Origins and implications of soil layering

Layering is common in soils, due to a variety of pedologic and geologic processes, and has important consequences for the interpretation of soils and landscapes. Layering can derive from original sedimentary layering; depositional upbuilding; episodic surface erosion, deposition, and stability; soil production by weathering; vertical or lateral translocation; bioturbation; and various combinations of these. Complex and polygenetic models incorporate both geogenic and pedogenic processes, and allow for physical and biological processes, as well as both vertical and horizontal movements. We review these conceptual frameworks and synthesize them into a vertical contrast model (VCM) for interpreting layered surficial materials. The VCM incorporates a variety of geologic and pedologic processes which may create, destroy, enhance, or obscure vertical contrasts. The model is illustrated via application to sites in the Ouachita Mountains, USA, and northwest Saxonian Lowlands, Germany. The examples illustrate the importance of a comprehensive pedogeomorphic interpretation of layering, since neither standard stratigraphic or top–down pedogenetic principles necessarily apply. The examples also show that the same process can, sometimes contemporaneously, both create and destroy vertical contrasts.