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Noon seminar - Edges matter: Erosion of landscapes by migration of edges

Monday, April 16, 2012, 12:00PM - 1:00PM

Noon seminar - Edges matter: Erosion of landscapes by migration of edges


Bob Anderson



ARC room 620

Many natural processes remove sharp corners from landscapes. Diffusion reigns, and we have well-developed models to address the resulting landforms.  But several landforms display sharp corners, or edges, that are maintained as the feature evolves.  I argue that we have been looking at these landscapes incorrectly. I present examples demonstrating the importance of edges in several landscapes, and attempt to show how one might treat these in models.  Most obvious among the examples are coastal bluffs, which we have long acknowledged evolve by horizontal erosion.  But more subtly, examples can be drawn from glacial landscapes in which the upvalley migration of bedrock bumps accomplishes the lowering of the glacial bed; from fluvial landscapes in which upstream migration of knickpoints big and small do the trick; and from hillslopes, in which migration of ledges leave blockfields in their wake.  I will attempt to draw the parallels between these systems.

Our crafting of landscape evolution models in terms of patterns of vertical erosion has blinded us to the relevance of processes that operate chiefly in the horizontal (or when we are not blind, we cheat). Simplifications embedded in our present models also require that we ignore the blocky nature of many landscapes.  These are two pieces of the same elephant. Many landscapes evolve by removal and transportation of discrete blocks of rock. Removal of a block maintains a sharp edge. None of our weathering rules, hillslope transport rules, fluvial erosion rules, or glacial erosion rules acknowledge this. If we fail to accept this challenge we will remain incapable of addressing the roles of rock type, fracture density, and geologic structure in geomorphology, and will be prevented from understanding the evolution of iconic landscapes that include the flatirons that grace our own backyard.