TERRACETTES: ANIMAL, VEGETABLE, OR MINERAL?

For nearly a century researchers have debated the origin of terracettes or steps, which are roughly parallel to one another and generally follow the contours on many moderately sloping (15-35 degrees) hillsides. Some researchers contend that terracettes generally originate from mass wasting processes such as slumping or solifluction. Others argue that large grazing mammals create the treads of the steps. The range of spacing (wavelength = λ) of biological terracettes is less than the variation in wavelength for similar landforms produced by mass wasting. We present evidence from various regions showing that some terracettes have a biological origin and others have a strictly mass wasting origin. Probable biological terracettes have been surveyed in southeastern Washington State (λ = 1.6-2.1 m), the South Island of New Zealand (λ = 1-1.5 m), and the Mongolian Altai (λ = 0.8-1.1 m). Examples of probable mass wasting phenomena of similar size and morphology exist in northwestern Wyoming and in northern and western Mongolia. Compared with terracettes caused by large herbivores, those of mass wasting origin have steeper, unvegetated risers and greater variability in tread slope and width. Biological terracettes exhibit characteristics related to grazing animal behavior; close to obstacles that might impede normal travel, parallel steps commonly merge and do not follow contours. In cases where fences are oriented obliquely to “path” directions, terracettes often terminate against a clear animal trail along the fence.