BLOCKS CONTROL HILLSLOPE EVOLUTION IN LANDSCAPES DEVELOPED IN LAYERED ROCK

Rocky hillslopes covered by a thin, non-uniform mobile regolith and dotted with large blocks of rock are common in both steep landscapes and arid environments, as well as on other planets. While the evolution of soil-mantled, convex hillslopes in uniform lithology is well-understood, the influence of lithology and geologic structure on hillslope form and evolution remains enigmatic. Landscapes developed in layered rocks feature landforms such as mesas and hogbacks that exhibit steep, linear-to-concave up ramps scattered with blocks derived from the resistant rock layers. We present a numerical model that shows that interactions between resistant blocks and underlying easily weathered rock explain the form and evolution of a hogback, a tilted feature that exemplifies this class of landforms. Our model consists of a dipping hard rock layer sandwiched between less resistant layers. The hard layer releases resistant blocks that then armor the underlying rock from weathering. In addition, blocks interfere with the flow of regolith, damming it upslope, and developing a wake of thinning regolith downslope, into which the block eventually moves. We find that these feedbacks between block release, weathering of blocks and of soft rock they armor, and sporadic downslope movement of blocks are necessary to capture the essence of hogback evolution. Insights from the numerical model lead to a simple analytical solution that predicts steady-state hillslope form and slope angle from block size, spacing, rate of weathering, and the efficiency of regolith transport. Our results illuminate previously unrecognized hillslope feedbacks, improving our understanding of the detailed geomorphology of rocky hillslopes and the large-scale evolution of landscapes developed in layered rock.