Migrating Transverse Escarpments in Strike Valleys on the Colorado Plateau

Erosion in strike valleys separating parallel cuesta escarpments controls the evolution of staircase‐like landscapes developed in layered rocks. We explore an important erosional process in strike valleys developed in the Mancos Formation of central Utah, USA. These shale‐floored valleys are segmented by transverse (strike‐perpendicular) scarps, which we call migrating transverse escarpments (MTEs). The MTEs separate axial stream networks that join larger transverse rivers from gullies that drain across adjacent sandstone layers. The MTEs mark a state of transience within the strike valleys as the axial stream networks expand along strike and adjust the gullies to a new base level at the transverse rivers. Here, we describe MTE morphology, consider the geomorphic processes that control their evolution, discuss their impacts on regional landscape evolution using field relationships, and explore how variable geologic parameters may affect their behavior using numerical simulations. Our simulation results suggest that fluvial pour points through the adjacent hard layers serve as internal base levels that modulate MTE evolution. Once a stable axial stream network forms, new MTEs cannot form until the channel network reroutes through the hard layers. This likely occurs over glacial‐interglacial timescales as debris deposition reroutes channel networks and variable hydrologic conditions accelerate transverse river incision. However, our results also suggest that MTEs can form autogenically as downdip strike valley retreat causes the lower hard layer to migrate underneath the axial stream. Our findings highlight the influence of lithologically controlled base levels on landscape evolution, particularly in regions of variable layered rocks.