Nature Notes

Vol. X September, 1932 No. 9

Roches Moutonnees

This formidable-appearing glacial term, like many others applied to glaciers and forms produced by them, has come into our language from the French. In the early days glaciers were best known and more extensively studied in that part of the Swiss Alps where French is spoken - hence today our glacial terminology has quite a number of original French words. Translated, this term means "sheep rocks". It is applied to certain masses of solid rock which have been rounded by ice action.

Ice is the most active and effective of all erosive agents, but its dynamic action is by no means caused by the ice alone. During the process of snow accumulation on rocky hillsides and the subsequent downward movement and change of this snow to solid ice, rock fragments of various size are torn loose and engulfed. These included boulders, acting under the tremendous pressure of a large mass of moving ice, scrape and scour the overridden rocks. This abrasive action pulverizes the rock to a flour fineness, and the particles are swept away by the streams flowing in channels under the ice. And, since the minute rock particles are swept away before becoming discolored by weathering, the streams emerging from beneath glaciers are gray to an almost milk-white color.

Where this sort of erosive action is equally intense under all portions of a glacier, roches moutonnees are not formed: where the rocks have variable hardness, or where ice eddies go around a certain rock area, the rock between continues to be overridden, but is subjected to less effective abrasion and erodes into prominence. Later, when the ice is gone from the valley or slope, these rounded masses of solid rock have a superficial resemblance to the backs of sheep, hence the term "roches moutonnees". This resemblence is especially striking where there are a large number of these rounded rock bumps close together. One can easily imagine that he is looking over the backs of a flock of sheep - or better yet, a herd of elephants.

Roches Moutonnees

In Paradise Valley and below Paradise Glacier, isolated roches moutonnees are conspicuous. Those below Paradise Glacier have been uncovered by ice so recently that deep scratches and grooves are indicative of the process which made them. In the basin-like amphitheaters of upper Paradise Valley, below the Skyline Trail, some of these smoothed rock masses may be found, but greater lapse of time since the ice occupation has given the elements a chance to nearly obliterate the scratches. Over some of them a thin peaty soil has formed and today flowers and trees are growing there. Some very excellent "sheep rocks" may also be found in the vicinity of the Box Canyon of the Cowlitz River.

In the midst of the Nisqually Glacier ice, near the 5500 foot level, one may observe a large and representative example of a roche moutounnee in the process of formation. With deep ice channels on both sides and a thin sheet extending over the top, this example will be a hundred or more feet high when it is completely exposed to view in a few hundred years - if the Nisqually continues to recede at its present rate of about 70 feet a year. Ordinarily a roche moutounnee is not seen in the formative stage, but in this case a local increase in gradient has favored the exposure of part of the prominent rock mass.

Walter M. Chappell,

marten, a dweller of upper Canadian and lower Hudsonian regions

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