Clendennin Mountain Geology Field Trip and Hike

Sunday, July 16, 2006

Clendennin Mountain is the mountain on the far skyline with the treeless talus-covered slopes and is the south end of the Clendennin-Peterson Laccolith.  Mixes Baldy is the high peak on the right and is part of the Mixes Baldy-Anderson Peak Laccolith.  The reclaimed former lower tailings pond at Barker is in the foreground.  The yellow-colored outcrop on the right is a porphory sill that underlies the south half of the tailings ponds.  The 6-story mill building was located beyond the upper end of the lower tailings pond (on the yellow bench partially hidden in the trees.  The Barker Railroad Station was to the lower left of the lower left corner of the photo.

• Location -- This is an introductory level field trip and hike to look at natural features in the Little Belt Mountains, located about 60 miles southeast of Great Falls, Montana, U.S.A. Take U.S. Highway 89 to the meeting place at the Canyon Café, located about 2˝ miles south of Monarch.  There will be discussions of the flora, fauna, and geology of the area.



• The Hike -- After taking care of the car shuttle, we will drive to Kibbey Divide above Hughesville,  hike across Clendennin Mountain, and descend into Lone Tree Park.
  Clendennin Mountain is on the far skyline in the first two photos.  The route follows the skyline, climbing 1300 feet in elevation and descending 2000 feet over a distance of about 4 miles.  There is no trail.  Bring your camera.  There will be great views of the Highwood Mountains and other features from Clendennin Mountain.


• Geology of the Barker/Hughesville Area -- Hughesville is at the center of a cluster of igneous intrusions.  The small Hughesville Stock is surrounded by three laccoliths.  These are the Barker Laccolith, which forms  a prominent peak seen from Great Falls, the Clendennin Mountain/Peterson Mountain Laccolith shown in the photo below, and the Mixes Baldy/Anderson Peak Laccolith.  The largest body of lead/zinc/silver ore is located in the Hughesville Stock.  The age of the igneous rocks is about 50 million years.




Clendennin Mountain as seen from the old Clendennin Townsite.  The yellow outcrop to the right of the cabin with the white fence was mapped as the Gold Run Tuff.  It is actually a diatreme that is about a kilometer in diameter.  The diatreme has been drilled to a depth of nearly 3000 feet.  The gulch known as Gold Run is to the right of the lower right corner of the photo.  (Although diamonds occur in diatremes, no diamonds were found in this diatreme.  There is, however, some lead/zinc/silver mineralization.  The "gold" found in Gold Run is "fool's gold" or iron pyrite.)



• Ore Deposits -- The Barker/Hughesville Mining District is a classic lead/zinc/silver locality.  There are many smaller deposits at the contact between the igneous rock--porphory--and limestone.  The largest ore body is at the Block P Mine.  It has the shape of an inverted cone within the Hughesville Stock.  The major ore minerals are galena (=lead sulfide), sphalerite (=zinc sulfide), and pyrite (=iron sulfide).
  
  
• Reclamation -- The Barker/Hughesville Mining District is a Superfund site.  Reclamation of the tailings pond shown in the upper photo will be mostly completed this summer.
  
  
• Scenic Views -- The photo below shows the view from Kibbey Divide, the starting point of the hike.  Hikers will be able to see many prominent peaks formed by igneous intrusions--laccoliths. Thunder Mountain, Tiger Butte, and Tillinghast Mountain are laccoliths to the west of Belt Creek.  The highest peak in the Little Belt Mountains is formed by 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.




View from Kibbey Divide looking north down into Big Otter Creek.  The Highwood Mountains on the far skyline consist of a stack of lava flows on top of Cretaceous-age sedimentary rocks.  The lava flows are covered with forest whereas the sedimentary rocks below lack forest cover.  The prominent cliff of Madison Limestone in the center of the photo dips 30° to the left due to the forceful igneous intrusion--the Clendennin-Peterson Laccolith--inside of Clendennin Mountain (on the right side of the photo).   Limestone Butte is on the skyline on the left side of the photo, covered with dense forest.  A laccolith inside the butte has bulged up the sedimentary rocks, including the Madison Limestone on the surface, to make a dome.



• Flora and Fauna of the Barker/Hughesville Area -- Lists of the mammals, birds, and plants found in Paine Gulch, the valley to the south of Dry Fork, are available on the internet. Both the common names and the scientific names are listed. To see photos of these plants and animals, use the Image Search Engine at Google and search for the common name or the genus and species.



• Field Trip Leaders -- Dr. David Baker, Earth Scientist in Monarch, and Les Nilson, hunting and fishing guide, will lead the hike.




Peterson Mountain as viewed from Clendennin Mountain.  The entire ridge is underlain by the Clendennin-Peterson Laccolith.  On the left side the beds are tilted to the left.  On the right side of the ridge, the beds are tilted to the right.  The heat from the igneous intrusion has "cooked:" the shale and limestone on the crest of the ridge.  Look for yellow-green epidote formed as a result of the contact metamorphism.




• Registration and Cost -- There is no charge for the field trip; however, prior registration is required. The group size is limited. To register, call (406) 236-5934 or (406) 727-2240 or fill out our on-line registration form. Meeting time is 9:00 a.m.

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