File:Gneiss with mafic enclave (Archean; Norris South roadcut, Madison County, Montana, USA) 5 (45479834612).jpg

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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.

Geologic unit & age: unit Agg of Kellogg (1994) - plagioclase-microcline-quartz-biotite gneiss, Archean, probably 3.1 to 3.2 Ga (= age of peak metamorphism during intrusion of Spanish Peaks area trondhjemites and tonalites)

Locality: roadcut on the eastern side of Route 287, Burnt Creek-Bradley Creek Valley, south of the town of Norris, northern Madison County, southwestern Montana, USA (45° 31' 29.67" North latitude, 111° 41' 50.54" West longitude)

Reference cited:

Kellogg (1994) - Geologic map of the Norris quadrangle, Madison County, Montana. United States Geological Survey Geologic Quadrangle Map GQ-1738.
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Source Gneiss with mafic enclave (Archean; Norris South roadcut, Madison County, Montana, USA) 5
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@N05/45479834612. 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.

7 November 2018

James St. John

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current00:28, 7 November 2018Thumbnail for version as of 00:28, 7 November 20183,000 × 4,000 (6.88 MB)Lu Brito (talk | contribs)Transferred from Flickr via #flickr2commons

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