John Day Fossil Beds NM,Sheep Rock unit,Cathedral Rock, photo

Cathedral Rock

John Day Fossil Beds NM

Majestic Cathedral Rock rises above the John Day River on a rainy cloudy day.The colors you see are due to the chemicals in the layers of basalt and claystones including ignimbrites and tuff. BASALT: The top layers are dark, dense, iron-rich rock from lava flows. CLAYSTONE: Beneath the basalt layers are deposits composed largely of volcanic ash, deposited by air and water. Much of these ash deposits were worked into clay-rich soils, then buried and changed into hard rock and may be in shades of white, pale green, red or brown. IGNIMBRITE: Ignimbrites are made of a very poorly sorted mixture of volcanic ash (or tuff when lithified) and pumice lapilli, commonly with scattered lithic fragments. Ignimbrites may be white, grey, pink, beige, brown or black depending on their composition and density. TUFF: Relatively soft, porous rock that is usually formed by the compaction and cementation of volcanic ash or dust. Those in the Blue Basin area contain a rare green-blue mineral called clinoptilolite. The blue color seems to leach out in rains and wash away, leaving the greenish clay. Recent rain and overcast skies definitely saturate the colors!

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