Trail Guides
Integrating Research and Education > Trail Guides > Bear Basin

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

A view looking northwest into the spectacular Bear Basin Photo courtesy of Tyson Berndt.

Introduction

Bear Basin is carved into the Spanish Peaks uplift of the Madison Range. This extraordinary landscape of towering jagged peaks, untainted remote wilderness, and lush hidden streams can be found just southwest of Bozeman, Montana in the Lee Metcalf Wilderness Area. Many people daydream about ski lines or trophy elk hidden in its vastness, while others lace up their hiking boots, strap on a pack, and hike through some of the most diverse geology and breath taking views in the world. Whatever the case, get ready to experience Montana at its finest!

Bear Basin drainage. Image created on Google Earth.


Directions

Take highway US 191 west out of Bozeman until reaching the junction known as Four Corners. At this junction, turn left (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!

Bear Basin Directions from Bozeman, MT.

Bear Basin seen from Highway 64 near Meadow Village. The steep cliffs on the right (east) are the Mississippian (~340 million year old) Madison Limestones that were tipped up into their near vertical orientation by the Spanish Peaks-Gardiner Fault. This fault passes through the low saddle on the ridge to the northwest (upper left part of the photo). Photo by David Mogk.

Step by Step Guide to the Trail

The Trail

Meandering Bear basin trail
Photo courtesy of Nathan Danz.

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, backcountry 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 backcountry skiers. Bear Basin is a remarkable wilderness preserve that is in the shadow of Big Sky and close to Bozeman. As with any backcountry 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!

Flat Irons Photo courtesy of Tyson Berndt.
Crossing a few open parks, it is possible to see the surrounding mountain sides from the trail. Looking to the north and east, you can see near-vertical dipping strata that have a triangular erosive pattern to them. This is called triangular faceted spurs or flat irons. These are commonly found along faults. This boundary marks the massive Laramide-aged Spanish Peaks reverse fault that runs northwest-southeast along the southern margin of the Spanish Peaks. This fault juxtaposes 300 million year old sedimentary limestones (the Mississippian Madison Limestone) against metamorphic rocks of Archean age (older than 2.5 billion years). So as you cross the Spanish Peaks Fault, you can step from the Madison limestone into the Archean basement, and you will straddle over 2 billion years of missing Earth history!

Bridge crossing the North Fork of the West Fork of the Gallatin River.

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 backcountry 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.


Three miles up the trail you leave National Forest and enter the Lee-Metcalf wilderness area. Photo courtesy of Tyson Berndt.

"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 is 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, chainsaws, 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 without the intrusion of mechanized, modern life.

Bighorn sheep grazing trailside in early July. Photo by David Mogk.

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, Rocky 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.


Exposed cavern eroded in Madison Limestone just past the second creek crossing. Photo courtesy of Tyson Berndt.

The trail diverges from the creek bottom onto a bedrock stratum made of the Madison limestone. If you watch closely, a small cave/sinkhole is visible to your right as you make your way up the trail. This was formed by underground channels from the North Fork that eroded caverns and caves in the adjacent rock.

Below, fording the North Fork during run-off season, early July. A bridge now crosses the creek at this location.

Fording the creek in high run-off season, early July. This part of the trail now has a bridge at the creek crossing. Photo by David Mogk.



View to the south across ski runs and towards Sphinx Mountain. Photo by David Mogk.


Entering the upper part of Bear Basin in early July. Photo by David Mogk.


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!


View of the east ridge of Bear Basin. Photo by David Mogk.

Intrusive Igneous Dike In Cirque head wall of Bear Basin. Photo Courtesy of Tyson Berndt.



These photos were 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 composition.


View to the north from the cirque headwall of Bear Basin. Photo by David Mogk.
This is the view from the cirque headwall in Bear Basin to the north into the Spanish Creek drainage. The large mountain on the skyline is Blaze Mountain. Walk quietly in this stretch and you just might run into a herd of mountain goats!
Mountain goats, on cliffs above South Fork of Spanish Creek. Photo by David Mogk.


View to the south from the Bear Basin cirque headwall. Photo by David Mogk.
View to the south from the Bear Basin cirque headwall. The many layers of the Archean (older than 2.5 billion years) crystalline rocks can be seen in the profiles along the ridges and by the color changes of the rock units. The U-shaped valley attests to sculpting by glaciers, and the talus slopes and hummocky topography show that active surficial processes continue to shape this high Alpine landscape.

Geologic Structures

The Spanish Peaks area is laced with 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 of ~750° 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.

Ductile Structures


Isoclinal fold in gneisses of Bear Basin.

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).


Metamorphic gneisses with strong layering, cut by a younger vein of granitic material. Photo by David Mogk.



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.


Sheared metamorphic gneiss. Photo by David Mogk.

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."


A mylonite fabric--a form of ductile deformation. Photo by David Mogk.
.
Karl Kellogg of the United States Geological Survey, taking notes while mapping in the Spanish Peaks area.


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 Faults

The 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).

Landsat image of the Spanish Peaks Area.
This is a Landsat (satellite) image of the Spanish Peaks area. The trace of the Spanish Peaks Fault is outlined in yellow. Ennis Lake and the Gallatin River are located for reference. Bear Basin is located by the yellow dot.


Fault diagram.
Photo courtesy of Google Earth.



We classify faults by how the two rock bodies on either side of a fault move relative to each other. The one you see here is a reverse fault. Along a reverse fault, one rock wedge is pushed up relative to rock on the other side.

Reverse Fault diagram. Image from the United States Geological Survey.

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).



Cropped Geological map Ennis, MT Quad. Map by Kellogg and others, United States Geological Survey.

Cropped Geological map key Ennis, MT. Quad Map by Kellogg and others, United States Geological Survey.

The trail continues across this massive Laramide-style (approx. 75 milion year old) 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 eons.

Igneous Rocks

To many people, igneous activity conjures the postcard image of a mountain bursting forth with fire and brimstone spewing from its daunting peak. However, large amounts of magma are generated deep in the Earth and some of these melts crystallize on their way to the surface. Melts migrate upward towards the surface because they are less dense than the surrounding "country rock." This process of injection of magma into the crust produces intrusive igneous rocks. These rocks are also known as plutonic rocks and they may occur as big, balloon shaped bodies, or as "sills" that are intruded into the layered structure of the country rock. These rocks typically have a coarse-grained texture because they crystallize slowly, given the minerals have enough time to add more and more material to their crystalline forms. Some of these magmas may have originated by partial melting of the mantle, and this is one of the ways new material contributes to the growth of continental crust. Partial melting also occurs within the crust, and re-melting of earlier igneous rocks and transport of these younger melts to higher crustal levels also contributes to the further differentiation and evolution of the crust.The igneous rocks of Bear Basin formed in a long series of discrete magmatic events that spanned a time interval of over a billion years, from ~3.5 to 2.55 billion years! (Details of how we determine the ages of rocks are provided below).

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.

The "porphyritic granodiorite" unit found in Bear Basin. Photo by David Mogk.
This is a close-up photo of a unit known as the "porphyritic granodiorite" unit found in Bear Basin. Porphyritic refers to the large grain size of the pinkish potassium feldspar crystals. In hand sample, you can also see dull gray quartz, white plagioclase, and dark platy biotite and prismatic hornblende (these dark minerals are <10% of the rock volume).

Tabular, sill-like igneous bodies in upper Bear Basin. Photo by Tom Isaacs.

Close up of an intrusive igneous dike in the cirque headwall of Bear Basin (Geologists for scale). Photo Courtesy of Tyson Berndt.

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. These features can be seen from outlooks on the trail as well as when nearing the summit; the trail will cross one prominent dike several times. Climbing to the top of the cirque headwall you will be rewarded with a spectacular view of untamed wilderness, looking north into the headwaters 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 Rocks

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 where they are subjected to high temperatures and the great pressure of the rock layers above them. More often, metamorphic rocks from in response to large-scale tectonic processes such as continental collisions, which cause horizontal shortening, or 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 banded appearance 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.

Typical "gray gneiss" of tonalitic composition, showing strong foliation (planar layers), and with small-scale folding seen in the cross-cutting veins. Photo by David Mogk.

Tight, isoclinal folds in gneisses in the upper part of Bear Basin. Photo by David Mogk.

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 Himalayan 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).

Metamorphosed Gneiss cliff above trail. Photo courtesy of Nathan Danz.

Metamorphic Gneiss.
Photo courtesy of Nathan Danz.

Metamorphic Schist.
Photo courtesy of Nathan Danz.


Minerals You'll See on the Bear Basin Trail

The metamorphic rocks of Bear Basin contain numerous types of fascinating minerals. As you walk along the 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.

Dime to quarter sized almandine garnets in a metamorphic schist photo courtesy of Tyson Berndt.
Almandine garnets are one of the most common metamorphic minerals. They are recognized because of their equant shape (golf-ball shaped), deep red color, and extreme hardness (8 on Mohs' Hardness scale). Garnets occur in a wide variety of metamorphic rocks including schists, gneisses, and metamorphosed basaltic rocks called amphibolites. Here, dime- to quarter-sized garnets occur in a mica schist.


Almandine garnets in mafic dike. Photo by Hanan Danz.
Almandine garnets. These garnets occur in the mafic (basaltic) dike on the cirque wall at the head of Bear Basin. Although the dike is an intrusive igneous feature, these garnets grew under metamorphic conditions under high pressure. This indicates that this dike was emplaced in a fracture at great depth. We estimate that the rocks in this dike were emplaced at ~10 Kilobars of pressure (10,000 atmospheres of pressure, which is equivalent to a depth of burial of 30 Km or ~18 miles!). Consider how much uplift and erosion had to occur to expose these rocks in their present setting at the top of the Spanish Peaks!


Elongated blades of blue kyanite.Photo courtesy by Nathan Danz.
This picture shows elongated blades of a blue mineral, kyanite. Kyanite is an "aluminosiliciate mineral' with formula Al2SiO5. It is very common in mica schists. It only occurs in rocks of very high pressure (see section below).


Sillimanite gneiss in Bear Basin. Photo by David Mogk.
The white prismatic minerals in this picture is the mineral sillimanite. It is also an aluminosilicate mineral with the same composition as kyanite (Al2SiO5; these minerals have different crystalline structures even though they have the same composition; we call these minerals polymorphs because they occur in more than one form).


Gedrite, a type of amphibole, in a metasedimenatry unit in Bear Basin. Photo courtesy by Nathan Danz.
The dark mineral in this photo is gedrite, an amphibole mineral similar to hornblende. It commonly occurs as very long and thin individual minerals (needle-like minerals are termed "acicular"), and the individual minerals often occur as radiating splays of minerals in a "plumose" texture. Gedrite is typically brown to green, with a "waxy" luster, and typically occurs as needle-like crystals.


How do we know the rocks came from 30 Km below the surface?

Kyanite forms under high pressures. Photo courtesy of Nathan Danz

Phase diagram showing the physical stability fields of the aluminosilicate minerals, kyanite, sillimanite, and andalusite.


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 co-exest. These types of figures are called phase diagrams.

A pressure-temperature phase diagram showing the stability fields of common metamorphic minerals. Figure from Winter, An Introduction to Igneous and Metamorphic Petrology, Prentice Hall.


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 analyzing 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).

An example of a pressure, temperature, time PTt path, showing an example of a collisional even in which rocks were initially buried deeply to obtain high pressures, were subsequently heated to obtain higher temperatures in response to heating of the rock body at great depth as the geothermal gradient brought more heat into the system, and the rocks subsequently were uplifted and decompressed.




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 750° C.

How do we know the rocks are as old as 3.5 billion years?

Back-scattered scanning electron micrograph of a zoned zircon. Photo courtesy of Darrell Henry, Louisiana State University, used with permission.

To date igneous and metamorphic rocks geologists often use a mineral called zircon. Zircon is a very resilient mineral, capable of surviving billions of years of violent geological history. Incorporated into zircon's crystal structure is uranium. Being radioactive, uranium eventually decays into lead. This decay process happens over a predictable amount of time. By measuring the amounts of uranium and and lead in the zircon crystals geologists can estimate the age of the rock. The photo on the left was taken on a scanning electron microscope, using a technique called "back scatter electron" (BSE) imaging. This technique helps to show compositional differences in a cross section of a mineral, as shown here. Each of the color zones represent a different stage of growth of the zircon. For example, the zones in this zircon grain may represent "inherited" zircon from some ancient source area, zircon that grew during igneous crystallization in a magma chamber , and zircon that grew during metamorphic recrystallization. So much history contained in a grain that is only a few 10s of microns across!

A U-Pb "concordia" diagram for a tonalitic gneiss in Bear Basin yielding an age of 3.05 billion years. Figure from Paul Mueller, University of Florida.
This figure is a U-Pb "Concordia" diagram that was determined for sample SP 105, which is one of the tonalitic gneisses in Bear Basin. Zircons were separated from this sample, and these were then analyzed using the 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 web pages 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?

Glaciation

Glaciers are iconic images of high mountainous regions world wide, and in Bear Basin, as well as much of the Rocky Mountains, are the primary agents of landscape evolution. Glaciers are literally great rivers of ice that flow and grind slowly down the mountainsides, carving and sculpting as they go.This erosion is achieved via two processes , abrasion and plucking. Plucking occurs when chunks of the bed rock freeze into the ice at the bottom of the glacier and are pulled away with the ice as it moves down hill. This is a major way that sediment is picked up and carried by the glacier. Abrasion is the result of to ice, as well as the plucked rocks, sliding over the bed rock. This process polishes and some times leaves striation on the bed rock that indicate the direction of ice flow.

Glacial flow direction, U-shaped valley cross section, and cirque of Bear Basin. photo courtesy of Thomas Rendle.

Mountain glaciers, like the one that formed Bear Basin, create distinctive land forms as they erode the landscape. Both the flow of streams and the flow of glaciers form valleys, but how can the savvy hiker, intent on impressing his buddies, tell the difference? The key is in the cross section of the valley. Rivers and streams create valleys shaped like a big V, with the creek at the bottom and slopes on either side. Glaciers carve U-shaped valleys with steep cliffs on either side sloping into a broad valley floor. Glacial valleys are also commonly ended with bowl-like cliffs called cirques. Glacial valleys look like they were carved with a gigantic ice cream scoop.

Glacial erratics scattered over basin. Photo courtesy of Thomas Rendle.

The hundreds of tons of sliding ice that filled Bear Basin not only scraped and carved the hard Precambrian bedrock into valleys and cirques, it also carried the resulting debris down slope. This rock debris ranges in size from boulders the size of small houses, to the finest powered rock known as rock flower. Rock flower is usually the result of abrasive glacial erosion, while the boulders are the result of ice plucking. This broad mix of grain sizes is distinctive of glacial sediments and is called glacial till. When this debris reached the terminus of the glacier, all the debris melts out and is deposited in piles of till called glacial moraines. Moraines come in several different flavors. Terminal moraines accumulate at the melting toe of the glacier called the terminus. The terminal moraine of the Bear Basin glacier is located near the trail head, but is difficult to see because of its size and the fact that is covered in trees. Lateral moraines are deposited along the edge of the glacier and medial moraines occur in the center of the ice flow, usually after two glaciers merge. An important type of glacial deposit are glacial erratics, which are the over-sized boulders which the glacier deposits randomly in places they wouldn't otherwise occur. Several examples of glacial erratics are visible along the trail, especially in the meadows.

U-shaped glacial carved valley from multiple past glaciations. The most recent glaciation filled the basin with ice approximately 20,000-10,000 years ago. Photo courtesy of Tyson Berndt.

The Bear Basin glacier begin its journey high above the snow line, where snow accumulated year round. Under the pressure of the new snow above, the base of the snow field re-freezes into ice. Under the persistent force of gravity, this ice mass begins to move down hill. Ice is a solid, but viewed over years and decades, ice can behave viscously, flowing just like water. In fact, the flow of ice in a mountain glacier behaves similarly to the flow of water in a stream. The ice flows the fastest in the center of the glacier and slower near the sides where the friction of the rock slows it down. As the ice moves to lower elevation, it encounters warmer temperatures and predictably begins to melt. However, the flow of the glacier carries the melting ice much lower until the rate of melting is equal to the rate at which new ice is supplied by the glacier. This is called the glacial terminus. The glacial terminus is the balancing point between the the rate the ice is melting and the rate at which new ice is flowing down hill to replace it. Where the glacial terminus is located is dependent upon climate. A colder climate allows the glacial terminus to advance down hill because there is less melting and more snow accumulation at the top of the glacier. A warmer climate forces the glacial terminus to retreat up hill because there is more ice melting and less snow accumulation. It is important to note that the ice in a glacier is always moving down hill, even if the terminus is retreating. The position and motion of the glacial terminus also affects the location distribution of the terminal moraine.

The majority of the glacial land forms at Bear Basin were most likely created 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 12,000 years ago and its greatest extent considered to be between 23,500 and 21,000 years ago.

Even though the Bear Basin glacier has long since melted, its geomorphological legacy is still very apparent to the knowledgeable eye.

Mass Wasting

land slides and slumps are important processes in the Spanish Peaks as well as any high relief topography. A mass wasting event is a movement of a quantity of material down slope. This can occur slowly or suddenly and can take on a variety of forms. Two of the more prevalent in the Bear Basin area are rock falls and slope slumps.In the Bear Basin aria, the major control on the location of these different mass wasting phenomena is the type of rocks in the aria, what geologists call the lithology. As stated above, the rocks to the north of the Spanish Peeks fault are hard Precambrian metamorphic rocks, which tend to produce rock falls and slides. To the south of the fault, the soft Cretaceous sand stones are predisposed to slumps.

Rock falls causing scree and talus slopes at the Bear Basin cirque Photo courtesy of Thomas Rendle.

Rock falls are pretty straight forward. Lumps of stone detach from steep cliff faces and fall and/or roll down hill. These events are sudden and sometimes dramatic. They can result in scree or talus slopes or boulder fields. Rock falls are common on the steep cliffs of Bear Basin's glacial valley.

A less obvious form of mass wasting is slope slumps. These usually occur over a period of years and are characterized by a large portions of hillside slowly slumping down hill under its own weight. Slump slopes are prevalent through out the Big Sky valley. Ground that moves is a terrible place to build anything, so the high concentration of both active slump slops and and rapid land development in Big Sky should be of serious concern

So why do some hills fall over while others don't? This could be an important question when considering where to build your house. Mass wasting events are controlled by a variety of factors. These factors are split into two main categories: conditions that help prevent mass wasting and conditions that make it more likely.

The big player, of course, is gravity. Nature abhors a gradient. A rock sitting on top of a mountain would be much happier in the valley, and thats where it will go given half a chance due to the driving force of gravity. Another major factor in the probability of mass wasting is the angle of repose. The angle of repose is the steepest a slope can be and remain stable given all other factors. Slopes steeper than the angle of repose are likely to mass waste while slopes shallower won't given constant conditions. The likelihood of mass wasting also has a lot to do with the cohesiveness or strength of the mass. For example, granite has more internal cohesion than unconsolidated sand, and therefore has has a higher angle of repose, nearly vertical in fact, where as loose sand has a angle of repose of about 30 degrees. A huge influence on internal cohesion is water. Wet stuff is more slippery. This is especially true of slump slopes. A previously stable hillside can begin to slump if more water is introduced into the system. Water is such an important influence that a good way to help identify slump slops is to look for aspen trees ,which require a lot of water, at the base of hills.



Feedback

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 (mogk@montana.edu), Professor of Geology, Dept. of Earth Sciences, Montana State University.

Panoramic view from trail as it crests cirque headwall of Bear Basin. View looking north into south fork of Spanish Creek Photo courtesy of Travis Courthouts

References--For Further Reading

Kellogg, 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.


« Previous Page