MT. OBERLIN CIRQUE AND BIRD WOMAN FALLS (HILEMAN PHOTO)
In plucking, the glacier actually quarries out masses of rock, incorporates them within itself, and carries them along. At the head of the glacier this is accomplished mainly by water which trickles into crevices and freezes around blocks of rock, causing them to be pulled out by the glacier, and also by the weight of the glacier, squeezing ice into the cracks in the rock. As the glacier moves forward these blocks of ice are dragged or carried along with it. Usually a large crevasse, the bergschrund, develops in the ice at the head of a glacier. The bergschrund of most glaciers in the park consists of an opening, usually 10 to 20 feet wide at the top and as much as 50 feet deep, between the head of the glacier and the mountain wall. On Sperry Glacier, however, it is more typical of that found on larger valley glaciers and consists of several conspicuous crevasses separating the firn area (where the snow is compacted into ice) on top of Gunsight Mountain from the glacier proper below (see [photo on the cover]). It is at this site that plucking is most dominant because water enters by day and freezes in the rock crevices at night. This quarrying headward and downward finally results in the formation of a steep-sided basin called a cirque or glacial amphitheatre. Because the cirque is the first place that ice forms and the place from which it disappears last, it is subjected to glacial erosion longer than any other part of the valley. Thus its floor is frequently plucked and scraped out to a comparatively great depth so that a body of water known as a cirque lake forms after the glacier disappears. Iceberg Lake lies in one of the most magnificent cirques in the Park. The lowest point on the crest of the wall encircling three sides of the lake is more than 1500 feet above the water. Prior to 1940 this cirque contained a small glacier. It has been shrinking rapidly for about two decades, and in the last two or three years of its existence was hardly recognizable as a glacier. Its disappearance is made more remarkable by the knowledge that in 1920 the front of the glacier rose in a sheer wall of ice nearly 100 feet above the surface of the lake. All that remains of this glacier which once kept the lake filled with icebergs each summer is a large bank of snow at the base of the cirque wall at the head of the lake. Other good examples of cirques are those which hold Hidden, Avalanche and Cracker Lakes. The tremendous cliff on the south side of the latter rises 4,100 feet from the lake to the summit of Mount Siyeh. Other notable cirque lakes are Ellen Wilson, Gunsight, Ptarmigan and Upper Two Medicine.
ST. MARY VALLEY FROM LOGAN PASS SHOWING GLACIAL PROFILE (HILEMAN PHOTO)
Rock fragments of various sizes frozen into the bottom and sides of the ice form a huge file or rasp which abrades or wears away the bottom and sides of the valley down which the glacier flows. The valley thus attains a characteristic U-shaped cross section, with steep sides (not necessarily vertical) and a broad bottom. A mountain valley cut entirely by a stream does not have such shape because the stream cuts only in the bottom of the valley, whereas a glacier, filling its valley to a great depth, abrades along the sides as well as on the floor. Practically all valleys of the Park, especially the major ones, possess the U-shaped cross section. This feature can best be seen by looking down from the head of the valley rather than from the valley floor. Splendid examples are the Swiftcurrent Valley viewed from Swiftcurrent Pass or Lookout; St. Mary Valley from east of Logan Pass; the Belly River Valley from Ptarmigan Tunnel; and Cataract Creek Valley from Grinnell Glacier.
FIGURE 1. IDEALIZED SKETCH OF A GLACIAL STAIRWAY FROM THE ARETE AT THE CENTER OF THE RANGE TO THE ICE AGE MORAINE AT THE MOUTH OF THE VALLEY.
Cirque wall Glacier Lake Moraine
The floors of many of the Park’s major U-shaped valleys instead of having a more or less uniform slope, steeper near the head than farther down, as is usually the case in a normal stream valley, are marked by several steep drops or “steps,” between which the valley floor has a comparatively gentle slope. Such a valley floor, throughout its entire course, is sometimes termed the glacial stairway. Most of the steps, particularly those in the lower courses of the valleys, are due to differences in resistance of the rocks over which the former ice flowed. On the east side of the Lewis Range, where the steps are especially pronounced, the rock strata of which the mountains are composed dip toward the southwest, directly opposite to the direction of the slope of the valley floors ([Figure 1]). Thus, as glaciers flowed from the center of the range down toward the plains, they cut across the edges of these tilted rock layers; where the ice flowed over weaker beds it was able to scour out the valley floor more deeply creating a “tread” of the glacial stairway. The more resistant rock formations were less easily removed, and the ice stream, in moving away from the edges of these resistant strata, employed its powers of plucking and quarrying to give rise to cliffs or “risers.” Lakes dammed partly by the resistant rock strata now fill depressions scoured out of the weaker rock on the treads ([Figure 1]). These are rock-basin lakes, and where several of them are strung out along the course of the valley they are referred to as paternoster lakes because their arrangement resembles that of beads on a string. Well-known examples of such bodies of water are Swiftcurrent and Bullhead Lakes, two of the long series which stretches for seven miles between Many Glacier Hotel and Swiftcurrent Pass. Resistant layers in the lower portion of the Altyn formation, the upper part of the Appekunny, and the upper part of the Grinnell[2] normally create risers.