The Geologic Story of Mount Rainier / A look at the geologic past of one of America's most scenic volcanoes

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.)

A glacier deposits most rock debris at its terminus. The steep snout of any major glacier is a dangerous place to approach closely because rock debris almost constantly falls, rolls, and slides down the melting ice faces. Much of this debris collects at the ice margin, and if the margin stays in one place long enough a ridge-shaped end moraine of rock debris forms along the ice front (fig. 14). If such a moraine forms across the front of a glacier at its farthest advance it is called a terminal moraine. End moraines that form as the ice recedes are called recessional moraines (fig. 15). Ridges of rock debris that form along the sides of a glacier are called lateral moraines (fig. 12).

Some recent moraines of modern glaciers are only a few feet away from the present ice margin; others, formed thousands of years ago during the most recent major glaciation, are on ridgetops and valley sides or floors miles away from modern glaciers. By examining the shape and location of these moraines, we can reconstruct the size and character of past glaciers, as we will see in the next section.

Glacier-smoothed and grooved rock along the Wonderland Trail between Indian Bar and Panhandle Gap. (Fig. 13)

A muddy grayish-blue lake several hundred feet long lies behind a small horseshoe-shaped end moraine of Flett Glacier, on the northwest side of Mount Rainier. The glacier is mostly out of view to the left. (Fig. 14)

Glaciers erode, transport, and deposit huge quantities of rock debris. So do their coworkers, the melt-water streams. These turbulent streams flow from tunnels beneath every glacier, and their degree of muddiness roughly shows how active the glacier is. Glaciers that move very slowly, or that are stagnant, produce relatively clear melt water because they are not actively eroding bedrock. In contrast, streams of muddy water that look like chocolate milk often come from very active or “live” glaciers. These streams carry rock debris ranging from flour-size particles to large boulders. You can sense the carrying power of this swiftly moving water on warm summer days, when large cobbles and boulders are bumping along in a stream swollen by rapid glacier melting. Although you can rarely see these boulders, you can hear their constant low thunder. Their repeated impacts on other boulders in the streambed will vibrate the nearby streambanks beneath your feet. Hikers often find that a melt-water stream safe to cross in the early morning of a warm summer day is an impassable torrent at the same spot by early afternoon.

Four curved recessional moraines are spread over a distance of 2,000 feet on the valley floor of Fryingpan Creek. They were formed within the last few hundred years as Fryingpan Glacier lost volume and shrank back toward its present position above a line of cliffs. (Fig. 15)

A melt-water stream generally deposits coarse material wherever the slope of the valley floor decreases and the stream loses some of its velocity and carrying power. Only a flood may move the boulders farther downstream. However, the current carries fine material far downstream to deposit it in lakes, in Puget Sound, or in the Pacific Ocean. The Puyallup River, for example, is still very muddy where it enters Puget Sound at Tacoma, more than 40 miles from its source in the glaciers on Mount Rainier.

During the last glaciation, when glaciers were much larger than they are now, melt-water streams carrying great quantities of sand and gravel built valley floors up to levels tens of feet higher than they are today. Later, as the glaciers grew smaller, the rivers cut down into their valley floors and remnants of the sand and gravel deposits were left standing in benches or terraces along the sides of the valleys. You can see a good example of such a terrace in the Nisqually River valley beyond Ashford, which is 5 miles west of the park. You cross it on the highway that leads to the park. Cuts beneath the terrace reveal deposits of sand, cobbles, and boulders that look the same as those deposits being formed today by melt-water streams. The terrace west of Ashford was formed a little more than 15,000 years ago, when a glacier extended down the Nisqually River valley to the vicinity of Ashford.

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