8/1 Stop 5: Twisp River Road Mile 5.4

The Winthrop Formation contains interbedded shale and arkose sandstone. The arkosic sandstone has greater than 90% quartz and feldspar. When it has this percentage in addition to biotite or muscovite mica, it indicates that Winthrop Formation was originally granite. The arkosic sandstone includes lots of plant fossils. Some of the sandstone contains of pyroxine.

Sandstone (above) and shale (below) of the Winthrop Formation.

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