The Karheen Formation of southeastern Alaska (Alexander terrane) is an 1800-m-thick redbed molasse sequence that formed during the Late Silurian–Early Devonian in the wake of the Klakas orogeny. Previous research interpreted thin, platy limestone in the Karheen Formation as marine in origin. We describe new evidence, including a lack of marine fossils, which indicates that the rhythmically laminated limestone formed in deep, stratified lakes. Intercalated siliciclastic, lake-margin facies record cycles of lake expansion and contraction. Comparative analysis with coeval Lake Orcadie deposits in Scotland shows that the Karheen Formation is strikingly similar in age, thickness, lithology, paleontologic characteristics, and tectonic setting to the Old Red Sandstone. Together with other published geologic data from southeastern Alaska, the Karheen's lacustrine deposits strengthen paleogeographic connections between the Alexander terrane and the N. Atlantic–Caledonide region in the mid-Paleozoic.