. Montana Bureau of Mines and Geology.

Fritz, William J. and Thomas, Robert C. 2011. Roadside Geology of the Yellowstone Country, Second Edition Missoula: Mountain Press Publishing Company. 149 pp.

llustrated Glossary of Alpine Glacial Landforms. https://www4.uwsp.edu/geo/faculty/lemke/alpine_glacial_glossary/index.html

Montana-Yellowstone Geologic Field Guide Database. http://serc.carleton.edu/research_education/mtroadlogs/logs/NWGeol-1995-SWMT-1.html

U.S. Department of the Interior. National Register of Historic Places Registration Form for the John Hepburn Place, September 5, 2005.

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