New Sm–Nd and Rb–Sr data are presented from the volcanic member of the Huckleberry Formation, a unit near the base of the upper Proterozoic Windermere Supergroup in northeastern Washington. The isotopic data are compatible with a rift setting during deposition of Windermere strata. A best-fit line through all the data yields an age of 795 Ma with a large error of ±115 Ma (2σ). However, a preliminary Sm–Nd mineral–whole-rock age of 762 ± 44 Ma (2σ) obtained from a least-altered basalt is within error of other age information from the Canadian Cordillera that broadly constrains the initiation of Windermere deposition to ~750 (±30) Ma. A synthesis of isotopic data, of the results of tectonic subsidence analyses, and of geologic relations within the Windermere Supergroup and the immediately overlying uppermost Proterozoic – Lower Cambrian strata leads to the conclusion that the Windermere Supergroup does not represent the rift deposits that led directly to the initiation of the early Paleozoic miogeocline. Instead, Windermere sedimentation occurred as a result of a protracted, episodic rift history that culminated with latest Proterozoic – Early Cambrian rifting associated with the onset of thermal subsidence in the ancient passive margin.