Crops out on both sides of White Pass HWY from summit to 3 mi east of Spiral Butte and on west and northwest of Tumac Plateau, Yakima Co, WA. Consists of dense, light-gray lava with strong linear flow-banding which gives rock characteristic platy rock cleavage. Geologic sketch map and structure section show Spiral Butte Andesite unconformably overlies Russell Ranch Formation. Age given as Quaternary.
Source: GNU records (USGS DDS-6; Menlo GNULEX).
Described as light-gray, platy, flow-layered hypersthene-hornblende-augite andesite lava flows and minor breccia. Erupted from several vents, the youngest being Spiral Butte. Age given as Pliocene to 10,000(?) yr B.P.
Source: GNU records (USGS DDS-6; Menlo GNULEX).
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