Slopes in southwestern Wisconsin, U. S. A., periglacial or temperate?

Abstract

Since 1960 near Madison studies have been carried on of mass movement rates and processes affecting slope angles and terracettes on Ordovician dolomite and Cambrian sandstone. Within fenced areas measurements of vertical, horizontal, and downslope components of movement are referred to bench, marks cemented to bedrock and protected from the overlying soil and rubble by casings. Measurement techniques include taping between larger stones and use of point gages, dial indicators, linear-motion transducers, and SR-4 strain gages. Slope segments range from horizontal to almost 40 degrees. Annual downslope components of movement range generally from 2 to 20 mm. Consistant seasonal patterns of movement for different kinds of material are masked partly be erratic behavior of many particles. Active formation of terracettes is going on along with rapid displacement downslope of soil and small stones. Late winter movements from alternate freezing and thawing above seasonal frost are largest; late spring and èarly fall wetting and drying moves particles almost as much.

From mass movements alone, slopes are losing up to 100 cm of material per 1000 years. Rates many times faster are reported in nearby agricultural areas. Solution of carbonates is also rapid. The steeper slopes in southwestern Wisconsin are related to structurally controlled stream incision and migration in gently dipping cuestas of alternating sandstones and dolomites. Except for a thin silty alluvial fill related to deforestation and agricultural practices during the last century, local stream channels contain little rock waste. Periglacial climates existed at different times during the Pleistocene but are not considered to have produced any major slopes still identifiable. The present températe climate acting through stream incision, ground water solution, and mass movements suffices to explain most features.