The late Quaternary Old Crow tephra is a two-pyroxene, calc-alkaline dacite whose known areal extent is broadly delimited by a triangle with apices at the Seward Peninsula, the Wrangell Mountains, and the Old Crow Basin in the northern Yukon. Everywhere the tephra is fine grained (Mdσ = 5.33 ± 0.32) and moderately to poorly sorted (6σ = 1.33 ± 0.29). Samples from the extremities of the fall-out zone have similarity coefficients of 0.95.A palaeomagnetic excursion is recorded in sediments just below Old Crow tephra in the Fairbanks area and at Imuruk Lake on the Seward Peninsula. A short, full reversal of the magnetic field has been preserved at the corresponding stratigraphic level near Old Crow in the northern Yukon. Chronological controls based on 14C and fission-track dates and sedimentation rates show that these palaeomagnetic features are almost certainly partial records of the complex geomagnetic Blake Event, which occurred 100–120 ka ago.New petrochemical data demonstrate that the source of Old Crow tephra is in the eastern Aleutian arc.