Geology of the Okanogan Highlands 2012 - Stop 1

From the corner of highways 97 and 20 in Tonasket, drive toward Republic. The first stop offers a close-up view of pictographs as well as layers of mylonite.

Stop 1: Mylonite

We are in the Okanogan Metamorphic Core Complex. The layers in this mylonite (Greek for “milled rock”) result from shearing, grinding and recrystallization of an original granite subjected to high temperature and stress within a deep-seated fault zone in the crust. This mylonite is part of the now inactive, 50 million year old Okanogan fault zone, along which igneous and metamorphic rock from deep in the crust rose to form a dome as overlying rocks slid to the west. Layered rocks like these crop out along the east side of the valley on the drive from Omak to Tonasket, forming what people in the area call “the flat-irons.” 

Ralph Dawes of Wenatchee Valley College provides an explanation of the history of the Okanogan fault zone.

The group gathers at Stop 1.

Steve Box of USGS describes the layered rocks outcrop.

The group takes some time to explore and observe.

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