Geological Survey 18th Annual Report (Part II) Glaciers of Mount Rainier Rocks of Mount Rainier

GLACIERS OF MOUNT RAINIER.

By ISRAEL C. RUSSELL.

The State of Washington presents marked geographic and climatic diversities. The Cascade Mountains cross the State in a general north south direction and divide it into two portions which are as different from each other in nearly all their features as two adjacent regions can well be.

The Cascade Mountains begin at the south in northern California and extend northward across Oregon and Washington and into British Columbia. In northern Washington the range is composed of granitic rocks, and its scenic features are here much more diverse than in its southern extension, where only igneous rocks, mostly andesite and basalt, have as yet been reported to occur. Many of the summit peaks of the Cascade Range in Washington are from 6,000 to 8,000 feet or more in altitude. At least five passes are known which are considered practicable for railroads. These range in elevation from 3,100 to 3,500 feet above the sea. The Columbia River, flowing front east to west, passes through the mountains, forming in part the boundary between Washington and Oregon, and dissects the range nearly to sea level. The elevation at Cascade Locks is but 106 feet above the ocean.

East of the foothills of the Cascades lies the Great Plain of the Columbia, as it is termed, which merges on the south with a vast plateau, forming southeastern Washington and extending far into Oregon. The elevation in this region ranges from about 300 feet along the Columbia to 2,500 feet near the Washington-Idaho boundary. Central and southeastern Washington, embracing a region about 20,000 square miles in area, was once a level plain of basalt formed by many successive flows of molten rock. In the neighborhood of the Cascades the once level basaltic sheets have been broken by numerous lines of fracture and the intervening blocks have been variously tilted, but in the southeastern portion of the State the lava sheets are still horizontal and form a nearly level plateau, in which there are many deeply eroded stream channels, some of which, termed coulees, are no longer lines of drainage.

The vast lava flows which covered central and southeastern Washington, and a still greater region to the southward, met the mountains of metamorphic rock of Idaho and Washington in an irregular line coinciding in general with the Washington-Idaho boundary from the Oregon line northward to Spokane River; thence westward the border of the lava is followed by the Columbia in making its westerly detour, known as the Big Bend. To the north of the Big Bend of the Columbia are rugged mountains of granite rocks which merge with the northern Cascades. When the geology and geography of Washington are more thoroughly studied it will probably be found that the granitic portion of the northern Cascades belongs in reality to the great series of highlands and mountains north of the Big Bend, which is designated in general as the Okanogan Mountains.

The Cascade Mountains are precipitous on the west and descend abruptly to a region of mild relief in which the extremely irregular basin occupied by Puget Sound is sunk. This low country, deeply covered with glacial drift, extends westward to the Olympic Mountains and to the Pacific.

As shown in this brief outline, the main geographic features of Washington are a great mountain range trending approximately N. 15° E., a roughened lava plateau on the east, and a dissected plain of glacial drift on the west.

Associated with the Cascade range in Washington, but of later date, and distinct from it both geographically and geologically, are four especially prominent volcanic mountains. About 15 miles east of the crest line of the Cascades in south-central Washington stands Mount Adams, 9,570 feet high. This is a volcanic cone much defaced by disintegration and erosion, but steam escaping from its summit reveals the fact that the rocks within the snow- and ice-covered cone are still hot. Fifty miles west of the Cascades and near the Canadian boundary, rises Mount Baker, 10,877 feet high, also a volcanic mountain, with a well-defined crater near its summit. Similar to Mount Baker in geographical position, but in the southern part of the Puget Sound country, stands Mount St. Helens, 9,750 feet high. Mount Baker and Mount St. Helens have been in eruption in modern times, and it is believed, although perhaps not proved, that eruptions from them have occurred within the last fifty years. The fourth great volcano referred to, and the grandest of all, is Mount Rainier, to be described later, on which the glaciers forming the subject of this paper are situated.

CLIMATE.

Of special interest in the study of the glaciers of Mount Rainier is the character of the climate of Washington. The plateau country to the east of the Cascades is arid and partakes of the desert-like nature of the Great Basin region, of which in many respects it is a northern extension. The rainfall is small. Over an area of 20,000 square miles the mean annual precipitation varies from 5 to 20 inches. The summers are long and hot; the winters mild and rainy. Snow lies on the higher portions of the plateau for several weeks each year, but seldom remains more than a few days in the valleys. The plateau is treeless. In the lower valleys sagebrush and other desert shrubs impart their neutral tints to the landscape. The higher portions of the plateau in a state of nature were covered with luxuriant bunch grasses. Much of the former prairie is now under cultivation. The deep, rich soil formed by the decay of basalt absorbs the rain and retains it in such a manner as to render wheat raising possible with an annual precipitation of less than 20 inches.

West of the Cascades the entire country, except certain limited areas where soil conditions are unfavorable, was originally clothed with magnificent forests. The forests of fir, cedar, spruce, etc., from which the Evergreen State derives its popular name, extend from the shore of the Pacific over the Cascade Mountains and down their eastern slopes to an elevation of about 2,000 feet, leaving the higher summits hare. The annual rainfall to the west of the mountains ranges from 50 to over 100 inches. On the higher mountains it is probable that the mean annual precipitation, mostly in the form of snow, is in excess of the heaviest precipitation observed in the valleys. The temperature throughout the year is surprisingly mild and equable. The winters are humid, but with little snow in the valleys.

The prevailing winds in Washington are from the west, and come from the Pacific charged with moisture. In passing Mount Rainier and other lofty mountains they are forced upward, rarefied, and chilled, and part with much of their humidity before reaching the plateau east of the Cascades.

Glaciers are no less dependent on climatic conditions than forests. We find them especially where high mountains rise in the paths of warm, humid winds. On the mountains of Washington, and particularly on Mount Rainier, these conditions are abundantly fulfilled.