The Goat Rocks are the remnants of a very old volcano that first erupted about 2.5 million years ago. Lava, rock, and ash gradually built a peak as large as nearby Mt. Rainier. About half a million years ago, eruptions ceased, and glaciers later carved the mountain into the peaks and crags seen today, leaving only a fraction of the original volcano. Glacial erosion shaped the mountain through freezing water, moving ice, and transported rock. Water seeped into cracks, froze, expanded, and loosened rock that became embedded in the glacier. As gravity pulled the glacier downward, the moving ice carried rock along and reshaped the mountain. The process formed features such as cirques, crevasses, lateral moraines where rocks fell onto the glacier's sides, and medial moraines where two glaciers merged. Valley cross-sections changed from V-shaped forms before glaciation to broader U-shaped valleys afterward. Remnants of andesite lava flows are exposed around the Goat Rocks volcano. One eruption sent andesite lava about 50 miles eastward, filling valleys where the Tieton and Naches Rivers once ran. It is identified as the longest known flow of andesite lava on earth.