Pseudocraters Never Die (Phillips County, Montana)
On the Internet, known pseudocraters ( craterwrongs )
never seem to die. They seem to get endlessly recycled
as seen in a rambling article, “The Younger Dryas Impact
Event and the Cycles of Cosmic Catastrophes,” reprinted
by RH on July 5, 2012 at:
This web page cites:
Geologists may have found 'new' meteor crater – Montana
by Annette Hayden, Havre Daily News, June 11, 2007.
This circular feature is located at Latitude: 47.804896°:
Longitude: -108.631878° in Phillips County, Montana,
about 14.5 km southwest of Zortman, Montana.
as can be seen at:
Unfortunately, these circular features are not impact craters.
These features, which partially surround the Little Rocky
Mountains, are eroded surfaces of circular structural domes
created by the uplift of sedimentary strata overlying laccolithic
intrusions composed of syenite porphry.
These laccoliths are discussed in "Geology and Physiography
of Fort Belknap" at:
A geologic cross section of these circular features can
be found at:
According to the above web page, a laccolith is a “concordant
igneous intrusion that has domed the overlying rocks and has
a known or assumed flat floor and a postulated dikelike
feeder beneath its thickest point. It is roughly circular in plan,
less than five miles in diameter, and from a few feet to several
hundred feet in thickness.”
The geology of these features is illustrated by:
Porter, K. W., and E. M. Wilde, 2001, Geologic map of the
Zortman 30' x 60' quadrangle, eastern Montana, Montana
Bureau of Mines and Geology. Open-File Report 438,
16 p., 1 sheet(s), 1:100,000.
This report is available from:
The 4.2 MB PDF file of the geologic map is available from:
Publications about these circular geologic structures are:
Collier, A. J., and S. H. Cathcart, 1922, Possibility of finding
oil in laccolithic domes south of the Little Rocky Mountains,
Montana. Bulletin 736-F (pp. F171-F178) United States
Geological Survey, reston, Virginia.
Knechtel, M. M., 1944, Oil and gas possibilities of the plains
adjacent to the Little Rocky Mountains, Montana. Oil and Gas
Investigations Map no. OM-4, scale 1:48000, U.S. Geological
Survey, Reston, Virginia.
Knechtel, M.M., 1959, Stratigraphy of the Little Rocky
Mountains and encircling foothills, Montana. Bulletin
no. 1072-N, U.S. Geological Survey, Reston, Virginia.
Laccolith - http://en.wikipedia.org/wiki/Laccolith
2. What's a laccolith? - http://formontana.net/2a.html
Syenite - http://en.wikipedia.org/wiki/Syenite
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