|
Monarch Area Community Association |
||
|
Location Map Topo map Air photo Video cam Weather Restaurants
Motels Camp/Conference Ctr Businesses Recreation Government Services Arts, Crafts & Publications History Landmarks
Natural History Community Issues Contact Us |
Geology - Stratigraphy The mountain range in Central Montana known as the Little Belt Mountains is a dome-shaped uplift that exposes a sequence of layers similar to the layers in an onion. The oldest rocks (up to 2.7 billion years) are in the center of the range and progressively younger formations are found on the flanks. Some of the distinctive rocks are shown in the images below.
The Devonian-age Jefferson Dolomite consists of dolomite and limestone. The large cliff on Sun Mountain, south of Monarch, in the photo above is made of this rock. This cliff is also called the "Man Face."
The Mississippian-age Madison Limestone is approximately 1700 feet thick. Madison Limestone is now a Group name, which includes two formations--the Mission Canyon Formation and the Lodgepole Formation. Several generations of caves formed in this limestone. Madison Limestone is the largest artesian aquifer in the U.S. The large cliff in the photo is in Sluice Boxes State Park. Morrison Formation and Kootenai Formation
Black coal beds at the top of the Jurassic-age Morrison Formation are visible from the old highway south of the town of Belt. The coal was mined in the late 1800's and early 1900's and was the basis for developing the town. The sandstone beds on top of the coal belong to the Cretaceous-age Kootenai Formation. Resistant sandstone beds in the Kootenai Formation form the series of waterfalls on the Missouri River in the Great Falls area.
The Arrow Creek Bed is a thick bed of bentonite in the Bootlegger Member of the Cretaceous-age Blackleaf Formation. It is clearly visible from highway at Geyser as shown in the photo above. The bentonite formed from volcanic ash that came from a volcano in Eastern Oregon or Eastern Washington about 95 million years ago. The Arrow Creek Bed is also found just below the top of Belt Butte. However, there it has been "cooked" by an igneous intrusion--a sill--to a porcellainite. Square Butte and Round Butte are two prominent igneous intrusions--laccoliths--on the east end of the Highwood Mountains.
|
The 
The basal formation in the 
There are more than 30 igneous intrusions in the Little Belt Mountains. Most have the shape of laccoliths and the composition of porphyry. Wolf Porphyry, as shown on the left, has large crystals of white to cream-colored feldspar, smaller dark gray crystals of quartz, and occasional small flakes of black mica and needles of hornblende. The fine-grained groundmass consists mostly of small crystals of feldspar and quartz. This rock was intruded as molten magma and froze (turned into solid rock) at about 600° C. or 1000 ° F. These igneous intrusions were emplaced between 54 and 48 million years ago. The outcrop shown in the photo is in the Mixes Baldy-Anderson Peak Bysmalith close to the Blankenship Divide.