Abstract

A distinctive sorted pattern of soil mounds and stone pavement, which resembles patterned ground of cold regions, is widely distributed in southern Idaho on gravel fans, basaltic lava flows, and marginal rocky colluvium, as well as on some outcrops of lake beds and welded tuff.

Soil mounds in flat areas are closely packed, monotonously uniform circular heaps 50–60 feet across and 3 feet high, but on slopes they are elliptical and lie in rows along paths of soil that trend downhill. Each mound is composed of a lens-shaped cap of silt about 18 inches thick, abruptly underlain by 1–2 feet of brown clay that rests on siliceous hardpan.

Stone pavement that surrounds mounds in rocky places consists of a shallow layer of well-sorted stones derived from underlying parent rocks. It is marked by vertical sorting (large stones on smaller ones), by tabular stones turned on edge (commonly parallel to the pavement border), and by numerous broken stones—all encrusted with lichens.

The pattern varies with local slope. A net pattern, consisting of pavement woven between equally spaced mounds on flat terrain, gradually lengthens into a striped pattern, consisting of alternate rows of soil and pavement on hillsides. Striped patterns are first perceptible on grades of 3 per cent, dominant on grades of 10 per cent, and still recognizable on grades of 50 per cent.

The patterned ground could have formed under a periglacial climate during the late Pleistocene when mass-wasting was presumably favored by freeze and thaw of saturated soil, but this origin is based only on analogy with patterns in cold regions, and the evidence of age is inconclusive. If this opinion is correct, the cold aspect of the Pleistocene extended several hundred miles farther south than has been generally assumed.

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