Big Sky, Montana

 Big Sky, Montana

Big Sky Montana geology Lone Mountain travel copyright RocDocTravel.com
Discover the unique bedrock that shaped this northern Rockies treasure!

Travel to Big Sky, Montana

Big Sky is located in southwestern Montana just north of Yellowstone along U.S. highway 191.  The nearest airport is at Bozeman, about an hour's drive away.


Lodging is abundant, but expensive during ski season.  More is available in the Bozeman - Belgrade area to the north, but would involve a long canyon drive.  Camping is available in Gallatin Canyon -- make summer reservations early.

Geology of Big Sky

Big Sky Montana geology Lone Mountain travel copyright RocDocTravel.com
Sunrise on Lone Mountain, seen from highway 191.  The peak is dacite, which is a slightly darker igneous rock like granite.  Its parent magma rose from below about 68 million years ago, spreading between the sedimentary rock layers as sills and dikes.  Lone Mountain is the thickest part of the dacite, which explains why it has not been eroded down like the rest of Big Sky.

Big Sky Montana geology Lone Mountain travel copyright RocDocTravel.com
The north side of Big Sky is a dramatic fold (a syncline) that formed along a major fault, the Spanish Peaks fault.  The bedrock north of the fault was lifted up by thousands of feet, bringing some of North America's oldest rocks (gneiss, schist, and granite) up against the Paleozoic to Mesozoic sedimentary rocks of Big Sky.

Big Sky Montana geology Lone Mountain travel copyright RocDocTravel.com
This is a panorama of the fold and Spanish Peaks fold and fault east of Big Sky.  Big Sky is off to the left.  The cliffs up high are steeply tilted Mississippian limestone.

Big Sky Montana geology Lone Mountain travel copyright RocDocTravel.com
The bedrock in most of Big Sky is Cretaceous sandstone, siltstone, and shale.  It was deposited just east of a major mountain range we refer to as the Sevier mountains.  Those mountains have long since eroded away, but the sediments remain.  Only the sandstones form cliffs, while the shale and siltstone form gentle slopes, making these hills ideal for ski runs.  Lone Mountain and its diorite are in the background.

Big Sky Montana geology Lone Mountain travel copyright RocDocTravel.com
The diorite is pinkish to brownish, and forms cliffs and steep slopes because it is hard -- it resists erosion.  This kind of mushroom-head-shaped intrusion is called a laccolith.

Big Sky Montana geology Lone Mountain travel copyright RocDocTravel.com
Some of the Cretaceous sedimentary rock layers are visible on the north side of Lone Mountain (the gray and light tan layers in the cliffs).  The diorite intruded above and below these layers like creme filling in a pastry.

Big Sky Montana geology Lone Mountain travel copyright RocDocTravel.com
A broader view of Lone Mountain from central Big Sky.

Big Sky Montana geology Lone Mountain travel copyright RocDocTravel.com
View from a little farther north.

Big Sky Montana geology Lone Mountain travel copyright RocDocTravel.com
In these ski runs, dacite sills form the cliffs and steepest slopes.

Big Sky Montana geology Lone Mountain travel copyright RocDocTravel.com

Big Sky Montana geology Lone Mountain travel copyright RocDocTravel.com
This is the central resort area of Big Sky.  Most of the forest is on gentle slopes on the Cretaceous sedimentary rocks.

Big Sky Montana geology Lone Mountain travel copyright RocDocTravel.com
Lone Mountain from the north side.

Big Sky Montana geology Lone Mountain travel copyright RocDocTravel.com
A tram serves the 11,167 foot peak.  In this view you can see some of the darker sedimentary rock layers.

Big Sky Montana geology Lone Mountain travel copyright RocDocTravel.com
G is the granite (dacite), and S is the sedimentary rocks.

Big Sky Montana geology Lone Mountain travel copyright RocDocTravel.com

Big Sky Montana geology Lone Mountain travel copyright RocDocTravel.com
Sedimentary rock layers within the dacite on the east side of Lone Mountain.

Big Sky Montana geology Lone Mountain travel copyright RocDocTravel.com
The Cretaceous sedimentary rocks are part of the "foreland basin" that extends from Alberta to Arizona.

Big Sky Montana geology Lone Mountain travel copyright RocDocTravel.com
Here's a dacite sill intruded into the sandstone.

Big Sky Montana geology Lone Mountain travel copyright RocDocTravel.com
More dacite.

Big Sky Montana geology Lone Mountain travel copyright RocDocTravel.com
The prominent little cliff is a dacite sill in shale and siltstone.

Big Sky Montana geology Lone Mountain travel copyright RocDocTravel.com
A good roadcut of shale, siltstone, and sandstone.

Big Sky Montana geology Lone Mountain travel copyright RocDocTravel.com
Lone Mountain viewed through the ski resort entrance.

Big Sky Montana geology Lone Mountain travel copyright RocDocTravel.com

Big Sky ski area is absolutely enormous!  I can't wait to ski down from the peak!

Related Posts:

Yellowstone, Gallatin Canyon


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