Big Baldy Geology Field Trip and Hike

August 13, 2005

• Location -- This is a geology field trip and hike to look at features in the Little Belt Mountains, located about 75 miles southeast of Great Falls, Montana, U.S.A. Take U.S. Highway 89 to the meeting place on Kings Hill (in the southeastern corner of Cascade County). Dirt roads provide access to the Big Baldy Trailhead. These roads are not suitable for very low clearance street vehicles. Big Baldy is the highest peak in the Little Belt Mountains.






• The Hike -- The hike climbs from the trailhead to the top of Big Baldy--about a thousand feet higher in elevation--where there is a good view of Rhoda Lake, nestled in a glacial cirque on the east side of the peak. The hike descends the 750 feet to Rhoda Lake, returns to the ridge and goes to a point overlooking Twin Lakes.  Rhoda Lake is in the cirque with the sunlit snowbank in the top center of the photo above.  Twin Lakes are in the cirque on the right.



• Big Baldy Laccolith -- Big Baldy is a laccolith--a sand dollar-shaped igneous intrusion that was emplaced about 50 million years ago. The base of the laccolith is marked by the tree line in the photo above. The current curved surface that forms the top of Big Baldy is probably close to the upper surface of the laccolith. Erosion removed the overlying sedimentary rock and some but not most of the igneous intrusion. Most of the high peaks in the Little Belt Mountains are laccoliths. Mountain glaciers cut glacial cirques into the east side of Big Baldy. This is the only occurrence of glacial features in the Little Belt Mountains. The field trip will also examine the Yogo Peak Stock with its outstanding examples of shonkinite.



• Scenic Views -- Hikers will be able to see many prominent peaks formed by igneous intrusions--laccoliths. Thunder Mountain and Tillinghast Mountain are laccoliths to the west of Belt Creek. The Barker laccolith, the Clendennin/Peterson Mountain laccolith and the Mixes Baldy/Anderson Peak laccolith are near Hughesville/Barker. The highest peak in the Little Belt Mountains is in the Big Baldy laccolith. Long Mountain and Neihart Baldy are prominent peaks because they are capped by the very resistant Neihart quartzite and are uplifted as part of the Pioneer Ridge Uplift--a block of basement 8 miles long by 6 miles wide that was uplifted about one and half miles vertically.



• Field Trip Leaders -- Dr. David Baker, Earth Scientist in Monarch, will conduct the field trip. Co-leader is Vernon Cromwell, Forest Service geologist.



• Registration and Cost -- There is no charge for the field trip; however, prior registration is required. The group size is limited. To register, fill out our on-line form. Meeting time is 9:30 a.m.

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