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File:Narrows Basalt (Lower Pleistocene, 1.5-1.6 Ma) (Narrows of the Yellowstone River, northeastern Yellowstone National Park, Wyoming, USA) 2 (20137105071).jpg

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Columnar-jointed basalt lava flow in the Pleistocene of Wyoming, USA.

The horizon with closely-spaced vertical fractures shown above is a 1.5 to 1.6 million year old basalt lava flow exposed along the eastern walls of the Narrows of the Yellowstone River in Yellowstone National Park. Basalt is a mafic (dark-colored), aphanitic (finely-crystalline), extrusive igneous rock composed principally of plagioclase feldspar and pyroxene. Upon cooling and slight contraction, many basalt lava flows develop closely-spaced subvertical fractures called columnar jointing. The fractures define columns having (usually) pentagonal to hexagonal transverse cross-sections.

Below the lava flow is a Pleistocene glacial outwash unit, deposited during a glacier's melting phase. Above the lava flow is Pleistocene till, which is a poorly-sorted, non-bedded glacial deposit formed during a glacier's advancing phase.

Stratigraphy: upper Narrows Basalt, Lower Pleistocene, 1.5-1.6 Ma

Locality: Narrows of the Yellowstone River, just north-northeast of Tower Falls, northeastern Yellowstone National Park, Wyoming, USA (44° 53' 52.31" North, 110° 23' 07.47" West)
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Source Narrows Basalt (Lower Pleistocene, 1.5-1.6 Ma) (Narrows of the Yellowstone River, northeastern Yellowstone National Park, Wyoming, 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@N05/20137105071 (archive). It was reviewed on 10 October 2019 by FlickreviewR 2 and was confirmed to be licensed under the terms of the cc-by-2.0.

10 October 2019

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current00:15, 10 October 2019Thumbnail for version as of 00:15, 10 October 20194,000 × 3,000 (3.81 MB)Ser Amantio di Nicolao (talk | contribs)Transferred from Flickr via #flickr2commons

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