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• Overlapping crater populations constrain the total era of martian valley formation. • Martian valleys may have formed intermittently over hundreds of millions of years. • Long-term erosion rates would have been similar to the modern Atacama Desert. A number of studies have constrained the cessation of martian valley network activity to around the Noachian-Hesperian boundary, but the total time period over which these features formed remains poorly constrained. Most attempts to ascertain valley network formative timescales rely on an intermittency factor that varies widely for terrestrial rivers and is unknown for Mars. In this contribution, superposition relationships are used to identify crater populations that pre- and postdate valley network incision, resulting in maximum cumulative valley formation times of ∼108 years. These timescales correspond with long-term erosion rates of ∼0.25 m/Myr, similar to those calculated for the Middle to Late Noachian as well as the central Atacama Desert since the Late Miocene. Assuming sediment transport rates previously calculated for other martian valley networks, this implies a minimum fluvial intermittency of ∼10−5, several orders of magnitude lower than modern day Earth rivers but similar to values independently calculated for some martian fluvial deposits. This low intermittency suggests that martian valley incision may have been significantly hindered by erosion into existing bedrock or by boulder armoring of the valley floor. Alternatively, unlike river valleys on Earth, runoff events driving martian valley network activity may have been non-seasonal, Milankovitch-like cycles, with long eras of quiescence between periods of intensified fluvial activity. [ABSTRACT FROM AUTHOR]

The concept of the geomorphic cycle is a foundational principle in geology and geomorphology, but the topographic evolution of a single landscape from inception to maturity has been difficult to demonstrate in nature. The onlapping dunes of the Cooloola Sand Mass (CSM) in eastern Australia provide an ideal chronosequence to evaluate landscape evolution. Here commonly assumed properties on which landscape models are based (i.e., conservation of mass and major factors contributing to landscape change) can be physically measured and accounted for. Our field based measurements and forward numerical models demonstrate that dunes, like other landscapes, relax in an exponential manner. The emplaced dunes evolve through an initial phase of rapid topographic adjustment associated with the dominance of landsliding. This phase continues for ∼1 kyr until hillslope gradients are lowered below their angle of repose (0.65 m m−1 or 33°). Once sufficiently lowered, the dunes evolve through slow, soil creep processes. These findings of dual transport regimes are validated by stratigraphic records at all excavated dune foot-slopes and we propose that this evolution can be measured by the distribution of curvature (C) of a landform, specifically its standard deviation (σ C), as a measure of surface roughness. Surface roughness smooths with time through diffusional sediment transport that lowers local relief. The value and its rate of smoothing can define the stage in evolutionary development and help infer processes, which makes it an important morphometric tool for understanding landscapes. These observations highlight that under stable evolutionary conditions, the development of the landscape is governed by the physical properties of the dune's parent material. In addition, our findings support landscape evolution inferences from numerical and physical models and the coupling of granular material physics with landscape change. • Measured evolution of a natural dune landform from emplacement to maturity. • Episodic mass-movement dominates slope evolution on young emplaced dunes. • Soil creep processes controls slope evolution on older emplaced dunes. • Angle of repose defines the transition from non-linear to linear slope processes. • Surface roughness is directly related to variability in erosion rates. [ABSTRACT FROM AUTHOR]