Bear Basin Trail, Spanish Peaks Area, Northern Madison Range, near Big Sky, MT
By Tom Isaacs, Nathan Danz, and Thomas Rendle, geology majors, Dept. of Earth Sciences, Montana State University
DirectionsUpon leaving Bozeman, take highway US 191 west out of Bozeman until reaching the junction known as Four Corners. At this junction, turn south and continue to follow US 191 into the Gallatin canyon. The road will take you through 40 miles of breath-taking views and will bring you to the Big Sky ski resort exit. Turn right (west) on Highway 64 and follow it towards Big Sky for 5.5 miles. Turn right onto the North Fork Forest Service road (just past the entrance to Lone Mountain Cross Country Ski Ranch) and follow it for two miles to the first trail head parking area. Upon reaching the trail head, prepare your legs, ready your inner self, and make sure your camera is ready!
Step by Step Guide to the Trail
Bear Basin is tucked in a remote part of the Spanish Peaks of the towering Madison Range just south of Bozeman, Montana. Like the saying goes, "The journey is the destination," so the compliment of the scenic drive with the hike will be the inspiration for those first time pioneers of this amazing landscape as well as those extreme recreationalists. Opportunities abound for hiking, back country skiing, horseback riding, mountain biking (up to the wilderness boundary), photography, wildlife and bird watching, berry picking...The trail is a relatively easy hike for the first 3 miles, and steepens a bit in the upper stretches--so the trail should be accessible to all, from families with children to senior citizens; casual hikers to elite trail runners or back country skiers. Bear Basin is a remarkable wilderness preserve, that is in the shadow of Big Sky, and close to Bozeman. As with any back country hike, be prepared: the weather can be quite variable so bring layers of clothing for cold, wind, and rain; sunglasses; first aid kit; bear spray; stay hydrated--bring water and a filter pump; lots of high energy snacks; and leave a travel plan with someone so they know where you're going and when to expect your return. This is a trail you don't want to miss--so, walk softly, leave no trace, and enjoy the wonders of Nature!
The initial part of the trail consists of linked mature logging roads that wind through some dense, at times towering timber, that skirts the Lone Mountain Nordic Ski Ranch. A constructed pine bridge, hand-crafted by the Forest Service, crosses the North Fork of the West Fork of the Gallatin River. At this point the trail leaves two-track roads, and follows a rolling footpath to the north. The Bear Basin trail intersects a connector trail to Beehive Basin (popular with mountain bikers in the summer, cross country back country skiers in the winter). Further up the trail you cross the boundary into the Lee Metcalf Wilderness area, and travel is restricted to foot or horse travel. Continuing to the head of Bear Basin (about 6 miles), you can continue to the north and connect to the trail to Summit Lake to Indian Ridge, to Mirror Lake and the Spanish Peak Ranger Station, or the connector trail to Hellroaring Creek to the east.
"A wilderness, in contrast with those areas where man and his own works dominate the landscape, is hereby recognized as an area where the earth and its community of life are untrammeled by man, where man himself is a visitor who does not remain." - Wilderness Act of 1964, U.S. Congress.
The Lee Metcalf wilderness area was designated in 1983 by the United States Congress. Encompassing 259,000 acres in the Madison Range, this wilderness area covers some of the last niches where human influence are still at a minimum. Wilderness areas help preserve the untamed areas from urban development, logging, and preserves the landscape from any mechanical mechanisms including bikes, chain saws, all terrain vehicles, and all other motorized vehicles. The only method to cross country travel in the Lee Metcalf Wilderness is by foot, horse, skis, or dog sled. Since these wilderness areas do a spectacular job in preserving the natural environment for future generations, a hike up to Bear Basin will produce an unforgettable day in a pristine atmosphere with out the intrusion of mechanized, modern life.
This lush basin and entire Madison ecosystem supports a diverse selection of flora and fauna, so as you continue up the trail keep your eyes peeled for wildlife. The area has a healthy population of mule deer, elk, moose, black bear and grizzly bears (so be 'bear aware"),mountain goat, bocky Mountain Big horn sheep, and several species of grouse. If your are stealthy enough, you just may be able to get a peek at some of the species that dwell in streams. The best way to see these elusive creatures is to creep up to a small pool located by log jams and see the infamous brook trout feeding on the aquatic insects emerging from the river.
Below, fording the North Fork during run-off season, early July. A bridge now crosses the creek at this location.
Take a minute to look around! The view in the left image is looking to the south from the trail--see the ski runs on the near slopes and Sphinx Mountain on the skyline. After traveling through switchbacks in the forest, Bear Basin open up into this high Alpine wonderland!
These photos are taken in the upper part of Bear Basin. On the left, the east ridge of Bear Basin has a jagged skyline which shows the location of different rock units in the area--each unit is more or less resistant which results in differential weathering that produces this rugged landscape. Layers of rock are oriented at a high angle and dip to the left (northwest). On the right, the saddle to the Spanish Creek drainage is occupied by a big, black intrusive dike of basaltic compositiion.
Geologic StructuresThe Spanish Peaks area is laced by geologic structures that reveal a long, tortured history of deformation and uplift. These include ductile structures that formed deep in the Earth at very high temperatures and pressures ~750o Celsius and up to 10 Kilobars pressure--that's over 10,000 atmospheres of pressure that occurred at depths of ~ 30 Km/18 miles!) that allowed rock to flow like toothpaste, and brittle faults that ruptured the Earth while bringing deep seated rocks up to the surface. The main fault style in the Spanish Peaks area is the Laramide-style high angle reverse fault that brings deep-seated crystalline rocks up and over the overlying Paleozoic sedimentary layers. The pictures below show a variety of these geologic structures that you will see and walk over on the Bear Basin trail.
There are a variety of folds that occur in the crystalline rocks of Bear Basin--these form on all scales (from cm to Km), and in a variety of shapes. This fold is called an isoclinal fold because the two limbs are close to parallel, i.e. they have the same (iso) inclination (-clinal).
The photo on the left shows darker, metamorphic gneisses that have developed a strong foliation (layering) due to ductile deformation. These layers are cut by veins of lighter granitic material. These relationships show the Law of Cross-Cutting Relations: the unit that cuts across an existing structure must be relatively younger (you can't cut across something that isn't already there!). This is a relationship that was established by Nicolas Steno in the 17th Century! By looking at the sequence of cross-cutting relations, geologists can construct the sequence of events (the relative ages, older to younger) that describe the geologic history of an area. We'll explain how to determine the absolute ages of rocks in a section below.
The photo on the right shows what happens when rocks are subjected to deformation under great temperatures and pressures. The rock in this image once looked like the one on the left--only it was "sheared out" such that the cross-cutting layers are now rotated into a nearly parallel orientation with the original layering. Imagine that you had a deck of cards on a table, and draw a circle on the edge of the card deck. Then "smear" the cards across the table top. Each contact between the cards represents a finely-spaced shear plane, and if you look at the original circle on the card deck edge, this will now appear to be a very elongate ellipse. This is exactly what happens in the deformation of these types of rocks--all of the structural elements are rotated into near-parallel orientations during "ductile shearing".
The photo on the left shows another type of ductile deformation known as a mylonite. Note that there are layers that have a marked reduction of grain size. This is the result of intense deformation at high pressures and temperatures. Earlier interpretations of this type of structure suggested that the reduction in grain size occurred by physically breaking the mineral grains into smaller parts (the term mylonite is from the Greek root mylos, meaning "to mill"). However, recent studies of this type of texture show that the reduction of grain size is due to reorganization of defects in the minerals to produce new grain boundaries and thus smaller grain size in a process known as "dynamic recrystallization." The photo on the right is Karl Kellogg of the United States Geological Survey, taking notes while mapping in the Spanish Peaks area.
Brittle FaultsThe Spanish Peaks are bounded by large fault systems that are responsible for the uplift and high topography of this area. The bounding fault on the south of the Spanish Peaks is the high-angle, reverse fault known as the Spanish Peaks Fault. This is a mighty structure that is responsible for the big flatirons of limestone that you see on the south side of the range, and the trace of this fault can be seen trending NW-SE from south of the Beartooth Mountains through the Madison Range, and terminating in the Tobacco Root Mountains (this fault is also known as the Spanish Peaks-Gardiner Fault, in recognition of its continuation across much of southwest Montana).
Here's a way to tell a reverse fault from any other fault. Take a look at the side that shows the fault and arrows indicating movement. See the block farthest to the right that is shaped kind of like a foot? Now look at the block on the other side of the fault. See how it's resting or hanging on top of the foot wall block? Think about this: if we hold the foot wall stationary, where would the hanging wall go if we reversed gravity? The hanging wall will slide upwards, right? When movement along a fault is the reverse of what you would expect with normal gravity we call them reverse faults! (USGS)
The trail continues across this massive Laramide-style (approx. 75 miilion) reverse fault that leads you into the Archean basement. The Archean includes rocks that are 2.5 billion to 4 billion years old. These Archean rocks can give us a tiny window into processes that take place deep within the Earth, and they also contain information about how the crust of the Earth has evolved over the Eons.
Once you cross the Spanish Peaks fault on the Bear Basin trail, most of the rocks in this basin were originally of igneous origin. In Bear Basin, most of the igneous rocks occur as tabular, sill-like bodies because they were injected into pre-existing layered metamorphic units. These rocks range in composition from dark gray tonalites (rich in black hornblende and white plagioclase), to very white and sugary-textured granites (rich in potassium feldspar, plagioclase, and with very little dark minerals like biotite or hornblende). In addition, there are some late-stage dikes that cut across the older rocks, and these dike rocks are typically of basaltic composition. These are some photos of the type of igneous rocks you will likely see on the Bear Basin Trail.
Dike rocks are some of the most prominent intrusive bodies that occur in Bear Basin. A dike is a body of igneous rock that is intruded into an older country rock in the form of a large sheet that runs perpendicular to boundaries between older rocks. In the basin itself, there are two easily identifiable dikes. They are large vertical bands of darker tinted rocks that climb to the crest of the mountaintops. From outlooks on the trail these features can be seen, as well as nearing the summit the trail will cross one prominent dike several times. Climbing to the top of the cirque head wall you will be rewarded with a spectacular view of untamed wilderness, looking north into the head waters of the South fork of Spanish Creek. If you continue along the trail you will meander over to Summit lake or down to Spanish Creek were a Gallatian National Forest Ranger station is located at the Spanish creek trail head. Although most of the bedrock in Bear Basin was originally of igneous origin, these rocks have also been subjected to deformation and recrystallization during metamorphism. Most of these rocks now have a very strong planar arrangement of their minerals (a texture known as foliation), so most of the crystalline rocks in Bear Basin are actually "meta-igneous" rocks, or are commonly referred to as "quartzofeldspathic" gneisses(quartzofeldspathic refers to the composition that is mostly quartz or feldspar, granitic to tonalitic in composition; gneiss refers to the strong, planar fabric of the rocks, sometimes with obvious compositional layering)
Metamorphic rock is the result of the transformation of an existing rock type, the protolith, in a process called metamorphism, which means "change in form". The protolith may consist of any of the three main types of rocks: sedimentary, igneous, or another older metamorphic rock. The protolith, is subjected to the most important agents of metamorphism, heat and pressure. These factors causes profound physical and/or chemical alteration resulting in both deformation (explained above) and chemical recrystallization that produces new sets of mineral assemblages. It's important to note that these transformations, deformation textures and new mineral assemblages, occur while the rock stays in a solid state. Metamorphic rocks make up a large part of the Earth's crust and may be formed simply by being buried deeply beneath the Earth's surface, subjected to high temperatures and the great pressure of the rock layers above it. More often, metamorphic rocks from in response to large-scale tectonic processes such as continental collisions, which causes horizontal shortening, stacking of rock units on big thrust faults which mechanically thickens the crust, and the emplacement of igneous bodies also adds a lot of mass to the crust and as a result increases both temperature and pressure.
Metamorphic rocks are classified according to their constituent minerals and texture. The mineralogy of metamorphic rocks reflects the mineral content of the precursor rock and the texture which can be seen by the pressure and temperature at which metamorphism occurred. There are two basic types of metamorphic rocks: 1) foliated metamorphic rocks such as gneiss and schist have a layered or bandedappearance that is produced by orientation of minerals due to intense pressure and deformation; and, 2) non-foliated metamorphic rocks such as marble and quartzite which do not have a layered or banded appearance, and simply recrystallize minerals like quartz and calcite. Some of the most common types of metamorphic rocks present in the Bear Basin area are: gneiss and schist. Gneiss is a foliated metamorphic rock that has a banded appearance due to compositional layering of light and dark minerals. It typically contains abundant quartz or feldspar minerals. Schist is a metamorphic rock that has a high abundance of platy micas, which are strongly aligned into a planar texture. So, if you stumble across some of these rocks don't be afraid to take a guess on the type of rock at hand.
In Bear Basin, most of the crystalline rocks are meta-igneous rocks (metamorphosed plutonic rocks, such as granites or tonalites), although there are some minor layers of metamorphosed sedimentary rocks. The metasediments produce some of the most interesting metamorphic minerals (see next section on Mineralogy). These rocks are also significant because their geologic history is quite complex: 1) originally, some sort of quartz-rich igneous rocks had to be formed, eroded, and the sediments must have been transported and deposited in some sort of basin; 2) these surficial sedimentary rocks then had to be buried quite deeply in the crust (we estimate ~ 30 Km burial, and this most likely occurred in some sort of collisional process, similar to the modern collision of the Indian and Asian plates to form the Himalaya Mountains); 3) the deeply buried sedimentary rocks were metamorphosed into schists and gneisses; and 4) numerous stages of faulting and uplift brought these rocks to the surface where they are exposed today (Laramide reverse faults and Basin-and-Range normal faults contributed to this most recent period of uplift).
Minerals You'll See on the Bear Basin Trail
The metamorphic rocks of Bear Basin contain numerous types of fascinating minerals. As you walk alongthe trail, you'll see some of these in the rocks and boulders on talus slopes. These minerals can also be seen on outcrops. Please enjoy these minerals where you find them, and leave samples for others to discover and enjoy! Protect this precious resource, and refrain from breaking up the outcrops to collect samples.
How do we know the rocks came from 30 Km below the surface?
Minerals are indicators of the physical conditions (temperature and pressure) that were present during metamorphism. We often refer to minerals such as kyanite as "index minerals" because their occurrence is restricted to certain physical conditions; kyanite is a mineral that most often forms from the high pressure metamorphism of clay-rich sedimentary rocks, sillimanite occurs at higher temperatures, and andalusite forms at lower temperatures and pressures. The figure below on the left shows the "stability fields" of these three aluminosilicate minerals. However, more important than the occurrence of individual minerals, is the metamorphic assemblage of minerals.
The figure below on the right shows how different mineral assemblages are limited by bounding curves that define the physical limits where sets of minerals can coexest. These types of figures are called phase diagrams.
Because the Earth is dynamic and has evolved over long periods of time, geologists attempt to determine the "pathways" that rocks have traversed through the crust throughout geologic time. It turns out that many minerals are capable of including different elements in their structure (this is known as "solid solution", and is very similar to liquid solutions only the elements mix in the crystalline structure rather than in fluids). The composition of many minerals is very sensitive to changing pressure and temperature, and they will exchange elements with their neighbor minerals in a rock. By analyizing the changes in composition of minerals using an instrument called an Electron Probe Micro Analyzer (EPMA), we can calibrate the changes in composition with changes in pressure and temperature. This allows us to create a pressure-temperature-time path for a given rock, as shown below; and this then can be interpreted in terms of tectonic environments (such as a collisional plate margin).
The metamorphic rocks in the Spanish Peaks area contain mineral assemblages (kyanite, garnet) that indicate that they were once buried as deep as 30 Km (10 Kilobar, or 10,000 atmospheres of pressure) and were heated up to temperatures on the order of 750oC.
How do we know the rocks are as old as 3.5 billion years old?
Sensitive High Resolution Ion Microprobe (SHRIMP) facility run by the U.S. Geological Survey and Stanford University. This instrument measures the uranium (U) and lead (Pb) isotopic ratios preserved within the zircon grain. The SHRIMP uses a very small ion beam (~10 microns across) which can be focused on the individual growth zones in a zircon grain (seen by BSE imaging), and thus, a single zircon grain can record a whole sequence of geologic events. Numerous grains are analyzed in this manner to get a statistical determination of the age. Because zircon is quite a robust mineral (it can't easily be melted or weathered), the radioactive "parent" atoms of U, and the "daughter" atoms of Pb (produced by radioactive decay) are preserved as a closed system (U and Pb can't enter or escape the zircon). Then the measured isotopic ratios of 206Pb/238U and 207Pb/235U are plotted on this type of Concordia diagram, and the intersection of the data points with the curve result in the age determinations.
The age of this Bear Basin tonalite is ~3.3 billion years. We've obtained ages from the various units analyzed in Bear Basin that range from 3.5 billion years, to 2.55 billion years--representing an intrusive history that occurred at different stages over about 1 billion years (or ~1/4 of the Earth's history). Now, that is a long and distinguished geologic history!
To learn more about geochronologic techniques, visit our webpages on Thermal ionization Mass Spectrometry, Secondary Ion Mass Spectrometry, Multi-Collector Inductively Coupled Plasma Mass Spectrometry and a related site on Zircon Chronology: Dating the Oldest Material on Earth from the American Museum of Natural History.
Geomorphology and Landscape Evolution
As hikers reach the upper elevation of the trail, they are rewarded with spectacular views of Bear Basin. What could have formed such massive topography? A clue to the answer can be found a few miles back down the trail. In some of the lower meadows, the ground is strewn with large, rounded, irregularly sized boulders (see right). These boulders are called glacial erratics. They were transported here from higher up the valley and deposited by a receding glacier. This hypothesis of a glacial origin is supported by the shape of Bear Basin itself. Bear Basin is a classic U-shaped valley. Imagine for a moment Bear Basin filled with a massive river of ice a couple of thousand feet thick!
How was the valley formed?
Based on how fresh the glacial land forms are at Bear basin, it was probably last glaciated during what geologists call the Pinedale glaciation. The Pinedale was the most recent active glacial period to affect the Spanish Peaks and is responsible for many of the glacial land forms in the northern Rockies. The Pinedale glaciation is estimated to have lasted from 30,000 to 10,000 years ago and its greatest extent considered to be between 23,500 and 21,000 years ago.
I've heard of retreating glaciers. Dose this mean they're flowing backwards?
A retreating glacier is not flowing back up hill. The edge of a glacier is called the glacial terminus. The terminus is where the glacier is melting. When it is said that a glacier is retreating it means that the terminus is melting back faster than new ice is flowing forward.
Let us know if you took the hike to Bear Basin, and if you used this Field Guide. Comments and feedback of any type are appreciated. Please contact: David Mogk (firstname.lastname@example.org), Professor of Geology, Dept. of Earth Sciences, Montana State University.
References--For Further ReadingKellogg, Karl S. , and Williams, Van S. , 1998, Geologic Map of the Ennis 30 X 60 minute quadrangle, Madison and Gallatin counties, Montana: U.S. Geological Survey Open-File Report 98-0851, U.S. Geological Survey, Denver, Colorado.
Salt, K.L, 1987, Archean geology of the Spanish Peaks area, Southwestern Montana: Bozeman, Montana State University, M.S. thesis, 81 p.
Spencer, E. W. and Kozak, S. J., 1975, Precambrian evolution of the Spanish Peaks, area, Montana: Geological Society of America Bulletin, v. 86, p. 785–792.