Nature Notes

Vol. XII September, 1934 No. 9

Glacial Signature on the Sourdough Range

Glaciers leave symbols upon the mountains which they have carved that we may learn to interpret. Upon the flanks of Mt. Rainier we may, today, see these symbols being inscribed. Upon the slopes of the lesser peaks that are clustered about the base of the old volcano one may read the record that has already been written.

Such is the case on the north slope of the Sourdough Range which rises above the beautiful plateau which is Yakima Park. From the notches and low peaks of this ragged ridge, only a few feet above the camp ground and Sunrise Lodge, the view extends northward, unbroken, to the distant peaks on the horizon such as Mt. Baker, Shuksan, Glacier Peak, the Selkirks of British Columbia and the Olympics that guard the Strait of Juan de Fuca on the south. Broad and fascinating as this scene is, however, the record to be read in the rugged wall of this ridge at ones feet is equally interesting. The symbols here are boldly inscribed.

Willis Wall

When a glacier heads against a cliff it undermines the wall. The rock at the base is plucked away. The ice may freeze to fragments of the rock, and, beginning its downward journey, carry away these fragments. Alternate freezing and thawing of the ice in crevasses will break the rock and permit more material to be removed. Eventually the cliff will be sufficiently undermined that portions will collapse and this debris will again be carried out. This action will be greater at the head of the ice mass and the resulting cliff will have a circular or amphitheater-like form. The ice will dig deeper at the base by slowly plucking at the rocks over which it rides - thus increasing the relative height of the cliff. Willis Wall, on the north flank of "The Mountain", is a cliff of this type in the process of formation while the north wall of the Sourdough Range is a series of such amphitheaters or cirques which have, long since, been freed of glacial ice. Here nature has written finis to a chapter on glaciation.

If the process of plucking and undermining continues upon one side of a ridge the cliffs will eventually be worn back beyond the crest of the ridge, more deeply at the center of each cirque, less so at the sides. Briefly then, that explains the activity of the former glaciers of the Sourdough. They have cut back beyond the crest, and the gaps into which the trail dips, and from which one gets the first views northward are the heads of the various cirques, and the many spire like peaks upon the ragged skyline of which Dege Peak is the highest, from the intervening walls. From each of these peaks ridges extend northward, separating the basins below.

portion of Sourdough Range
A Portion of the Sourdough Range

At the base of the cirques where the great mass of snow and ice accumulates, and from which it begins its downward journey, there is a typically gouged out basin or depression. The basins in the cirques upon "The Mountain" are filled with ice but upon the north face of the Sourdough, directly under the gaps which represent the heads os the cirques, these depressions now invariably contain a small lake or a series of small lakes or ponds; or at least poor drainage is indicated by an area of more luxuriant vegetation.

Leaving the basin the glacier begins to carve its channel, more deeply in its center, with a shelf-like formation at each side, at the level of, or slightly lower than th basin. When several tongues of ice join to form the larger field of ice, the lesser ones will cut more shallow channels. When joining larger channels at an appreciable angle a sudden drop will appear at the junction of the two. Where the many tongues of ice joined to form the glacier which flowed northward from the Sourdough toward the present canyon of the White River, these steps show clearly and the small streams which are flowing from the snow fields and ponds which linger in the basins, drop in a series of small cascades and falls.

Smaller glaciers joining the main ice stream will cut less deeply and when the glaciers have melted away these lateral canyons will be found as hanging valleys, fully formed to their junction, but with their terminals suspended high above the floor of the main valley. Streams flowing from these hanging valleys join the main stream in beautiful waterfalls.

The canyon extending north from the Sourdough lies at right angles to the ridge; from the trail one looks directly down its ever increasing depth. Its U-shape is obvious and several hanging valleys may be observed, two large ones joining the main canyon a few miles to the north, one from each side. The huge canyon once carved by the stream of ice of which the Emmons Glacier is now a remnant, describes a wide curve around the end of the Sourdough. In this canyon the White River, rising at the snout of the Emmons Glacier, now flows. From the Sourdough the course of the canyon can be traced from the glacier which lies to the southwest, eastward toward the Sound, its depth lost in a maze of lower ridges and peaks.

Because of its accessibility and the boldness in which the story has been inscribed in the rocks, the Sourdough Range is among the most easily read and most clearly illustrated chapters on the glaciation of this region.

Earl U. Homuth

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