A section of lake beds exposed in the sides of the trench of Ana River below Ana Spring near the northwest corner of Summer Lake basin in Lake County, Oregon, reveals near the top at least six layers of pumice. Four of these appear to record eruptions of Mount Mazama which led finally to the formation of Crater Lake; the source of the fifth is not known; the sixth is attributed to a later eruption of Newberry Crater. As certain layers in the associated sediments imply shallow-water conditions, these eruptions must have occurred when the last pluvial lake, formerly about 215 feet deep, had been reduced by evaporation to a depth of about 85 feet, probably about 14,000 years ago. The data appear to extend back the ages of Mount Mazama pumice, of Crater Lake, and of Paleo-Indian occupation of the area by several thousand years.

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