Trail Guide to Sacagawea Peak, Northern Bridger Range, MT

By Travis Corthouts and Donald Bent, geology majors, Department of Earth Sciences, Montana State University

Jump down to: Introduction | Directions | Trailhead | Fossils | Cirque Basin Area | Structural Geology |Active Geomorphology | More Fossils | Cirque to Summit | Stratigraphy and Geologic History

*In this trail guide you can single click any image for a larger, full resolution view of any of the pictures.

Introduction

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The hike from Fairy Lake Campground to Sacagawea Peak is a short hike (~2 miles) of moderate grade which takes you through hundreds of millions of years of geologic time. (This is also the start of the famous Bridger Ridge Run). The Bridger Range has a long, distinguished geologic history, and includes rocks that range in age from Archean (greater than 2.5 billion years) metamorphic gneisses, to conglomerates of Protrerozoic age (~1.4 billlion years), and Paleozoic sedimentary rocks that were deposited in the warm shallow seas that once covered this area (~540-100 million years ago). The Bridger Range was formed by at least 5 major mountain-forming events, which caused the uplift of the range, and these structures are evident in the rocks exposed at the surface today. Along the trail to Sacagawea peak you'll see mostly the massive limestones of the Madison Formation (Mississippian in age, ~360-320 million years) which form the prominent cliffs that outline the skyline of the Bridger Mountains. Within these rock formations are a plethora of invertebrate fossils (corals, brachiopods, crinoids, bryozoans) which can be found along the trail all the way to the summit. On the western slopes of the range, units of the Cambrian to Devonian limestones and shales are present, and these contain fossilized remnants of mats of algae called stromatolites. On the lower western slopes, the rocks that are present are Proterozoic (~1.4 billion year old) conglomerates of the Lahood Formation of the Belt Supergroup north of Ross Pass, and in the south, the "basement rocks" are Archean (greater than 2.5 billion year old) metamorphiic gneisses. Take a look at the Stratigraphic Column for Montana (Acrobat (PDF) 4.6MB Dec8 09). In addition, there are many landforms that attest to the ever-changing dynamic landscape that were produced from the last glaciations (~12,000 years ago) through ongoing erosional processes that are still active today. Take a walk through geologic time (http://3dparks.wr.usgs.gov/coloradoplateau/timescale.htm) on the way up to Sacagawea Peak! Also check out the geologic map of the area by Betty Skipp.

Directions from Bozeman

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Directions to the trail head from Bozeman are fairly simple:

Starting in downtown Bozeman at Main Street and Rouse Avenue, head north on Rouse Avenue (state Highway 86). Continue as Rouse Avenue curves east onto Bridger Canyon Drive, past the "M" trailhead, and continues up Bridger Canyon. Continue past Bridger Bowl (~16 miles from Main Street), Bracket Creek, and Battle Ridge Campground to Fairy Lake Road (National Forest Service Rd. 74). Turn left (west) and drive to the Fairy Lake Campground (~6 miles). Fairy Lake Rd is a dirt road and and can be a bit bumpy and slick in wet weather. You don't need four wheel drive to make the trip, but take your time driving to the trailhead. Please stay on the road, and obey all seasonal closings to motorized vehicles. The Sacagawea Peak trailhead is just uphill from the main campground parking area.

Here is what to look for on the trail. This is one of the great hikes in the northern Rocky Mountains. Close to town, but still easily accessible. Every step has something interesting to look for, and the views from the ridge are extraordinary.

Trailhead Through the Trees

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The first section of this hike meanders through old growth forest before a set of switchbacks take you above treeline and into the cirque basin area. While hiking below treeline there are some things of geologic significance to watch for. There are many layers of fossils preserved in the limestones on this stretch of trail--"horn" (rugose) corals, "brain" corals, numerous varieties of shelled organisms called brachiopods, byozoans (with branching and encrustation forms), and crinoids (also known as "lilies of the sea'). The rocks you'll be walking through on this stretch of trail are the Mississippian (~340 million years old) Madison Limestone, which has a lower, thinly layered unit called the Lodgepole Limestone, and an upper, much more massive unit called the Mission Canyon Limestone.

This section of trail also takes you across debris fields made of rock that was carved from the mountain above and deposited below by glaciers and other ongoing processes of erosion. As the trail steepens and begins to switchback, you will notice an increasing number of rock outcrops, and you will be mostly walking up the "dip slope" of tilted beds of limestone.

Fossils to look for on this stretch of the trail

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The rocks seen along the majority of the trail are limestone from the Mississippian Madison Formation, which include the lower thinly-layered Lodgepole Limestone, and the upper section which is the more massive Mission Canyon Limestone. These limestones were originally deposited in warm, shallow seas that covered this part of North America ~340 million years ago. This environment was similar to modern day sub-tropical carbonate platforms--thus, this geologic occurrence has been referred to as "Bahama Montana" (a term coined by Dr. David Lageson). Imagine this countryside once covered by coral reefs and all the organisms that typically live there....

Each of these samples of limestone formed in shallow marine basins which occupied western to mid-western continental North America during the Mississipian epoch 340 million years ago. Since that time, multiple mountain building events have uplifted and transformed the western half of North America into a region littered with mountain ranges. Now, where seas once spanned there are mountains, and the flora and fauna they once supported have been preserved as fossils in the rock formed by them.

Tabulate Coral:

Picture of modern, living corals (known as hexacorals or Scleractinian corals)(left) and fossil tabulate coral known as Syringopora (right). Tabulate corals are now extinct. This sample was observed on the trail near treeline, just before entering the cirque basin. Compare the modern and fossilized forms of these corals.

Rugose Coral:

This picture was taken on the trail near treeline just before entering the cirque basin. Shown are rugose coral fossils preserved in a limestone outcrop. This coral is also called "horn" coral due to its horn-like shape when alive. Rugose corals are now extinct and even though modern elkhorn corals have the same forms they are an entirely different genus.

Brachiopods

The photos on the right (taken in the switch-back section of the trail just below tree line) shows brachiopod fossils within Mississippian limestone which have been exposed from weathering. Brachiopods are a diverse, marine invertebrate organism which have existed since the Cambrian (542 million years ago). These brachiopod shells were part of a reef complex which was preserved in the rock record as fossils in limestone.

Bryozoans:

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Bryozoans are invertebrate fossils that typically live in colonial communities. Individual lifeforms are called "zooids". They make their living as filter feeders, scavanging organic detritus that floats by. Bryozoans first developed in the geologic record in the Early Ordovician (~480 million years ago), and many species continue to exist today. They mostly occur in warm, shallow tropical waters, but some species have been found in deep water oceanic trenches, cold polar waters, and even in fresh water. Bryozoans excrete a solid exoskeleten typically made of calcium carbonate, and the forms of these colonies occur either with branching forms or as encrustations on rocks or other reef organisms. Bryozoans typically have a "wheat chex" type of texture. The openings in the wheat chex are fenestrae (windows) - the animals (zooids) don't live in those, but in tiny, pin-sized chambers in the solid part of the skeleton called zooecia.

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Cirque Basin Area

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Moving above treeline, the trail brings you into a beautiful east-facing "bowl" which is actually a glacial cirque carved out of the mountain during the last ice age 18,000 years ago. Walking through the cirque basin and up the headwall you will notice continuous outcrops of rock on all sides of you. These rocks are now tilted in a near-vertical orientation, giving the Bridger skyline its jagged appearance along "The Ridge". As you approach the saddle on the ridge and continue to the west, you pass across the contact from the Mississippian Madison Limestone to the somewhat older units of the Devonian Sappington Formation yellowish sandy siltstones); Three Forks Formation (gray, thinly bedded dolomites); the Jefferson Formation (gray to brownish dolomite) the Maywood Formation (silty to sandy brownish-gray dolomite); and into the Cambrian units that include the Snowy Range Formation (the Sage Pebble Conglomerate member contains limestone pebbles, chert nodules and siltstones that are rich in algal laminations), Pilgrim Limestone (gray, with fossil fragments and oolites--rounded carbonate grains), Park Shale (gray-green to maroon), Meagher Limestone (massive dark gray with bluish mottling), Wolsey Shale (gray green); Flathead Sandstone white to yellow quartz sands strongly cemented); and on the western slope you will cross an unconformity (gap in time) into the LaHood Formation of the Belt Supergroup (middle Proterozoic in age, ~1.4 billiion years old).

Structural Geology

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This picture was taken from the trail just before the headwall switchbacks, looking at the north wall of the cirque. The folding exhibited in the photo in the Lodgepole Limestone is known as chevron folding and was produced by compressional stress associated with mountain building events. The layers that were once deposited horizontally in continuous layers are now "kinked" and tilted into their present orientation.

The parallel lines in this photo represent bedding planes, or "contacts" between successive layers of rock which were deposited on top of one another, with the younger strata deposited on top of the previous (older) rock layer. These contacts, now tilted at a steep angle, were originally deposited horizontally.

These pictures illustrate two fundamental principles of geology: 1) The Principle of Original Horizontality, which states that sedimentary layers were originally deposited in horizontal layers--an idea that originated with Nicolas Steno in the 17th Century! 2) The Law of Superposition, which states that sedimentary layers are deposited with the oldest units towards the bottom, and younger units are deposited at successively higher levels. These principles have important applications to understanding the Bridger Range. The sedimentary rocks that once must have been deposited horizontally are now nearly vertical--requiring mountain building events to have occurred sometime after the time of deposition. Also, the stratigraphic record must have accumulated over long periods of time, recording ever-changing conditions of deposition. And, fossils that must have been deposited in a marine environment (in some sort of low-lying basin) now occur at the top of the highest mountains. There must have been huge geologic forces at work to cause the uplift and deformation (folding) of these sedimentary rocks.

Contacts between sedimentary rock units are produced when new, lithologically different sediment is deposited on top of older sediments and the contact itself represents the change from one composition to another. A contact doesn't strictly represent compositional change, it can also signify a change in age or character of the rock. In this picture, finely laminated layers of the (older) Lodgepole Limestone (Ml) occur below the more massive layers of the (younger) Mission Canyon Limestone (Mmc).

The picture the above shows a contact within the Mission Canyon Limestone which differentiates thin to medium bedded limestone below the red line and thickly bedded limestone above. The presence of many thin layers is what indicates that the rock unit below the red line is thinly bedded, while the above unit is comprised of thick layers, making it thickly bedded.

This cross-section drawing shows how the sedimentary layers seen in the Bridger Range have been tilted up into their current orientation. The Bridger Range is underlain by an east-verging system of thrust faults that have been modified by reverse faults of moderate to steep dip (just west of the ridgeline). Refer to the geologic stratigraphic column (below) for identification of key rock units. This figure is from Lageson (1986), used with permission.

This figure is a generalized tectonic map of the northern Bridger Range and surrounding areas. Note the main Bridger thrust fault has transported the older Paleozoic rocks that make up most of the range up and over the younger Cretaceous rocks that are exposed lower on the slopes and in Bridger Canyon; and the "back thrust" (movement to the west) that created the impressive Battle Ridge monocline (the big sweeping folds you can see looking to the east from Bridger Bowl); and the important fault that divides the northern and southern Bridger Range, the Pass Fault, that occurs just to the north of Ross Peak in Ross Pass.

One of the major structural features of the Bridger Range is the Pass fault, which has been reactivated for over a billion years of geologic history. It was initially active in the Precambrian when it uplifted and exposed the Archean basement rocks on the south of the range, and resulted in deposition of thick sequences of conglomerates in the Belt Basin (contributing to the Lahood Formation) to the north about 1.4 billion years ago. It was also active in the Paleozoic as the thicknesses of the sedimentary layers are different north and south of the fault, indicating that there must have been differences in ancient topography at the time of deposition. The Pass Fault has also been reactivated during Sevier-style thrusting and has acted as a "lateral ramp" for the big, underlying Bridger thrust system. This picture shows a profile of the Bridger Range taken from the west (Amserdam Road); the Pass Fault resides in Ross Pass, the low saddle on the skyline.

Geomorphology and Landscape Evolution

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Active surficial processes of erosion are what shape mountains into the forms we see today. Glaciers, landslides, rock fall, soil creep, streams and rivers are just a few of the processes working endlessly to reduce mountains one particle at a time. Nowhere on this hike are these processes of mass wasting better seen then along the section of trail taking you through the cirque basin. This large cirque was carved out by glaciers into the Mississippian limestone and the Devonian Shales. Even though the most recent, the Pinedale iceage, ended more than 10,000 years ago, you can still clearly see its effects on the landscape in the creation of the cirque basin, U-shaped valley, and polished rocks on the floor of the valley that were sculpted by glaciers. The ridge of debris in the foreground is a "pro-talus" rampart, which consists of angular rock fragments that have broken off of the overlying cliffs during freeze-thaw processes (mechanical weathering), and are deposited away from the face of a cliff by transport on underlying snow layers.

These photos show the deposits that formed by active debris flows in the past year. Mechanical weathering of the high outcrops, typically by freeze-thaw action in these high elevations, cause the bedrock to break up into smaller, angular pieces. This debris accumulates throughout the year forming talus slopes or an "apron" of debris sitting at a high angle. When there is a big rainfall event (like the thunderstorms that commonly build up in mid-afternoon in this area), the runoff gets concentrated into gullies and channels, and is capable of transporting large volumes of material in a very short time in these channelized debris flows.

The photo at the left shows a great example of differential weathering. The higher rock units are more resistant to weathering than the surrounding rock. This causes the less resistant units to weather faster, therefore isolating the higher units.

More Fossils

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As you approach the saddle on the ridge, you will encounter entirely different fossilized life forms in the Devonian and Cambrian rocks that are exposed there. Most prominent are numerous preserved remnants of algal mats that tend to grow along the margins of shallow seas. These types of fossils are known as stromatoporoids (the root "stroma" means layered), and are interpreted as the remains of calcareous sponges--the dominant reef-builders in the Devonian. Stromatolites are also present on the trail, seen in the first outcrops on the west side of the pass. Stromatolites form as the result of layer upon layer of algae growing in a series of thin layers, and each layer traps a small amount of marine sediment thus forming the very thin rock layers. Columnar stromatolites can be seen in the figure below. Stromatolites record some of the most ancient life forms on earth, and evidence exists that stromatolites formed as far back as 3.5 billion years ago. In fact, its believed that cyanobacteria are responsible for oxygenating our atmosphere, as they are some of the first known photosynthesizing organisms. Here are numerous examples of stromatoporoids and stromatolites that you will encounter towards the top of the Sacagawea trail:

These columnar stromatolites are formed in much the same way as the ones in the picture above. They were deposited as algal mounds. Debris collects on the algae and hardens as the algae dies. Another layer of algae then starts to grow on top of the previous dead layer. This can continue for quite some time as can be seen from the height of the columnar stromatolites at left. These columnar stromatolites are found in the Cambrian Sage Pebble Conglomerate member of the Snowy Range Formation. These outcrops can be seen just to the south and west of the cairn on the saddle.

Above the Cirque and to the Summit...

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Once you reach the saddle just above the headwall, your only about 3/4 of a mile from the summit of Sacagawea Peak. The remainder of the trail takes you through more Mississippian limestone, mostly the Lodgepole Limestone, although at the saddle exists a contact between Mississippian and Devonian (and older) age rocks to the west. The Devonian time period came before the Mississippian and the contact stands for the end of this time period about 359 million years ago. Just up-trail from the saddle cairn is a west-facing outcrop of stromatolite columns shown above.

As you go downhill to the west, you'll pass through the lower Paleozoic--through the Pilgrim Limestone, Park Shale, Meagher Limestone, Wolsey Shale, and Flathead Sandstone of Cambrian age, and you will eventually cross the contact with the Proterozoic Lahood Formation which consists of coarse conglomerates made of cobbles of Archean metamorphic rocks. In both the following pictures, the lower wooded slopes are underlain by the conglomerates of the Lahood Formation, and the upper slopes without forestation are the Cambrian limestones and shales (trees don't like to grow on the carbonate limestones and impermeable shales!).

Looking down the western slopes of the Bridger Range, you will also see more evidence of active surficial processes. Here are examples of rock glaciers and a mass wasting process called solifluction.

"Rock glaciers are tongue-like or lobate masses of angular debris that resemble small glaciers, and which move downslope as a consequence of deformation of internal ice or frozen sediments" (Benn and Evans, 1998). The motion of a rock glacier is similar to an ice glacier, though rock glaciers will often appear more fluid than an ice glacier.

The picture above shows the "mass wasting" processes of solifluction and soil creep. Solifluction occurs in a saturated soil on top of a relatively non-porous layer. The saturated soil creeps down the slope due to the force of gravity and instability caused by excess water in the soil. The slope required depends on the type of soil and how deep the layer is, but if the slope is too steep, there generally isn't enough soil development to facilitate solifluction. This is a common process in Alpine areas where there are daily freeze-thaw cycles that cause the soil to become saturated with melt waters by day, but which can't infiltrate into the soil because of frozen layers at depth.

Avalanche!

Don't forget that the steep Alpine country of the Bridger Range is prone to avalanches. Avalanches also play a big role in sculpting the landscape. This picture was taken just after the first big fall storm in mid-November, 2009, just north of the Fairy Lake Campground. Have fun playing in the mountains in the winter, but BE SAFE! Know what to look for with respect to avalanche dangers; always venture out with a group; take the avalanche safety course; wear a transponder and know how to use it; shovels and probes are essential; bring a first aid kit; stay hydrated; wear layers of outdoor clothing and be prepared for changing weather conditions. And, be sure to check the Gallatin National Forest Avalanche Center for regular updates on the snow conditions in the backcountry, technical information, calendars of classes, and archives of information and photos.
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So what does this all mean?

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Each feature along the trail is a clue to the past: fossils can tell us about past environments while rock structures yield clues to the tectonic setting which produced the mountains themselves. The evolution of the Bridger Range is a complex record of multiple major tectonic events which have shaped the Northern Rocky Mountains into what they are today. These events span from more than 2.5 billion years ago until the present and consist of multiple phases of evolution of the North American continent.

For a more detailed look at the geologic evolution of the Bridger Range see "Reactivation of a Proterozoic Continental Margin, Bridger Range, Southwestern Montana" (Lageson, 1989).

Geologic History

Here is an overview of the geologic history of the Bridger Range, summarized from Lageson (1989):

• Some of the oldest rocks in North America are exposed in the high mountain ranges of southwestern Montana. These rocks are typically metamorphic rocks that occur as varieties of schists and gneisses. These rocks are older than 2.5 billion years, and constitute the "basement" on which younger younger rocks have been deposited. These rocks are present in the Bridger Range south of Ross Pass.
  
• Pass fault, which is exposed on Ross Pass, was active in the middle Proterozoic (~1.4 billion years ago), and helped to delineate the southern margin of the Belt Basin. Archean rocks south of the Pass fault were uplifted, and shed their detritus into the Belt Basin and formed the conglomeritic deposits of the Lahood Formation of the Belt Supergroup.
  
• Paleozoic seas covered most of the northern Rocky Mountain region between the Cambrian and Cretaceous. These shallow seas deposited thick sequences of sediments as the ancient sea level rose and fell. Depending on the depositional environment, thick sequences of carbonate rocks (limestones and dolomites), shales, and sandstones were deposited. For details, see
  Show Stratigraphic Column for Montana
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• The Pass Fault had to be active in the Paleozoic as well because the thicknesses of the sedimentary units in the nortern and southern Bridger Mountains are different.
• Starting in the Cretaceous (and continuing through the Eocene), there was a regional contractional event that resulted in the thin-skinned (relatively shallow) fold-and-thrust belt style of deformation (also known as Sevier-style). This style of deformation is part of a major period of mountain building that was experienced from the Yukon to the Yucatan! The direction of thrusting in this area is mostly eastward, and the thrusting gets younger and eventually dies out on the western margin of the Crazy Mountains to the east.
  
• Contractional deformation also produced "deep-seated" reverse faults that were responsible for uplift of the deep, Archean basement rocks. This type of faulting is known as Laramide-style faults, and is somewhat younger than Sevier-style faulting in this area (Paleocene - Eocene). Most of the major mountain ranges of southwestern Montana have experienced Laramide-style faulting, exposing Archean basement rocks in the Beartooths, Madison, Tobacco Root, and Ruby Ranges. basement-cored faulting.
  
• As the mountain ranges were uplifted by Sevier- and Laramide-style faulting, they shed a tremendous amount of material due to erosion, transportation and deposition in the adjacent basins. These deposits can be seen along Bridger Canyon Drive, and in fact, cover most of Montana to the east.
  
• Deformation didn't end in the Cretaceous! Cenozoic "basin and range" style extensional (normal) faulting has continued to create the high topographic relief between the crest of the Bridger Range and the Three Forks Basin to the west. This extensional style deformation is still active today (we are just at the "tip" of the extensional tectonics that created the great Basin and Range Province in Nevada, and this deformation is also being influenced by the Yellowstone Hot Spot just to the south). People living in this part of southwestern Montana should know that we are in a zone of active seismicity known as the Rocky Mountain Seismic Belt.
• The Bridger Range continues to be sculpted by a number of surficial processes including glaciation during the past Ice Ages, and active mass wasting processes that are constantly at work.
  

Feedback

Let us know if you took the hike to Sacagawea Peak, and if you used this Field Guide. Comments and feedback of any type are appreciated. Please contact David Mogk, Professor of Geology, Dept. of Earth Sciences, Montana State University.

Refrences

• Lageson, D.R. 1989. Reactivation of a Proterozoic Continental Margin, Bridger Range, Southwest Montana. Montana Geological Society - 1989 field Conference Guidebook: Montana Centennial Edition 1:279-298

• Locke, W.W., Lageson, D.R. 1989. Geology and Geomorphology of the Rocky Mountains/Great plains Transition. Trip 4 Road Log.
  

• Skipp, B., Lageson, D.R., McMannis, W.J. 1999. Geologic Map of the Sedan Quadrangle, Gallatin and Park Counties, Montana. U.S. Geological Survey