Fichier:Lower colonnade & entablature of the Overhanging Cliff Flow (Junction Butte Basalt, Upper Pliocene, 2.01-2.15 Ma; northeastern Yellowstone National Park, Wyoming, USA) 2 (19836730692).jpg
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DescriptionLower colonnade & entablature of the Overhanging Cliff Flow (Junction Butte Basalt, Upper Pliocene, 2.01-2.15 Ma; northeastern Yellowstone National Park, Wyoming, USA) 2 (19836730692).jpg
Basalt lava flow in the Pliocene of Wyoming, USA.
This is a famous outcrop of a thick basalt lava flow in the northeastern portion of Yellowstone National Park. The lowest part of the flow has nice, regularly-spaced, vertical fractures. Eroded, detached blocks from this layer show that it is composed of hexagonal columns. This is columnar jointing, formed by cooling, solidification, and slight contraction of molten lava into basalt rock. A columnar-jointed lower portion of a lava flow is called the lower colonnade. A thick layer overlies the lower colonnade - it is part of the same basalt lava flow. The regular columnar jointing of the colonnade is absent in that unit. Instead, an irregular fracture pattern is present - this is called the entablature.
Locality: roadside exposure along Grand Loop Road just north of Tower Falls, northeastern Yellowstone National Park, Wyoming, USA (44° 53' 49.06" North, 110° 23' 22.20" West)
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