From Wood and Kienle (1990): "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."