This geological formation is one of the largest comprising the Mount Meager massif. Formed during an onset of volcanic activity about 100,000 years ago, the Plinth Assemblage consists of light grey porphyritic rhyodacite with phenocrysts of plagioclase, quartz, minor biotite and rare hornblende. The of Mount Meager itself is made of Plinth Assemblage rocks and was the southern source of Plinth Assemblage lava flows and breccias.