We frequently hear the question: “Why are there glaciers on Mount Rainier?” A glacier forms wherever snowfall repeatedly exceeds melting over a period of years. Above 6,500-7,000 feet on Mount Rainier, more than 50 feet of snow falls each winter, and not all of it melts before the next winter. The survival of this snow from one year to the next depends partly on the cooler temperatures at the higher altitudes, and perhaps also on the somewhat deeper snowfalls there. [Illustration: Two ice streams meet to form the half-mile-wide Cowlitz Glacier. One heads on the flank of the volcano and the other (Ingraham Glacier) at the summit. The firnline is a short distance above the junction of the glaciers. The high bare embankment at the extreme right is a lateral moraine that was formed about 100 years ago when the glacier was thicker and about 1½ miles longer. (Fig. 12)] LITTLE TAHOMA PEAK MOUNT RAINIER INGRAHAM GLACIER COWLITZ GLACIER A line that marks the limit on a mountain above which snow persists from one winter to the next is called the annual snowline, and this line on a glacier is called the firnline (fig. 12). Above the firnline, snow that falls each year packs down and changes into glacier ice as air is slowly forced out of it. This part of the glacier is its accumulation area, where more snow falls each year than is lost by melting. Below the firnline is the ablation area, where melting predominates. The firnline on Mount Rainier’s glaciers has been well above 6,500 feet in recent years. But some glaciers extend to altitudes below 5,000 feet—that is, far down into the ablation area. They do this by slowly flowing downhill. Solid ice flows by sliding on the hard bedrock under the glacier and by slipping along the innumerable surfaces within the ice crystals that make up the glacier. The rate of flow and the rate of ablation govern the distance a glacier extends down into the ablation zone. If these rates remain fairly constant, the glacier will be in balance and its size will be about the same from year to year. But if changing weather patterns affect the rates of ablation or accumulation, or both, the glacier will either become smaller or grow larger. The change you are most likely to notice is in the position of the glacier’s terminus, which may either recede or advance, but precise measurements of the upper reaches of a glacier also show volume changes there, some of which may not affect the glacier’s terminus for many years, if ever.