Abstract

The Mt. Kosciusko area of Australia contains a variety of soil and landscape features attributed to periglacial conditions. Among the features are several types of solifluction terraces, one of which is the nonsorted step. The steps occur in groups restricted to the tops of wind-exposed slopes of metasediment outcrops. The slopes range in declivity from 4 to 25 degrees and the vegetation is a mosaic of alpine herbfield and fjaeldmark (dwarf open heath).

Nonsorted steps are more often lobate (width > length) than tongue-shaped (length > width), with 10–77-foot widths, 9–46-foot lengths, and risers up to 3 feet high. They consist of stony soil, a poorly sorted mass of silt to small stones, which overlies a layer of tabular rock fragments, up to 18 inches long and oriented downslope. This stony layer grades downward into fractured phyllite, schist, and slate with nearly vertical dip and cleavage. Between the stony soil and stony layer within several of the steps is an organically rich lens of stone-free soil.

Radiocarbon analyses of this material indicate that steps 50–400 feet below the mountain crests are 2000–3000 years old and that further upslope, within 50 feet of the crests, episodic movement occurred during the last 300 years. Botanical and microtopographic data also support the view that downslope steps are relict features, whereas closer to the crests increased activity is apparent. Mass movement of an entire step requires the perennial or seasonal impedance of subsoil drainage, which permits the soils' frost susceptibility to promote active solifluction. Local variations in this susceptibility across a slope could account for differential slope movement leading to the development of steps. Frost penetration of the slope mantle, sufficient for the steps to form, could have been produced by either a decline of at least 4° F in mean annual temperature or by stronger winter winds blowing more snow off exposed sites. The convex land forms on which the steps occur are not due to glacial smoothing, as previously thought, but to periglacial frost shattering of bedrock followed by solifluction. The last main period of solifluction 2000–3000 years ago produced most of the steps. The mechanisms, however, by which the steps assumed lobe and tongue forms or asymmetry, are not understood.

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