At Mount Rainier, the interaction of lava and glacial ice shaped the mountain for hundreds of thousands of years. This site stands on the toe of an ancient lava flow where flowing lava met glacial ice. Unable to melt through the thickest part of the glacier, the lava moved along its side and against a ridge where the ice was thinnest, then cooled and hardened into rock. When the glaciers retreated, they left high ridges of resistant rock like this one. During the ice age, glaciers filled Mount Rainier's deep canyons, and the White River Valley was filled with glacial ice about a thousand feet thick. About half a million years ago, andesite lava, the most common lava on Mount Rainier, flowed along the margin of a large glacier in the White River Valley. The ice cooled the lava rapidly, causing it to shrink and crack into narrow hexagonal columns that pointed toward the cool glacial ice. As the ice age ended, the glaciers slowly melted away and exposed ridge-capping lava flows that now separate deep valleys. Today's glaciers are small remnants of the much larger ice masses that once occupied the mountain.