File:Isoclinal fold in gneiss (Archean; Ennis Lake North roadcut, Madison County, Montana, USA) 2 (44609779485).jpg
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| Description |
Gneiss in the Precambrian of Montana, USA. Gneiss is a high-grade, foliated metamorphic rock. It forms principally by metamorphism of schist, granite, or granite-like rocks. The foliation in gneiss is obvious and consists of alternating bands of light and dark minerals. The gneiss in this Montana outcrop includes tightly folded foliation - the elongated fold in the middle of the picture is an isoclinal fold. Isoclinal folds, or "hairpin folds", have parallel flanks. Geologic unit & age: unit Aqfg of Vuke et al. (2002) - quartzofeldspathic gneiss, Archean, probably 3.1 to 3.2 Ga (= age of peak metamorphism during intrusion of Spanish Peaks area trondhjemites and tonalites; see Kellogg, 1994) Locality: roadcut on the western side of Route 287, north of Ennis Lake & south of the town of Norris, northern Madison County, southwestern Montana, USA (45° 30' 02.06" North latitude, 111° 41' 44.14" West longitude) References cited: Kellogg (1994) - Geologic map of the Norris quadrangle, Madison County, Montana. United States Geological Survey Geologic Quadrangle Map GQ-1738. Vuke et al. (2002) - Preliminary geologic map of the Bozeman 30' x 60' quadrangle, southwestern Montana. Montana Bureau of Mines and Geology Open File 469. |
| Date | |
| Source | Isoclinal fold in gneiss (Archean; Ennis Lake North roadcut, Madison County, Montana, USA) 2 |
| Author | James St. John |
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This image was originally posted to Flickr by James St. John at https://flickr.com/photos/47445767@N05/44609779485 (archive). It was reviewed on 7 November 2018 by FlickreviewR 2 and was confirmed to be licensed under the terms of the cc-by-2.0. |
James St. John
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| current | 00:29, 7 November 2018 | | 4,000 × 3,000 (8.69 MB) | Lu Brito (talk | contribs) | Transferred from Flickr via #flickr2commons |
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