Date of Award

Spring 1-1-2019

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

First Advisor

Robert S. Anderson

Second Advisor

Suzanne P. Anderson

Third Advisor

Peter Molnar

Fourth Advisor

Lizzy Trower

Fifth Advisor

Gregory E. Tucker

Abstract

Rocky hillslopes dotted with boulder-sized blocks and covered by a thin, non-uniform soil are common in both steep landscapes and arid environments on Earth, as well as on other planets. We have long known how to read geologic structure from aerial imagery; for example, folds in layered rock generate trains of hogbacks. Yet, while the evolution of soil-mantled, convex-upward hillslopes in uniform lithology is reasonably well understood, the influence of heterogeneous lithology and geologic structure on hillslope form and evolution has yet to be properly addressed at a process level. Landscapes developed in layered sedimentary rocks feature sharp-edged landforms such as mesas and hogbacks that exhibit steep, linear to concave- upward ramps with scattered blocks calved from resistant rock layers overlying softer strata. Here I pose the question: What roles do these blocks play in landscape evolution? Using a combination of numerical modeling, fieldwork, and mathematical analysis, I demonstrate that blocks profoundly alter the style and pace of hillslope evolution in rocky landscapes. First, in a numerical model of hillslope evolution I show that the presence of discrete blocks and their interactions with the production and transport of soil can explain the characteristic concave-up hillslope profiles observed in landscapes developed in layered rock on Earth. The presence of blocks increases both the relief and the persistence of topography in these settings. I use these numerical results to develop an analytical framework that characterizes the steady-state form of layered hillslopes in horizontal, tilted, and vertical rock. I find that hillslope weathering and transport processes in the presence of blocks lead to self-organization that allows hillslopes to maintain a steady relief and form through time. Next, I present the first process-based 2D numerical model of river canyon evolution that incorporates the roles of blocks in both hillslope and channel processes. The model reveals that channel-hillslope feedbacks driven by the delivery of large blocks from hillslopes to the channel are necessary and sufficient to develop the cross- sectional and planview morphologies of river canyons observed on Earth. Block feedbacks lead to persistent unsteadiness in the landscape, strongly modifying erosion rates over long periods of time, even under steady forcing conditions. Finally, I explore the common triangular and scalloped mapview patterns developed in tilted rocks when incised by dip-parallel streams. The work presented here demonstrates the importance of large blocks of rock in governing hillslope processes and rates, and advances our understanding of landscape evolution in layered rocks.

Available for download on Wednesday, November 13, 2019

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