Cuesta escarpment retreat is a principal mode of exhumation in regions of layered sedimentary rock. On the Colorado Plateau, this process acts as a mechanism for maintaining high‐relief topography and facilitating drainage divide migration. Quantitative estimates of cuesta evolution are difficult to evaluate over glacial‐interglacial timescales, and thus rates of geomorphic change along individual escarpments have mostly been constrained over millions of years. Several studies have addressed this problem by dating colluvium‐mantled talus flatirons. However, this technique has not been applied systematically on the Colorado Plateau. This study quantifies geomorphic change along a single Colorado Plateau cuesta using 36Cl surface exposure dating. We present 33 ages from a single generation of talus flatirons below the Coal Cliffs of central Utah. Landscape evolution is further constrained using 14 ages from in‐situ bedrock, 3 ages from boulders on gully interfluves, and two ages from terrace alluvium. Results suggest a colluvial apron was deposited below the cuesta beginning as early as Marine Isotope Stage 3, and the latest depositional phase occurred near the Last Glacial Maximum. A switch from apron deposition to incision initiated flatiron formation sometime between 19.7 ± 2.5 and 11.8 ± 1.6 ka, broadly coincident with the transition from glacial to interglacial conditions. Our results have several important implications. Climatic changes during the end of the last glacial period appear to have shifted the balance between deposition and erosion below the Coal Cliffs, emptying the sediment reservoir at their base and increasing their height via bedrock incision. The climatic forcing could be imparted by several mechanisms, including local controls on debris generation / mobilization and base level changes exerted by transverse streams. Similar processes may have occurred during previous glacial‐interglacial transitions, implying that the escarpment retreat processes may be partially modulated by orbitally‐controlled variations in Earth's climate over larger timescales. Copyright © 2018 John Wiley & Sons, Ltd.

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.

The assumption that landscapes evolve toward topographic steady state is one of the key frameworks used in geomorphic analyses, and it has proved useful for identifying transience in bedrock river channels (Whipple & Tucker, 1999) and soil-mantled hillslopes (Hurst et al., 2012), correlating transient features with exogenic driving events (Berlin & Anderson, 2007), and quantifying the geomorphic influence of tectonic (Snyder et al., 2000; Wobus et al., 2006) and climatic forcings (Willett, 1999). However, as nonvertical lithologic contacts are gradually exhumed, their surface exposures retreat downdip; this prohibits topographic steady state and autogenically induces spatially and temporally varying erosion rates (Cook at al., 2009; Forte et al., 2016; Perne et al., 2017). In some situations, such autogenically induced transient features could easily be misidentified as tectonic or climatic signals. These effects are likely exemplified in structurally controlled landscapes where dipping, layered rocks are exposed at the surface, such as fold and thrust belts (e.g.

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