rock glacier

In high mountain country, one may stumble upon a bulging, glacierlike mass of rock rubble spreading out from a steep, perpetually frozen amphitheater: a rock glacier. Around 1900, geologists first described— and hotly debated the origins of—rock glaciers. Three types are now recognized: lobate, tongue-shaped, and spatulate. Unlike scree slopes, landslides, or rock avalanches, rock glaciers slide downhill at three to six feet per year by means of ice creep. Some contain interstitial ice a few feet or so below their surfaces; others simply ride on top of ice glaciers. Standing on a rock glacier, the surface appears to be a chaotic jumble of boulders. Viewed from high above, patterns emerge: furrows, parallel ridges, pits, crevasses, lobes. Actively moving rock glaciers, free of vegetation, can be 150 feet thick. Dormant or fossil rock glaciers are thinner, and lichens and other scruffy plants grow on their faces. Other names: chrystocrene, coulée de bloc (French), talus glacier, rock river. Rock glaciers are most common in polar or subpolar regions, or in the high mountains of the midlatitudes. A dramatic rock glacier shaped like a lava flow oozes down the forested slopes of Sourdough Mountain, near McCarthy, Alaska. The genesis of rock glaciers continues to provoke controversy among geologists. Some argue that true glaciers are essential precursors of rock glaciers. Others point to rock glaciers found in the southwestern United States, a region with no evidence of glacial history. Most concede that lobate rock glaciers, as evidenced by their shape, almost certainly derive from true glaciers. Geologist Dale Ritter’s final pronouncement on the topic is that “enough disagreement exists . . . to refrain from sweeping conclusions at this time.”

Eva Saulitis