Catastrophic flooding has eroded a discontinuous network of oversized anastomosing channels on the northwest flank of the Medicine Lake volcano. Most of these previously unrecognized channels were cut into an andesitic ash-flow tuff; boulders as large as 2 m in intermediate diameter were moved in terrain where little rain falls today and stream erosion is nonexistent or minimal. The flooding was probably triggered by eruption of andesite tuff through a late Pleistocene ice cap on the volcano, about 60,000 to 70,000 or about 130,000 B.P.

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