A tephra bed within the Salmon Springs Drift at its type locality near Sumner, Washington, has a zircon fission-track age of 0.84 ± 0.21 m.y., and the immediately overlying silts are reversely magnetized. The same distinctive tephra at Auburn, Washington, has a glass fission-track age of 0.66 ± 0.04 m.y. and a zircon age of 0.87 ± 0.27 m.y. The bracketing silts are likewise reversely magnetized. Therefore, the Salmon Springs Drift, as defined at its type section, is of middle Quaternary age and is much older than the currently accepted age of about 70,000 yr.

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