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

Introduction

Directions

Upon 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

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!

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!

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.

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.

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.


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!


View to the south from the Bear Basin cirque headwall. The many layers of the Archean (older than 2.5 billion year) 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 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 ~750^o 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

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".


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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).

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 Gallatin River are located for reference. Bear Basin is located by the yellow dot.