Box Canyon in Mount Rainier National Park is a narrow gorge cut into granite bedrock. It began as a crack in the rock, and the cutting continues as cobbles and sometimes boulders in the powerful current batter the canyon walls. The stream flows from the terminus of the Cowlitz Glacier, three miles up canyon. Its milky color marks it as glacial meltwater, clouded with rock fragments scraped from the mountain's flanks. Although the glacier has melted back from this area, the glacier-fed river continues to cut downward through the canyon. The gorge is 180 feet deep and narrows to only thirteen feet at its tightest point.