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The two dark lava flows seen here in the Narrows of the Grand Canyon of the Yellowstone River erupted about 1.3 million years ago at about the time of the second caldera-forming rhyolitic ash-flow eruption, west of Yellowstone National Park. Thick layers of stream-deposited gravels lie beneath and between the two lava flows visible in this photo. The cliffs eroded into pinnacles below the lower basalt are older volcanic rocks of the Absorka volcanic field. Photograph by S.R. Brantley on 21 May 2001.
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View across The Narrows of the Grand Canyon of the Yellowstone. The lowest part of the cliff is composed of river-deposited gravel. Above that is a single columnar-jointed basalt flow overlain by glacial till. Photograph by S.R. Brantley on 21 May 2001.
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Columnar jointing is seen in The Narrows of the Grand Canyon of the Yellowstone. These columns formed as a result of a basaltic lava flow contracting while cooling. The basalt flow lies on fluvial gravels and later was erosionally truncated at the top of the columns, now overlain by glacial deposits. Photograph by David E. Wieprecht on September 1996.
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Outcrop of the Junction Butte basalt alongside the road at Overhanging Cliff. The zone of prominent columnar jointing at the base of this thick lava flow formed by contraction cooling; it overlies stream-deposited sediments and is overlain by a zone of irregularly intersecting joints. Photograph by S.R. Brantley on 21 May 2001.
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