Library of Congress Catalog-Card No. 79-601704

contents

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The changing landscape of 12-60 million years ago 3

Thumbnail biography of Mount Rainier 11

Results of recent eruptions 12

Why glaciers? 23

Work habits of glaciers 25

Yesterday’s glaciers 29

Landslides and mudflows—past, present, and future 35

The volcano’s future? 42

Further reading in geology 43

Frontispiece. Eunice Lake, northwest of Mount Rainier.

Figure Page

1. Outcrop of sandstone and shale in the Puget Group 6

2. Outcrop of welded tuff in the Stevens Ridge Formation 8

3. Granodiorite looks like granite 9

4. Geological cross section of Mount Rainier 10

5. An old lava flow which forms Rampart Ridge 13

6. Columns of andesite at the end of an old lava flow 13

7. Layers of pumice on the floor of a cirque 14

8. Generalized distribution of some pumice layers 16

9. Breadcrust bomb enclosed in a mudflow deposit 19

10. Pumice layer C, which consists of light-brown fragments 20

11. The recent lava cone lies in a depression 21

12. Two ice streams meet to form Cowlitz Glacier 23

13. Glacier-smoothed and grooved rock 26

14. A lake lies behind an end moraine of Flett Glacier 27

15. Recessional moraines on the valley floor of Fryingpan Creek 28

16. Extent of glaciers between 15,000 and 25,000 years ago 30

17. Lateral moraine at Ricksecker Point 31

18. Rock-glacier deposit at The Palisades 33

19. Hummocky end moraine in front of Emmons Glacier 34

20. Avalanche deposits in the White River valley 37

21. The northeast flank of Mount Rainier 39

Table

1. Characteristics, sources, and ages of pumice layers, Mount Rainier National Park 17

2. Summary of important geologic events in the history of Mount Rainier National Park 41

The Geologic Story of
Mount Rainier
By Dwight R. Crandell

WASHINGTON

Seattle

Tacoma

CASCADE RANGE

Mount Rainier

Mount Adams

Mount St Helens

OREGON

Portland

Mount Hood

Crater Lake

Ice-clad Mount Rainier, towering over the landscape of western Washington, ranks with Fuji-yama in Japan, Popocatepetl in Mexico, and Vesuvius in Italy among the great volcanoes of the world. At Mount Rainier, as at other inactive volcanoes, the ever-present possibility of renewed eruptions gives viewers a sense of anticipation, excitement, and apprehension not equaled by most other mountains. Even so, many of us cannot imagine the cataclysmic scale of the eruptions that were responsible for building the giant cone which now stands in silence. We accept the volcano as if it had always been there, and we appreciate only the beauty of its stark expanses of rock and ice, its flower-strewn alpine meadows, and its bordering evergreen forests.

Mount Rainier owes its scenic beauty to many features. The broad cone spreads out on top of a major mountain range—the Cascades. The volcano rises about 7,000 feet above its 7,000-foot foundation, and stands in solitary splendor—the highest peak in the entire Cascade Range. Its rocky ice-mantled slopes above timberline contrast with the dense green forests and give Mount Rainier the appearance of an arctic island in a temperate sea, an island so large that you can see its full size and shape only from the air. The mountain is highly photogenic because of the contrasts it offers among bare rock, snowfields, blue sky, and the incomparable flower fields that color its lower slopes. Shadows cast by the multitude of cliffs, ridges, canyons, and pinnacles change constantly from sunrise to sunset, endlessly varying the texture and mood of the mountain. The face of the mountain also varies from day to day as its broad snowfields melt during the summer. The melting of these frozen reservoirs makes Mount Rainier a natural resource in a practical as well as in an esthetic sense, for it ensures steady flows of water for hydroelectric power in the region, regardless of season.

Seen from the Puget Sound country to the west, Mount Rainier has an unreal quality—its white summit, nearly 3 miles high, seems to float among the clouds. We share with the populace of the entire lowland a thrill as we watch skyward the evening’s setting sun redden the volcano’s western snowfields. When you approach the mountain in its lovely setting, you may find something that appeals especially to you—the scenery, the wildlife, the glaciers, or the wildflowers. Or you may feel challenged to climb to the summit. Mount Rainier and its neighboring mountains have a special allure for a geologist because he visualizes the events—some ordinary, some truly spectacular—that made the present landscape. Such is the fascination of geology. A geologist becomes trained to see “in his mind’s eye” geologic events of thousands or even millions of years ago. And, most remarkable, he can “see” these events by studying rocks in a cliff or roadcut, or perhaps by examining earthy material that looks like common soil beneath pastureland many miles away from the volcano.

Our key to understanding the geology of Mount Rainier is that each geologic event can be reconstructed—or imagined—from the rocks formed at the time of the event. With this principle as our guide, we will review the geologic ancestry of this majestic volcano and learn what is behind its scenery.

The Changing Landscape of 12-60 Million Years Ago

The rocks of the Cascade Range provide a record of earth history that started nearly 60 million years ago. Even then, as today, waves pounded on beaches and rivers ran to the sea, molding and distributing material that formed some of the rocks we now see in the park.

You may find it difficult to imagine the different landscape of that far distant time. There was no Mount Rainier nor Cascade mountain range. In fact, there was very little dry land in the area we call western Washington. Instead, this was a broad lowland of swamps, deltas, and inlets that bordered the Pacific Ocean. Rivers draining into this lowland from the east spread sand and clay on the lush swamp growth. Other plants grew on the deposits, and they were covered, in turn, by more sand and clay. In this way, thousands of feet of sand and clay and peat accumulated and were compacted into sandstone, shale, and coal. We can see some of the rocks formed at that time in cuts along the Mowich Lake Road west of the park (fig. 1). Seams of coal were mined at Carbonado and Wilkeson, 10 miles northwest of the park, during the late 19th and the early 20th centuries.

These beds of sandstone, shale, and coal make up a sequence of rocks called the Puget Group, which is 10,000 feet thick. Wave-ripple marks and remains of plants show that the rocks were formed in shallow water fairly close to sea level. How could the rocks have piled up to this great thickness? The coastal plain and adjacent basin must have been slowly sinking, and the influx of sand and clay must have just barely kept pace with the downward movements.

A little less than 40 million years ago, the western Washington landscape changed dramatically. Geologists R. S. Fiske, C. A. Hopson, and A. C. Waters have discovered that volcanoes then rose on the former coastal plain at the site of Mount Rainier National Park and became islands as the area sank beneath the sea. When molten rock was erupted underwater from the submerged flanks of these volcanoes, steam explosions shattered the lava into countless fragments. The resulting debris, mixed with water, flowed as mud across great areas of the submerged basin floor.

You can see rocks formed from these layers of volcanic mud and sand in cuts along the highway on the east side of Backbone Ridge and between Cayuse Pass and Tipsoo Lake. Look there for alternating beds of grayish-green sandstone and breccia, a concretelike rock in which the pebbles have sharp corners. These rocks are known as the Ohanapecosh Formation. Like the Puget Group, the Ohanapecosh Formation is at least 10,000 feet thick. Yet, nearly all of it accumulated in shallow water as western Washington continued to sink slowly during the volcanic eruptions.

The long-continued sinking finally ended after the Ohanapecosh volcanic activity ceased. Western Washington was then lifted several thousand feet above sea level, and the Puget and Ohanapecosh rocks were slowly compressed into a series of broad shallow folds. Before eruptions began again, rivers cut valleys hundreds of feet deep, and weathering of the rocks produced thick red clayey soils similar to those that are forming in some areas of high rainfall and high temperature today. Look for the red rocks formed from these old soils in roadcuts as you drive along the Stevens Canyon road about 2 miles southeast of Box Canyon.

The next volcanic eruptions, which may have begun between 25 and 30 million years ago, differed from those of Ohanapecosh time. These volcanoes, somewhere beyond the boundaries of the park, erupted great flows of hot pumice that, being highly mobile, rushed down the flanks of the volcanoes and spread over many square miles of the adjacent regions. The pumice flows were “lubricated” by hot volcanic gas emitted from inside each pumice particle, which buffered it from other particles. Some hot pumice flows were 350 feet deep. The heat still remaining in the pumice after it stopped flowing partly melted the particles to form a hard rock known as welded tuff. Repeated pumice flows buried the hilly landscape and eventually formed a vast volcanic plain. The rocks, which are mostly welded tuffs, are now the Stevens Ridge Formation, which you can see along the highway in Stevens Canyon 1-2 miles west of Box Canyon. You can recognize the welded tuff by its light-gray to white color and its many flattened and sharp-edged inclusions of darker gray pumice (fig. 2).

Another period of volcanism followed, of still a different kind, when lava flowed outward from broad low volcanoes. The flows were of two kinds: basalt, the kind now erupted by Hawaiian volcanoes, and andesite, the type erupted by Mount Rainier. Individual flows 50-500 feet thick were stacked on top of one another to a total depth of fully 2,500 feet. We know these rocks as the Fifes Peak Formation. They form many of the cliffs and peaks in the northwestern part of the park. You can examine them in cuts along the Mowich Lake Road between Mountain Meadows and Mowich Lake. The time of the eruption of the Fifes Peak lavas may have been between 20 and 30 million years ago.

When the Fifes Peak volcanoes finally became extinct, this part of western Washington changed again. The rocks once more were uplifted and compressed into broad folds parallel to those formed at the end of Ohanapecosh time. The rocks buckled and, in places, broke and shifted thousands of feet along great fractures, or faults.

About 12 million years ago one or more masses of molten rock, many miles across, pushed upward through the Puget Group and younger rocks. When this molten rock cooled and hardened, it formed granodiorite, a close relative of granite. Although most of the molten rock solidified underground, some of it reached the land surface and formed volcanoes at a few places within the area of Mount Rainier National Park.

Granodiorite is probably the most attractive rock in the park. It is mostly white, but it contains large dark mineral grains that give it a “salt-and-pepper” appearance (fig. 3). The large size of the grains is a result of the molten rock cooling slowly at a considerable depth below the land surface—the individual minerals had a long time to grow before the “melt” solidified into rock. In contrast, the rocks formed from lavas that flowed onto the ground surface are generally fine grained because the lavas cooled too quickly for the mineral grains to grow appreciably.

Granodiorite underlies the White River valley, the Carbon River valley, and parts of the upper Nisqually River valley and the Tatoosh Range. You can see it in roadcuts between Longmire and Christine Falls and at several places along the road between White River Ranger Station and White River campground.

(left) High-resolution Diagram (right)

After the granodiorite solidified, the foundation of Mount Rainier was complete except for one other landscape change that preceded the birth of the volcano. Not long after the granodiorite was formed, the Cascade mountain range began to rise—not rapidly, but little by little over many thousands of years. As the land rose, rivers cut valleys into the growing mountains so that by the time the new volcano began to erupt, the Cascades had already been carved into a rugged range of high ridges and peaks separated by deep valleys. Deep erosion thus laid bare the rock layers in which we today read the geologic history of the park (fig. 4).

Thumbnail Biography of Mount Rainier

The life span of a volcano can be compared to that of an individual—after his birth and a brief youth, he matures and grows old. The birth date of Mount Rainier is not known for sure, but it must have been at least several hundred thousand years ago. We cannot tell much about the volcano’s complex youth because most of its earliest deposits are now buried under later ones. At an early age, well before the cone grew to its present size, thick lava, like hot tar, flowed repeatedly 5-15 miles down the deep canyons of the surrounding mountains. Because these lava flows resisted later erosion by rivers and glaciers, most of them now form ridgetops, as at Rampart Ridge, Burroughs Mountain, Grand Park, and Klapatche Ridge (figs. 5 and 6). Violent explosions occasionally threw pumice onto the slopes of the growing volcano and the surrounding mountains. As the volcano matured, the long thick flows were succeeded by thinner and shorter ones which, piled on top of one another, built the giant cone that now dominates the region. Even though Mount Rainier has grown old now, it has revived briefly at many times during the last 10,000 years or so and may erupt again in the future.

The events of the last 10,000 years, because they are so recent, in terms of geologic time, are better known than those of any earlier time, and we can examine this part of the volcano’s history in some detail. We will study three principal subjects: eruptions—because they have had widespread effects; glaciers—because they are such conspicuous features on the mountain; and landslides—because they have drastically changed the volcano’s shape.

Results of Recent Eruptions

While hiking, you soon become aware that there is a large amount of pumice along the trails in Mount Rainier National Park. Pumice is a lightweight volcanic rock so full of air spaces that it will float on water. The air spaces, or bubbles, originated when fragments of gas-rich lava were explosively thrown into the air above the volcano, and the molten rock hardened before the gas could escape. If you examine pumice deposits in a trail cut, in a streambank, or in the roots of blown-over trees, you may also note that there is more than one layer (fig. 7). If you circle the volcano on the Wonderland Trail, you may notice that the greatest number of pumice layers are on the east side of the park, but the thickest single layer is on the west side. The explanation lies partly in the source of the pumice deposits, because some pumice was erupted not by Mount Rainier but by other volcanoes in the Cascade Range of Washington and Oregon and brought to the park by strong southerly or southwesterly winds. The layers of pumice thrown out by Mount Rainier within the last 10,000 years lie mostly on the east side of the volcano. Strong winds evidently swept eruption clouds to the east during the outbursts and prevented the pumice from falling west of the volcano. This pattern of distribution, coupled with the coarsening and thickening of the pumice toward the volcano, reveals that the layers were erupted by Mount Rainier.

D. R. Mullineaux of the U.S. Geological Survey has studied in detail the pumice deposits of Mount Rainier National Park. One of his first and most important discoveries was that even though some pumice layers are spread widely over the park, they were erupted from other volcanoes. Strangely enough, one layer is thicker and more widespread than any recent pumice erupted by Mount Rainier. We can clearly see that these foreign pumice layers did not come from Mount Rainier, for they thicken and coarsen southward, away from the park. The oldest was erupted by Mount Mazama volcano at the site of Crater Lake, Oregon, about 6,600 years ago; this pumice forms a yellowish-orange layer about 2 inches thick nearly everywhere in the park. The pumice has a texture like that of sandy flour, and it feels grainy when rubbed between the fingers. It is so fine grained because of the great distance to its source, 250 miles due south of Mount Rainier. Near Crater Lake this same pumice consists of large chunks and is many feet thick.

Two other foreign pumice deposits in the park were erupted by Mount St. Helens, a symmetrical young volcanic cone about 50 miles southwest of Mount Rainier. The older of the two is between 3,250 and 4,000 years old; it forms a blanket of yellow sand-sized pumice that is as much as 20 inches thick in the western part of the park. The younger pumice layer is most conspicuous at the ground surface in the eastern part of the park, where it is as much as 4 inches thick and resembles a fine white sand. It is about 450 years old.

An inconspicuous bed of pumice records the first eruption of Mount Rainier that occurred after Ice Age glaciers melted back to the slopes of the volcano. It can be found on the east side of the mountain from Grand Park south to Ohanapecosh campground (fig. 8). In roadcuts near the east end of Yakima Park (Sunrise) the pumice forms a rusty-brown bed about 4 inches thick which contains fragments as much as 2 inches across. Wood from a thin layer of peat just above the pumice was dated by its content of radioactive carbon as about 8,750 years old; thus, the pumice is even older. We call this pumice layer R for convenience; other letter symbols have been assigned to the younger layers (table 1).

Generalized distribution of some pumice layers within Mount Rainier National Park. The pumice of layers W and Y was erupted by Mount St. Helens; all the other pumice originated at Mount Rainier. Letters represent the following localities: C, Cougar Rock campground; I, Ipsut Creek campground; L, Longmire; M, Mowich Lake; O, Ohanapecosh campground; P, Paradise Park; S, summit crater; T, Tipsoo Lake; W, White River campground; and Y, Yakima Park. Based on studies by D. R. Mullineaux. (Fig. 8)

TABLE 1.—Characteristics, sources, and ages of pumice layers,
Mount Rainier National Park
[Based on studies by D. R. Mullineaux]

The next two eruptions of Mount Rainier occurred between 5,800 and 6,600 years ago. Again, pumice spread over the area east of the volcano. The older pumice, which we call layer L, covers a band only a few miles wide that extends to the southeast from the volcano (fig. 8). The younger pumice, layer D, covers an area at least 10 miles wide directly east of the volcano. The distribution of both deposits shows that there were strong directional winds during the eruptions. The long, narrow pattern of layer L probably was caused by strong northwesterly winds during a short-lived eruption. The pattern of layer D was caused by winds from the west.

Some time during these eruptions, hot volcanic bombs and rock fragments were thrown out of Mount Rainier’s crater and fell onto surrounding areas of snow and ice. Wholesale melting resulted, and floods descended the east flank of the volcano carrying millions of tons of ash, newly erupted rock debris, and breadcrust bombs. Breadcrust bombs seem to be solid rock, but if you would break one open, you would find that the inside is hollow or is filled with a spongy mass of black glass. Their outer surfaces are cracked like the crust of a loaf of hard-crusted bread (fig. 9), so we call them breadcrust bombs. They originated as blobs of soft, red-hot lava which were thrown out of the volcano’s crater. As the masses arched through the air, they quickly chilled on the outside, and a hardened skin formed around the still hot and plastic core. As their outsides cooled, gas pressure in their hot interiors caused the bombs to expand slightly and their solidified outer skin to crack. When they struck the ground, many of the bombs became flattened on one side, but they were still plastic and sticky enough to remain whole.

Bombs can be found in two deposits that form the south bank of the White River about half a mile downstream from the White River campground. The deposits are mudflows caused by the mixing of hot rock debris with the water from melted snow and ice. As the mudflows moved down the valley floor they must have resembled flowing masses of wet concrete.

Mount Rainier erupted several times between about 2,500 and 2,000 years ago. During one of the first eruptions, a mass of hot ash, rock fragments, and breadcrust bombs avalanched down the side of the volcano and buried the floor of the South Puyallup River valley. Although this hot mass flowed like a wet mudflow, the temperature of the rock debris was above 600°F. Thus, if any water had been present, it would have been in the form of steam. You can see the resulting deposit in cuts along the West Side Road on both sides of the bridge across the South Puyallup River. Innumerable bombs have rolled from the cuts into the ditches beside the road. A charcoal log found in the deposit had a radiocarbon age of about 2,500 years.

Large amounts of pumice were thrown out of the volcano at the same time as the bombs or soon after. The pumice covers most of the eastern half of the park, and fragments are scattered as far southwest as Pyramid Peak and as far northwest as Spray Park. This pumice, called layer C, is especially thick and coarse at Yakima Park and Burroughs Mountain, where it lies at the ground surface (fig. 10). Here the light-brown layer is 5-6 inches thick and consists of irregularly shaped pumice fragments as much as several inches across. Mingled with the pumice fragments are fist-sized chunks of light-gray rock that probably were simultaneously thrown out of the volcano by violent explosions. Some of these angular rocks were hurled as far as Shriner Peak, 11 miles east of Mount Rainier’s summit.

The eruptive period was climaxed by the building of the volcano’s present summit cone, which is at least 1,000 feet high and 1 mile across at its base. Although dwarfed by the tremendous bulk of Mount Rainier, it is a little larger than the cone of the well-known Mexican volcano Parícutin that appeared in 1943 and erupted until 1952. Mount Rainier’s present summit cone was built within a broad depression at the top of the main volcano that had been formed nearly 4,000 years earlier (fig. 11; see p. 40). The cone consists of a series of thin black lava flows, and its top is indented by two overlapping craters. Rocks around the craters are still warm in places, and steam vents melt caves in the summit icecap. The first climbers who reached the top of the mountain, in 1870, spent the night in one of these caves, as have many benighted climbers since.

Even though the lava flows that formed the summit cone were relatively short, their eruption greatly affected some valleys at the base of the volcano. The hot lava melted snow and ice at the volcano’s summit, causing floods that rushed down the east and south sides. When the floods reached the valley floors, they picked up great quantities of loose rock debris and carried it downstream, sometimes forming mudflows. The resulting flood and mudflow deposits raised the floors of the White and Nisqually River valleys as much as 80 feet higher than they are today. These valley floors, as well as several others, then became broad wastes of bare sand and gravel that extended beyond the park boundaries. Later, the rivers cut down to their present levels, but they left remnants of the flood and mudflow deposits as terraces or benches along the sides of the valleys. You can camp on such a terrace in the Nisqually River valley at the Cougar Rock campground. The White River campground occupies a similar terrace in the White River valley.

When did Mount Rainier erupt last? The most recent pumice eruption was just a little over a century ago. However, between 1820 and 1894, observers reported at least 14 eruptions. Some of these may have been just large dust clouds, caused by rockfalls, that were mistaken for clouds of newly erupted ash. Other clouds may have been from genuine eruptions that left no recognizable deposits. D. R. Mullineaux has found that at least one eruption of that era did spread pumice over an area east of the volcano between Burroughs Mountain and Indian Bar to a distance of at least 6 miles from the crater. Pieces of the pumice, layer X, are light brownish gray and as large as 2 inches across. We find only scattered fragments of the pumice, and nowhere are they in a continuous layer. Where the X pumice is directly on top of layer C, we cannot tell them apart. The best areas for us to study the younger pumice, therefore, are glacial moraines formed within the last 150 years, because no pumice other than layer X is present on the moraines. Fortunately, R. S. Sigafoos and E. L. Hendricks of the U.S. Geological Survey have determined the ages of the moraines by counting the growth rings of trees on them. Their studies show that the pumice was erupted between about 1820 and 1854.

Captain John Frémont, an early explorer of the Oregon Territory, recorded that Mount Rainier was erupting in November 1843, but his journals give no details. Others have reported eruptions in 1820, 1846, 1854, and 1858. Pumice layer X probably was erupted during one or more of these times, but we do not know exactly when.

And will Mount Rainier erupt again? We think that it will, but we now have no sure way of predicting the time, the kind, or the scale of future eruptions.

Why Glaciers?

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.

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.

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.)

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.