Crevasses are a glacier’s most awesome features and are a constant hazard for climbers. They form where adjacent parts of a glacier are moving at different speeds. Some of Mount Rainier’s glaciers may be flowing at a speed of several thousand feet per year along their centers but at a much slower rate along their margins. This unequal rate of flow produces stresses in the ice that cause it to break. Groups of crevasses often form where the glacier flows over a steep place in its bed. The ice moves faster here, and pulls apart, and a crevasse is formed. Although a large crevasse may seem to be bottomless to the observer, most crevasses are less than 100 feet deep because ice pressure tends to close the open spaces in the ice below that depth.[3] Ice covers 37 square miles of the park today. Individual glaciers that make up this ice blanket are placed into three groups, depending on their place of origin. Those of one group originate at the volcano’s summit and flow far down the valleys that radiate from the cone. Most of the snow that nourishes these glaciers probably falls on them at altitudes well below the summit. The largest examples of the group are the Emmons, Nisqually, and Tahoma Glaciers. Glaciers of the second group originate on the flanks of the volcano, mostly at altitudes between 7,000 and 10,000 feet. This group is represented by the South Tahoma, Carbon, and Inter Glaciers. Glaciers of the third group are on north-facing slopes in the mountains around Mount Rainier. They are mostly at altitudes of about 6,000 feet, and they owe their existence to locations well protected from solar heat. Glaciers representing this group are the Unicorn and Pinnacle Glaciers in the Tatoosh Range, a small unnamed glacier near the west end of Burroughs Mountain, and the somewhat larger Sarvent Glaciers east of Mount Rainier. Work Habits of Glaciers Glaciers are extremely capable workers. Their work includes erosion, transportation, and deposition. The smoothed and grooved bedrock at Box Canyon and at many points along the trail to the ice caves near Paradise Park shows erosion of rock by glacier ice. Rock fragments carried by glaciers cut grooves in the hard bedrock and polish its surface (fig. 13). Although any one rock fragment might scrape away only 1 millimeter of rock along a single groove, the total effect is great when multiplied by countless thousands of similar fragments rasping a bedrock surface for hundreds or thousands of years. Glacier ice may also break chunks of rock loose as it overrides them and may even plow up sections of softer rocks. Glaciers transport not only the rocks that they quarry and scrape from their beds but also, more conspicuously, those that fall onto their surfaces from nearby cliffs. These falling rocks range in size from tiny particles to individual masses that weigh tens of thousands of tons, like those that fell onto Emmons Glacier from Little Tahoma Peak in 1963. (See p. 35.)