Description

From Wood and Kienle (1990) ^[1] : "Potassic alkali olivine basalt cinder cones and columnar-jointed flows occur as separate entities in the field, which is more or less centered on the Coast Range megalineament. Minor andesite and trachyandesite flows are locally present. There are also thin patches of ash and lapilli layers. The lava flows unconformably overlie glacial-fluvial sand and silt deposits and granitic gneiss of the Coast Mountains plutonic-metamorphic complex. Glacial striations and grooves indicate that some of the flows pre-date the last glaciation, but undissected cinder cones and flow surface features indicate that most of the activity was post-glacial. Flows are locally columnar-jointed, and some tuff and breccia deposits form void-rich spires and cliffs."
Wood and Kienle (1990) ^[1] report an eruptive history of “Possibly two periods: Older ~5 Ma, and younger ~1 Ma to 0.5 Ma.” Post-glacial features should be younger than 13,500, indicating possible Holocene ages for some of Behm Canal-Rudyerd Bay. Wanek and Callahan (1971) ^[2] state that basalt flows near Lake Grace could be either late Pleistocene or Holocene in age.