The Black Tusk, the oldest and most striking of the three principal volcanoes, is the glacially dissected remains of a stratovolcano that formed between 1.3 and 1.1 million years ago. Eruptions produced hornblende andesite lava flows and lithic tuffs. Prolonged erosion destroyed the original cone. The bluffs northwest, southwest and southeast of the main volcanic edifice are remains of this ancestral volcano. Renewed volcanism between 210,000 and 170,000 years ago produced hypersthene andesite lava flows, which locally terminate with precipitous thick ice-contact margins. This latest eruptive activity culminated with extrusion of an endogenous dome and related lava which form the present high summit spire. Later, the Cordilleran Ice Sheet carved a deep, north-trending U-shaped valley into the eastern flank of this edifice.