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| A SMALL LACCOLITH NEAR SUNDANCE, WYOMING. |
The Laramide-age Black Hills, the dominant landform
in western South Dakota extending into Wyoming, is surrounded along the
northern boundary by igneous intrusions.
The June 10, 2012, blog posting described
one of these intrusions near Sturgis, South Dakota, known as Bear Butte. Near Sundance, Wyoming, there are a series of
related, small Eocene laccoliths (an igneous mass intruding between layers of sedimentary rock and doming the overlying layers; at times the domed sedimentary rock is eroded away and igneous rock is exposed) intruding through the Mesozoic rocks; all are
easily visible from the highway--Sundance Mountain (5829 feet), Black Buttes, Inyan
Kara Mountain (6368 feet), Missouri Buttes (5055 feet) and the Bear Lodge Mountains (~6600 feet). A little further to the northwest of Sundance
is the best known of the intrusions, Devils Tower (may not be part of a laccolith but some sort of an igneous intrusion; 5112 feet).
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| SECTION FROM MISSOURI BUTTES EASTWARD TO BELLE FOURCHE RIVER THROUGH DEVILS TOWER. FROM USGS PROFESSIONAL PAPER 65, 1909. |
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| INYAN KARA MOUNTAIN SOUTH OF SUNDANCE, WYOMING, A SACRED PLACE TO NATIVE AMERICANS. THE PEAK WAS VISITED BY MEMBERS OF THE 1874 BLACKS HILLS EXPEDITION LEAD BY GEORGE ARMSTRONG CUSTER. A SCIENTIST IN THE EXPEDITION (DONALDSON) STATED THERE IS NO GRANITE OR OTHER PRIMARY ROCK IN THE MOUNTAIN, NEITHER BASALT OR TRAP. THE WHOLE IS AN IMMENSE UPHEAVAL OF NON-FOSSILIFEROUS, SEDIMENTARY, METAMORPHIC ROCK. PHOTO AND QUOTE FROM WYOMING STATE HISTORIC PRESERVATION OFFICE. | OFFICE. | | |
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| NORTHWEST-SOUTHEAST SECTION THROUGH INYAN KARA MOUNTAIN. FROM USGS PROFESSIONAL PAPER 65, 1909. |
I-90 then proceeds into South Dakota toward the city
of Spearfish. I have always enjoyed
camping around, and visiting, this community for a number of reasons. Spearfish Canyon leaves town and follows
Spearfish Creek southwest toward Savoy and a summit. A wonderful section of Paleozoic rocks are
exposed along the route and the Creek offers some great fly fishing
opportunities. One can also locate the “final
scene” from the movie, Dances with Wolves
(http://dances-with-wolves.purzuit.com/video/LR4j8iWu-OI.html).
At this locality, up Forest Road 222
(west of Savoy), Wind in His Hair shouts from the cliff (Pahasapa Limestone of
Mississippian age) as John Dunbar is leaving the tribe Shumanitutonka Ob Wachi! Although
I took exception to movie's rendition of Fort Hays (I lived there for 24 years),
this final scene in the Black Hills was pretty spectacular!
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| CLIFF OF PAHASAPA LIMESTONE AT FINAL SCENE FROM DANCES WITH WOLVES. |
I also am fascinated by the area’s small laccoliths
that just intrude and push up through the Paleozoic rocks. I am far from a petrologist or structural
geologist and really do not understand all of the emplacement mechanisms except
that somehow they seem related to the Laramide Orogeny; however, they are
enjoyable to observe and climb—Crow Peak (5760 feet), Citadel Rock (5410 feet),
Ragged Top (6220 feet) and Spearfish Peak (5764 feet). Most have a quartz latite porphyry (plagioclase
phenocrysts in a fine groundmass of orthoclase feldspar; Lisenbee and others,
2007) in the center surrounded by the Cambrian-Ordovician Deadwood Formation
(sandstone), some lower Paleozoic thin carbonates and the massive, cliff-forming
Mississippian Pahasapa Limestone. Most structures
are rather circular in nature and are generally “off the beaten path”. They evidently are Eocene in age and related
to the other small intrusions. Elkhorn
Mountain further east down I-70 is an intrusion where the igneous core has not
been exposed and additional small laccoliths are located south of Sturgis.
In the Spearfish area I am especially enamored with
Crow Peak. There is a beautiful “back
road” leading out to the summit trail (1560 feet elevation gain) and it appears
a fall drive would display some fantastic colors. I thank my guide Junior Geologist for introducing me to both
the peak and the aptly named Crow Peak Brewing Company.
mike
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| CROW PEAK, AN EOCENE LACCOLITH NEAR SPEARFISH. |
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| SECTION THROUGH CROW PEAK. FROM USGS PROFESSIONAL PAPER 65, 1909. |
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| GEOLOGIC MAP OF CROW PEAK SHOWING CIRCULAR NATURE OF LACCOLITH AND IGNEOUS ROCKS IN THE CORE. FROM LISENBEE AND OTHERS, 2007. |
REFERENCES
CITED
Lisenbee, A. L., J. A. Redden, and M. D. Fahrenbach,
2007, Geologic Map of the Maurice Quadrangle, South Dakota: South Dakota
Geological Survey, Vermillion.
