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The Geology of Mount Rainier National Park




The summit of Mount Rainier is characterized by three distinct peaks; all above 14,000 feet in elevation. They define a triangular summit area so broad that from no point about the base can one see the actual top of the cone. As viewed from Longmire, or Paradise, the highest point of the mountain is judged to he Point Success; while, if the observer happens to be in the Mowich Lake region, the prominence of Liberty Gap seems highest. From the Yakima Park side, the third point or the crater rim, is readily seen and appears to out-top any of the other points. This latter case is due more to the perspective than to any true evaluation of their respective heights. The actual summit is a small mound of snow on the northern side of this crater rim, and, because it was once thought to be the highest point in the United States, it is known as Columbia Crest.

A closer examination of the summit area leads to the determination of three more or less distinct craters. The one just mentioned is clearly and unmistakably defined. The black rocks protruding through the snow and ice mark out an almost perfect circle approximately one-quarter of a mile in diameter. The rim is composed of sub-rounded boulders several feet in diameter, intermingled with smaller pyroclastics. There is an abrupt drop for a distance of 30 feet or more on the inner side of the crater where the floor is lined to an unknown depth with snow, forming a shallow saucer-shaped depression. Jets of steam, not containing any detectable traces of sulphur, issue from the loose rocks and also from under the ice on the floor, melting out irregularly-shaped caverns adjacent to the rim. These provide a welcome haven of refuge from the icy blasts so everlastingly present at the summit. The steam jets are thought to be a most important factor in keeping the rim free of snow.

Another crater, slightly larger in diameter and somewhat lower in elevation, encloses the smaller one, just described, in an eccentric fashion. The larger one, however, is not so easily distinguished as the greater part of the rim has either been destroyed by erosion or has been completely covered by snow. The point where the two craters come closest together, and almost touch, is marked by a rounded mound of snow—Columbia Crest. Matthes (21) logically explains the mound as being due to furiously-driven westward winds,

... whipping through the breach in the west flank of the mountain between Point Success and Liberty cap, eddying lightly as they shoot over the summit and there deposit their load of snow.

A third crater, much larger and enclosing the other two, is now greatly modified, retaining only such remnants as Liberty Cap and Point Success, attesting to its former size. Russell (27) has advanced a tenable and plausible hypothesis for the formation of this entire summit area. The writer advisedly used the word "hypothesis" in this case for, in his experience, such field work as can be accomplished on the summit of Mount Rainier leads to few definite conclusions. The only points of actual outcrop are along the rim of the smaller and portions of the medium-sized crater, and at Point Success; all else is covered by snow or ice. The most fruitful information is concealed in inaccessible faces, such as Willis Wall and the head of the Sunset Ampitheater. This necessitates making observations from afar; a most unsatisfactory method.

In a brief summary, Russell (27) states:

The profiles of the mountain and the character of its summit show that at the time of its greatest perfection and beauty it rose as a tapering cone with gently concave sides to a height about 2,000 feet greater than its present elevation. At a later date it was truncated, probably by an explosion, which removed the upper 2,000 feet and left a summit crater from 2 to 3 miles in diameter.

The writer hesitates to attribute the present configuration of the summit area to one huge explosion removing the upper 2,000 feet. The distribution of the pyroclastics and their alternation with lava flows on the upper reaches of the mountain suggest intermittent explosive activity, probably breaching first one side of the crater and then the other.

Some doubt might also be raised to the suggestion that the volcano ever attained a height of 2,000 feet above its present elevation. To reconstruct the sloping flanks until the height approached 16,000 feet it would be necessary to diminish the diameter of the crater to a few feet. It does not seem possible that such thick and viscous flows issued from so small an orifice.

The crater might better be regarded as having a rather large diameter and modified at various intervals by a number of violent explosions, as well as by glacial erosion.

FIG 26. A view looking south from the crater rim at the summit of the mountain. (The snow and ice-covered floor of the crater is seen in the foreground. Mount Adams is in the distance.)


The entire upper portion of the mountain is covered by nevé fields. As these have been adequately and vividly described by Matthes and Russell, the reader is referred to these papers. (21) (27)

However, the effects of glacial erosion have not been treated so fully and, in the pages to follow, some of the resulting land forms will be described briefly.

Cleavers. Between the elevations of 10,000 and 13,000 feet are numerous long walls of rock, arranged in a radial manner, with the summit as the common center. Both on the map, and locally, these are known as "cleavers" for they stand immovable, splitting the ice in its descent. Most famous of the cleavers is the blocky mass of Gibraltar. This wall, as seen from Paradise, is sufficiently large to give the entire upper portion of the mountain a bulky, broad-shouldered appearance. When viewed from Camp Muir, or any points along the Cowlitz or Ingraham Glaciers, the form is not so imposing as one is looking parallel to the long axis of the mass.

It is interesting to observe that vertical walls more than 1,000 feet in height should be carved on either side while the top has remained relatively unchanged. That the upper surface of Gibraltar is a dip slope is obvious to anyone making the summit trip. Along the "chutes," where the upper ice is first encountered, and at Camp Comfort at an elevation of 12,679 feet, the dip slope of the loosely consolidated pyroclastics can be recognized as coinciding with the upper surface of the cleaver. Stripping could easily be effected in such loose material, but, judging from the height of Gibraltar and its position in regard to the summit, it is doubtful if the mass ever attained an elevation much greater than at present. The upper surface of Gibraltar has undoubtedly been covered countless times by snow and also been attacked by the agents of subaerial erosion.

Other cleavers, but slightly less magnificent than Gibraltar, are Cathedral Rocks, Success, Puyallup, and Wapowery cleavers. In the process of cirque and valley widening the cleavers are fashioned principally by the undercutting of the ice. Working most vigorously on either side, the glaciers carve sheer and almost parallel walls. As the undercutting continues, the walls come closer and closer together until the thin cleavers we see today are all that survive. In the not too distant future even these bold remnants will narrow to such an extent that they will finally collapse and, perhaps, eventually be smothered by ice. (Note the small rock outcrop at an elevation of 9,432 feet on Emmons Glacier.)

Wedges. At an elevation of about 10,000 feet, the wedges come into prominence. The wedges probably represent modified forms of the cleavers but, nevertheless, closely rival them in beauty and in interest. As the broad neves of the upper slopes descend, they choose certain paths, probably determined by previously formed valleys, and, sinking deeper and deeper into the rocks, their courses become firmly established. Because of the confinement in narrow canyons and the ablation effects of lower elevations, the longer or primary glaciers diminish considerably in area as their termini are approached. The interglacial divides now assume the form of giant V's with their apices pointing toward the summit. Two excellent examples are provided in The Wedge (Steamboat Prow) and Little Tahoma. Russell (27) suggested the use of the word "Tahomas" as a generic name for these rock masses from the type locality of Little Tahoma. The writer is inclined to favor The Wedge as an equally desirable type and to use the name "wedge," both as a generic term and a descriptive one, mainly because it is self-explanatory. Undoubtedly The Wedge formerly headed much higher on the mountain's flank, perhaps extending upward in the form of a long attenuated cleaver. Continued abrasion by the Emmons and Winthrop glaciers has reduced it to successively lower levels. Even today, the dividing ice is still sharpening, and shortening, the remaining stub. Little Tahoma is a more spectacular wedge because of its greater elevation and larger size and must be considered an equally worthy representative of this type of land form.

FIG. 27. Steamboat Prow and The Wedge. (Looking southeast from above the Winthrop Glacier, elevation 10,000 feet. This photograph was loaned through the courtesy of the 116th Photo Section of the Washington National Guard.)

On the southwest side of the mountain, glaciation has not progressed to the same extent as it has on the northeast side. The Success, Wapowery, and Puyallup cleavers, Ptarmigan Ridge, and the like, remain as lines of rock from the base of Rainier up to approximately 12,000 feet. With continued erosion these will be reduced in elevation by the impinging masses of ice until they, too, will become wedges. But slight changes are needed to cause the long rib of rock between Mineral Mountain and Avalanche Camp to alter from a cleaver into a wedge. With continued erosion at the ice fall of the Nisqually Glacier adjacent to Gibraltar, the Cathedral Rocks and the Cowlitz cleaver will be shaped into a striking wedge. Many other examples could be cited, indicating this same process.

Interglaciers. An interesting associate of the wedges are the interglaciers. These are formed, for the most part below the 9,000-foot contour line and occur on the back slopes of the wedges. Conforming to the general pattern of the wedges, the highest reaches of the interglaciers are represented by points; the lower portions are more extensive as the ice spreads out in thin aprons. The type example is the Interglacier lying on the back of The Wedge. Similar ones are the Fryingpan and Whitman glaciers on the back slope of Little Tahoma and the unnamed ice mass to the west of the Winthrop Glacier. Perhaps the Van Trump, Pyramid and many other glaciers could also be considered as belonging to this same class.

The effect of the interglaciers is to cover and sink into the back slope of the wedges in such a way that only a scant rim of rocks are exposed on either side and at the prow. The result is a skeletal form of a "V," composed of rocks projecting through the snow.

It may at first seem anomalous that glaciers as prominent as those just mentioned should form at elevations as moderate as 6,000 to 9,000 feet. Surrounded as they are by a rock wall and perched on the back slopes of the wedges, the manner of emplacement clearly precludes any chance of the longer, primary glaciers adding to their volume. The only other feasible explanation is to account for them by precipitation in sufficient amount to withstand the wasting effects of the lower elevations. Matthes has pointed out that the precipitation on lofty mountain regions is heaviest at moderate altitudes, while higher up it decreases markedly.

In the Rainier region, the height of the storm clouds is, in a large measure, regulated by the height of the Cascade Range, for it is really this cooling mountain barrier that causes the moisture-laden winds from the Pacific to condense. As the storm clouds are seldom much elevated above the skyline of the range, the greatest precipitation occurs at a relatively moderate height. The zone between 8,000 and 10,000 feet is perhaps most favorable for the development of glaciers. Below an altitude of 8,000 feet, the ice rapidly wastes away in the summer heat; while above 10,000 feet, the snowfall is relatively scant. The result is manifest in the distribution and extent of ice on the cone.

Asymmetrical Topography as a Result of Selective Glaciation. The glacial erosion of the base of Mount Rainier is so intimately connected with the ice sculpture of the Cascades that any attempt to describe the two separately is inadvisable. Hence the following discussion may apply equally well to the lower flows of the mountain or the upper flows on the range on which it stands.

One of the most interesting phenomenon is the selective manner in which the ice has attacked the previous topography. Recalling the description of the wedges given above, it will be noted that they represent more advanced stages of dissection than do the cleavers from which they are derived. The best examples of wedges are on the north and east sides of the mountain; while the cleavers attain their finest development on the south and west sides. This evidence suggesting greater ice erosion on the north and east slopes is supported by other facts.

The most extensive mass of ice at the present time is on the northwest side of the mountain at the heads of the Emmons and Winthrop glaciers. In broadest dimension this ice field is approximately 3 miles wide. The second largest width is on the north side where a 2-mile wide nevé finally divides into the Carbon and Russell glaciers. It is readily seen that the largest glaciers, and, as a consequence, the most intense glaciation is operative on the northeast side of the mountain. If conditions of elevation, prevailing wind direction, and distribution of precipitation were not markedly different during the time of maximum glaciation, then we may assume the northern slope has been favored in the amount of ice sculpture since Pleistocene time. Testimony of long-continued erosion is offered by the steepest face on the entire mountain—the cirque head of Willis Wall. Exposed to the northward, this wall drops 4,500 feet in elevation in a horizontal distance of approximately 1/2 mile.

About the base of the mountain the distribution of glacial erosion confirms the evidence offered higher up on the cone. On the steep northern face of the Tatoosh Range, small glacierets still persist as Unicorn and Pinnacle glaciers. Even better examples are the Sarvent glaciers on the ridge between the Cowlitz Chimneys and Panhandle Gap. These are the largest of the glacierets and are confined to the northern face of the ridge. In Spray Park, a small, unnamed glacieret originates at an elevation of 6,500 feet on a northern slope. Also, near the Colonnades, another small ice patch forms below 6,700 feet. Nowhere in the Park has any ice been encountered originating on a southern slope at so low an elevation.

Proof of selective glaciation, not involving actual ice masses, is also available. At a number of localities perfect cirques have been carved on the northern side of ridges whose southern faces are unmodified by glacial action. The frequently-visited Sourdough mountains are scalloped on the north by many cirques, extending from Mount Fremont to Dege Peak. The smooth, unscarred southern slope, however, makes an ideal location for the settlement of Yakima Park. Two small, incipient basins occur on the southern side of the Sourdough Range, but these can be accounted for easily as they lie to the west and north, respectively, of the overshadowing Burroughs Mountain. On the Panhandle Gap-Cowlitz Chimneys Ridge, the Sarvent glaciers are still nestled in their self-made cirques, facing northward, while the other slope is an undissected plane dipping gently to the southeast. The Cowlitz Chimneys are carved into spires and aretes by small glacierets formerly occupying the western slope. Many more instances could be cited showing the same relationships on the opposite sides of the various ridges. Quite a number of the higher promontories have been glaciated on both sides, as, for example, the Tatoosh Range. Butter Creek drains what may be considered the back slope of the range in a southeasterly direction. This river has been glaciated for the greater part of its course. At first this appears to be an exception to the general rule of having the northern slopes glaciated, while the southern slopes are either unglaciated or but slightly modified by ice. A close examination of the Mount Rainier quadrangle topographic map shows Butter Creek to lie immediately east and slightly north of Dixon Mountain—a long ridge averaging 6,000 feet in elevation. This mountain parallels Butter Creek, towers 2,000 feet above it and the horizontal distance between creek bottom and the crest of Dixon Mountain is but 1/2 mile. Hence the valley, which seemingly presents an exception to the general rule, is little different from its associates in the position of the steeper slopes and greater glaciation. The above localities suffice to show that an asymmetry of crest line does exist within the Park, both in regard to the surface of the Cascades, and to the volcano. Now, let us consider the reason.

A structural control of the Park's topography would easily account for the steep northern faces of the ridges. In the preceding pages, the Keechelus series was mentioned as containing an extremely massive lower portion, practically devoid of structure, and an upper series of lava flows, gently warped. The structure of the upper Keechelus undoubtedly has been very influential in shaping the present topography. The asymmetry, however, is developed whether aided (tilted to the south) or hindered (tilted to the north) by the attitude of the series. Unequal declivities also occur on perfectly horizontal structure and on massive granodiorite, both of which should be neither an aid nor a hindrance to the formation of asymmetrical divides. Yet the higher points in this region are decidedly asymmetrical in cross-section. The Sourdough Mountain may or may not attribute its shape to the underlying structure. An effort was made to determine if Yakima Park was situated on the upper Keechelus lavas or whether it owes its present configuration to selective erosional agencies. Proof was lacking to show conclusively either view but both factors seem to have had a hand in shaping the Sourdough Mountains. On Burroughs Mountain, where the Rainier lavas dip very gently to the northeast, the cirque at Berkeley Park on the northern side is in no way influenced by the structure; the southern slope, although over-deepened by the Emmons and another glacier once occupying the Interfork of the White River, is not modified by cirques. Other localities where structure has been a negligible factor in accounting for the steep northern (or western) cirque-marked faces are: Goat Island Mountain, Tamanos Mountain, Cowlitz Divide, Mount Ararat, Mount Wow, and others. It seems certain that a structural control will not account for all these abrupt northern faces. Perhaps a more abundant plant growth has been influential in protecting the snow-free slopes from erosion. The present distribution of plant life gives little light on this subject. The relative abundance of shrubs or grass on either side of the ridges is not striking enough to suggest this as a plausible reason. It is true that plants and soil are lacking from many of the northern slopes, but this might well be a result of their excessive steepness rather than a cause.

FIG. 28. Yakima Park from the Sourdough Mountains. (Looking southeast. Note the smooth and undissected surface of Yakima Park as compared to the rugged, glaciated, northward-facing slopes. Sarvent glaciers in distance on the right. Tamanos Peak in the center of the picture in the middle distance.)

It seems reasonable to assume that the steep northern or western faces we find today are caused by the glaciers which now occupy them or by glaciers now vanished but which have left abundant proof of their former presence. This explanation is proffered rather than one whereby the glaciers and snowfields are the result of landforms brought about by some other cause. Snow and ice are thought to have persisted longer, and been more concentrated on the northern sides because of two reasons: insolation, and, the accumulation of wind-driven snow on the leeward side of the ridges. The former reason is regarded as being the more important, especially at the lower elevations.

The prevailing wind direction in the vicinity of the Park is from the west or southwest. Driven by these winds, the snow accumulates on the leeward side of obstructions, whether they be large or small, and piles up in sizeable drifts. At moderate elevations (5,000-6,000 feet) and below the timber line, the transporting power of the wind is not so effective. The trees afford a certain measure of protection. The temperature is usually sufficiently high to assure a heavy, moist snow difficult of transport. Above timber line and, as one goes higher and higher, the conditions for drifted snow are more ideal. The wind velocity is many times stronger than at timber line and the temperature is low enough to maintain a dry, powdery snow readily capable of transport.

Insolation is somewhat analogous to the wind-driven snow in that both increase in effectiveness at the higher altitudes. It is a well known fact that as the elevation increases the air becomes more rarified and offers less resistance to the sun's radiant energy. As one goes higher the result is a wider divergence between sun and shade temperatures and a corresponding increase in the degree of insolation. As the snow and ice fields on the northeastern slopes are sheltered from the sun they endure much longer and are more effective eroding agents than their neighbors on the opposite slopes. As the small glaciers commence to deepen their beds and sink more and more into the protecting shadows of the ridges above, they prolong their own lives. The result is an additive process whereby a small glacier, once gaining the advantage of a slightly more sheltered position, will aid itself in accumulating more of the wind-driven snow and preserving it much longer from the sun's rays than those less favorably situated. The greater accumulation and protection makes the glaciers just that much more able to entrench themselves still farther and thus become even more protected.

The steep northern faces and the relatively moderate southern slopes produce an asymmetry of crest lines which is by no means an unusual feature, although it has never been described and explained in any of the literature on the Cascades. Gilbert (13) first mentioned the asymmetry of crests in the Sierras of California. Later, Bowman (2) described the asymmetry of the volcanic peaks in southern Peru. In this instance, the steeper faces were on the southern side as the area is south of the equator and the insolation effects would be reversed. Recently Tuck (37) published a paper on the asymmetrical topography in south-central Alaska. He points to insolation as being the dominant factor in causing the differential erosion of the present topography. Tuck also offers proof that the interstream divides have been shifted southward since pre-glacial times.

In summary, the asymmetrical topography of the Park is attributed to the more vigorous glacial erosion, as represented by the northward facing slopes, as compared to non-glacial or less glaciated areas, as represented by the southern slopes. The factors causing this selective distribution are: firstly, insolation, and, secondly, the greater accumulation of wind-driven snow on the leeward side of the ridges.

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Last Updated: 28-Mar-2006