Thunder Mountain Geology Field Trip and Hike

July 17, 2005

• Photo -- Thunder Mountain is on the skyline in the far distance. The photo shows a limestone cliff on Dry Ridge in the foreground. Belt Creek Valley is below. The Lazy Doe Restaurant (not visible) is left of center near Belt Creek. Monarch Mountain is the tree-covered ridge that extends across the photo from left to right beyond Belt Creek. The hike goes to the highest peak on Thunder Mountain (left of center).



• Time and Location -- This is a geology field trip and hike to look at features in the Little Belt Mountains, located about 50 miles southeast of Great Falls, Montana, U.S.A.  Meet at 9:00 a.m. on Sunday, July 17, 2005 in the parking lot of Cub's Den in Monarch.



• The Hike -- The hike leaves Divide Road at the Pilgrim Creek Trailhead, follows the ridge over to Thunder Mountain, and climbs to the top. Return is by the same route.



• The Thunder Mountain Laccolith -- The purpose of this field trip is to look at the Thunder Mountain Laccolith--a sand-dollar shaped igneous intrusion, 4 miles in diameter--and the rocks into which it intruded. The laccolith has a flat bottom (which is not exposed) and a gently updomed top. Erosion has removed the sedimentary rocks from the top of the laccolith but they can been seen on the flanks. The approximate location of the top of the laccolith is indicated by the red line in the photo above.  Igneous rock (=porphory) in the Thunder Mountain Laccolith is shown in the foreground of the photo below. The Big Baldy Laccolith is on the left side of the skyline.






• Scenic Views -- Hikers will be able to see many prominent peaks formed by igneous intrusions--laccoliths--which are like blisters under the sedimentary "skin" of the Little Belt Uplift. (The Little Belt Mountains have the structure of a large fold about 80 miles long and 30 miles wide.) Tiger Butte is to the northeast. Tillinghast Mountain is to the south, and Barker Mountain is to the east. Far to the northeast the Highwood Mountains (on the skyline in the photo below) consist of a stack of lava flows piled one on top of another.






• 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 requested. The group size is limited. To register fill out our on-line registration form.

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