Glaciers have played an important role in modifying the landscape of the Long Valley area before, during, and after the enormous caldera-forming eruption 760,000 years ago. Glacial striations, moraine deposits beneath the town of Mammoth Lakes and in major river valleys, jagged cirques, and ice-rafted boulders scattered on ancient shorelines of the large lake that once filled the caldera are clear evidence that several episodes of recent glaciation occurred here.