Six rhyolitic tephra layers from ancient loess and related detritus in the Cypress Hills, Saskatchewan, represent separate volcanic eruptions from the Snake River Plain. Idaho, U.S.A. The weighted mean age and uncertainty of the youngest tephra bed is 8.3 ± 0.2 Ma, using the isothermal plateau fissiontrack technique on its hydrated glass shards. The loess that hosts five of these tephra beds extends across the Cypress Plain, which is the oldest (Middle Miocene) and highest depositional surface in the Interior Plains, and also occurs on four juxtaposed erosion surfaces. It appears that the first and maybe the second erosion surface began forming before 10 Ma, and that formation of the second, third, and fourth erosion surfaces was completed between 10 and 8.3 Ma.