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Erratic Rock State Natural Site is a state park in the Willamette Valley of Oregon, United States. Featuring a 40-short-ton (36 t) glacial erratic from the Missoula floods, the small park sits atop a foothill of the Northern Oregon Coast Range in Yamhill County between Sheridan and McMinnville off Oregon Route 18. The day use only park is owned and maintained by the Oregon Parks and Recreation Department.

Location
Change title from "Data" to “Location” Located off Oregon 18, Erratic Rock State Natural Site lies mid-way between Sheridan to the west and McMinnville to the east.[3][7]

Geology
Change title from "history" to Geology

The pre-historic Missoula floods began in western Montana fifteen to twenty-thousand years ago.[2] These large floods altered the landscape of the Columbia River valley and flooded the Willamette Valley.[2] Many rocks were transported down the Columbia encased in icebergs and deposited from Montana through Idaho, Washington, and Oregon when the flood waters receded and the ice melted.[2]

One of these 40-short-ton (36 t) erratics was deposited on a 250-foot (76 m) tall hill in the Yamhill Valley portion of the Willamette Valley.[3] The rock comes from Canada and is the largest glacial erratic rock in the Willamette Valley.[8][9]

The rock is argillite believed to be 600 million years old and originally part of the sea-floor.[10] It is also the only rock of its type outside of Canada.[11]

Geology for the Youth Readers
Erratic Rock arrived in a giant, melting iceberg. The iceberg was part of a giant sheet of ice that covered a lot of Canada. The iceberg broke off the sheet in Montana and when it started to melt, it created a flood of water called the Missoula floods. The larger the iceberg, the longer it takes to melt in the same temperatures. This flood of melting ice and water moved from Montana, through Idaho and Washington and into Oregon some fifteen- to twenty-thousand years ago. Geologists believe Erratic Rock is about 600 million years ago, which is millions of years before the dinosaurs roamed the Earth. When the flood waters finally left, it left Erratic Rock behind.

Erratic Rock is the only rock of its type, outside of Canada. It is clues like this that help the Geologists figure out how the iceberg and flood waters traveled. Erratic Rock did not melt because it is made of rock that is stronger than ice and which won't melt at the same temperature. The rock is called argillite and Geologists believe it was originally part of the sea-floor.