The Rattlesnake Formation - Oregon Historical Markers on Waymarking.com

Sign about the Rattlesnake Formation along U.S. Hwy 26 between Dayville and Mount Vernon.

Marker Name: The Rattlesnake Formation
Marker Text: The rimrock is a volcanic ash flow about 5 million years old. The ash erupted as red hot pumice, highly charged with gas, which flowed like very dry fine sand; it probably filled the ancient John Day River valley to a depth of more than 100 feet over a distance of 60 to 70 miles. When movement ceased, the center of the flow was still so hot that the glassy particles of pumice were partly melted or welded together to form a hard rock called welded tuff. About 70 feet of tuff is exposed in the cliffs opposite.
The steep dark slopes below the rimrock are formed by gravels deposited before the eruption. The rounded hills above the tuff are gravels which were deposited after the eruption. The tuff is thin or missing over large areas where it has been stripped off by erosion. The gravels and tuff together make up the Rattlesnake Formation. Light-colored slopes below the Rattlesnake gravels are on ash beds of the considerably older Mascall Formation.
The John Day River valley originated as a downwarp when the Aldrich and Strawberry Mountains were raised by movements of the Earth’s crust. Many thousands of feet of rocks were eroded away, especially from the soft Mascall Formation, before the first gravels of the Rattlesnake Formation were deposited. When deposition of the Rattlesnake Formation was completed, the ancient John Day River was about 1000 feet higher than at present and occupied a valley 6 to 7 miles wide. The present narrow valley, ranging up to 1.5 miles in width, probably was carved by the river within the last 500,000 years.

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