Block-controlled hillslope form and persistence of topography in rocky landscapes

Abstract

Abstract Rocky hillslopes dotted with boulder-sized blocks and covered by a thin, nonuniform soil are common in both steep landscapes and arid environments on Earth, as well as on other planets. 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. 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 we show that blocks can control the persistence of topography and the form and evolution of hillslopes in these landscapes. We present a numerical model demonstrating that incorporation of feedbacks between block release, interruption of soil creep by blocks, and sporadic downslope movement of blocks are necessary and sufficient to capture the morphology and evolution of these landscapes. Numerical results are reproduced by a simple analytical solution that predicts steady-state concave hillslope form and average slope angle from block size and spacing. Our results illuminate previously unrecognized hillslope feedbacks, advancing our understanding of the geomorphology of rocky hillslopes. On a landscape scale, our findings establish a quantitative method to address the migration of sharp edges and the persistence of topography in layered landscapes.