On the northeast side of the mountain, descending from the same high névés as the Emmons Glacier, is the Winthrop Glacier. Not until halfway down, at an elevation of about 10,000 feet, does it detach itself as a separate ice stream. The division takes place at the apex of that great triangular interspace so aptly named "the Wedge." Upon its sharp cliff edge, Steamboat Prow (fig. 13), the descending névés part, it has been said, like swift-flowing waters upon the dividing bow of a ship at anchor. The simile is an excellent one; even the long foam crest, rising along the ship's side, is represented by a wave of ice.
Undoubtedly the Wedge formerly headed much higher up on the mountain's flank. Perhaps it extended upward in the form of a long, attenuated "cleaver." It is easy to see how the ice masses impinging upon it have reduced it to successively lower levels. They are still unrelentingly at work. It is on the back of the Wedge, it may be added here, that is situated that small ice body which Maj. Ingraham named the Inter Glacier. That name has since been applied in a generic sense to all similar ice bodies lying on the backs of "wedges."
Of greatest interest on the Winthrop Glacier are the ice-cascades and domes. (Figs. 13 and 14.) Evidently the glacier's bed is a very uneven one, giving rise to falls and pools, such as one observes in a turbulent trout stream. The cascades explain themselves readily enough, but the domes require a word of interpretation. They are underlain by rounded bosses of especially resistant rock. Over these the ice is lifted, much as is the water of a swift mountain torrent over submerged boulders. Immediately above each obstruction the ice appears compact and free from crevasses, but as it reaches the top and begins to pour over it breaks, and a network of intersecting cracks divides it into erect, angular blocks and fantastic obelisks. Below each dome there is, as a rule, a deep hollow partly inclosed by trailing ice ridges, analogous to the whirling eddy that occurs normally below a bowlder in a brook. Thus does a glacier simulate a stream of water even in its minor details.
The domes of the Winthrop Glacier measure 50 to 60 feet in height. A sample of the kind of obstruction that produces them appears, as if specially provided to satisfy human curiosity, near the terminus of the glacier. There one may see, close to the west wall of the troughlike bed, a projecting rock mass, rounded and smoothly polished, over which the glacier rode but a short time ago.
Another feature of interest sometimes met with on the Winthrop Glacier, and for that matter also on the other ice streams of Mount Rainier, are the "glacier tables." These consist of slabs of rock mounted each on a pedestal of snow and producing the effect of huge toadstools. The slabs are always of large size, while the pedestals vary from a few inches to several feet in height.
The origin of the rocks may be traced to cliffs of incoherent volcanic materials that disintegrate under the frequent alternations of frost and thaw and send down periodic rock avalanches, the larger fragments of which bound out far upon the glacier's surface.
The snow immediately under these large fragments is effectually protected from the sun and does not melt, while the surrounding snow, being unprotected, is constantly wasting away, often at the rate of several inches per day. Thus in time each rock is left poised on a column of its own conserving. There is, however, a limit to the height which such a column can attain, for as soon as it begins to exceed a certain height the protecting shadow of the capping stone no longer reaches down to the base of the pedestal and the slanting rays of the sun soon undermine it. More commonly, however, the south side of the column becomes softened both by heat transmitted from the sun-warmed south edge of the stone, as well as by heat reflected from the surrounding glacier surface and as a consequence the table begins to tilt. On very hot days, in fact, the inclination of the table keeps pace with the progress of the sun, much after the manner of a sun-loving flower, the slant being to the southeast in the forenoon and to the southwest in the afternoon. As the snow pillar increases in height it becomes more and more exposed and the tilting is accentuated until at last the rock slides down.
In its new position the slab at once begins to generate a new pedestal, from which in due time it again slides down and so the process may be repeated several times in the course of a single summer the rock shifting its location by successive slips an appreciable distance across the glacier in a southerly direction.
As has been stated, the slabs on glacier tables are always of large size. This is not a fortuitous circumstance; rocks under a certain size, and especially fragments of little thickness, can not produce pedestals; in fact, far from conserving the snow under them, they accelerate its melting and sink below the surface. This is especially true of dark-colored rocks. Objects of dark color, as is well known to physicists, have a faculty for absorbing heat, whereas light-colored objects, especially white ones, reflect it best. Dark-colored fragments of rock lying on a glacier, accordingly, warm rapidly at their upper surface and, if thin, forthwith transmit their heat to the snow under them, causing it to melt much faster than the surrounding clean snow, which, because of its very whiteness, reflects a large percentage of the heat it receives from the sun. As a consequence each small rock fragment and even each separate dust particle on a glacier melts out a tiny well of its own, as a rule not vertically downward but at a slight inclination in the direction of the noonday sun. And thus, in some localities, one may behold the apparently incongruous spectacle of large and heavy rocks supported on snow pillars alongside of little fragments that have sunk into the ice.
There is also a limit to the depth which the little wells may attain; as they deepen, the rock fragment at the bottom receives the sun heat each day for a progressively shorter period, until at last it receives so little that its rate of sinking becomes less than that of the melting glacier surface. Nevertheless it will be clear that the presence of scattered rock débris on a glacier must greatly augment the rate of melting, as it fairly honeycombs the ice and increases the number of melting surfaces. Wherever the débris is dense, on the other hand, and accumulates on the glacier in a heavy layer, its effect becomes a protective one and surface melting is retarded instead of accelerated. The dirt-covered lower ends of the glaciers of Mount Rainier are thus to be regarded as in a measure preserved by the débris that cloaks them; their life is greatly prolonged by the unsightly garment.
Last Updated: 07-May-2007