Mountain Lake is the only natural lake of significance in the unglaciated southern Appalachian Highlands. It is located near the summit of Salt Pond Mountain, Giles County Virginia, at an elevation of 1177 m. It is underlain by Ordovician and Silurian non-calcareous shale and sandstone of the Martinsburg, Juniata and Clinch formations. Historical (250 years) and sediment (6000 years) records suggest that the size of the lake has varied periodically. In the 1930s lake origin was proposed as due to valley damming by a lateral landslide (Hutchinson and Pickford, 1932) or damming by scree (Sharp, 1933). A later theory modified the landslide hypothesis to the primarily vertical collapse of a canyon feature in the Clinch (Parker et al., 1975). Fracture trace analysis now reveals a regional lineation feature associated with the lake. This feature is present surficially both downgradient from the lake to the northwest, and upgradient to the southeast. Sonar bathymetry and diver reconnaissance show it expressed as a (relatively sediment-free) narrow open crevice in the deepest (33 m) portion of the lake, probably a fault. Hydrologic observation and resistivity suggest preferential water movement along this fracture, as well as leakage directly from the lake. The present study suggests conduit erosion within this feature and periodic vertical downsettling of overlying Clinch material as the primary mechanism of lake origin and water-level fluctuations through time.

Extensive Noachian‐aged intercrater planation surfaces comprise much of the southern highlands of Mars. We mapped aggradational and stable to degradational surfaces in three study areas with diverse relief elements and ages: the high and rugged relief of Libya Montes, the well‐preserved intercrater plains of Noachis Terra, and the rolling relief with more drainage development in Terra Cimmeria. Here we describe four major geomorphic features that formed in these regions: debris‐mantled escarpments, regolith pediments, sloping aggradational surfaces, and depositional plains. We interpret that with tectonic stability and an arid paleoclimate, these features supported slow pedogenesis, sediment transport, and diagenesis over hundreds of millions of years during heavy impact bombardment. Slow aqueous weathering generated primarily fine‐ or medium‐grained particles from basaltic surfaces of impact ejecta and megabreccia. These sediments were collected in local lows, reducing surface roughness, permeability, and populations of small craters. Larger crater walls and structural escarpments retreated radially or linearly as ~5–20° slopes, indicating efficient removal of fine‐ or medium‐grained debris but little downslope transport of coarse material by fluvial erosion or creep. Gently to moderately sloping, composite intercrater planation surfaces evolved as regolith pediments with tectonic stability and little fluvial dissection. Noachian impact craters degraded in place on pediments and became embayed or buried on basin floors. The concentration of aggradational surfaces in low‐lying areas, lack of coarse‐grained alluvial fans in most locations, and resistance to later eolian deflation suggest intermittent low‐magnitude (hypo‐)fluvial erosion with aqueous cementation or development of a lag in basins.

In deserts, the interplay between occasional fluvial events and persistent aeolian erosion can form composite modern and relict surfaces, especially on the distal portion of alluvial fans. There, relief inversion of alluvial deposits by differential erosion can form longitudinal ridges. We identified two distinct ridge types formed by relief inversion on converging alluvial fans in the hyperarid Chilean Atacama Desert. Although they are co-located and similar in scale, the ridge types have different ages and formation histories that apparently correspond to minor paleoclimate variations. Gravel-armored ridges are remnants of deflated alluvial deposits with a bimodal sediment distribution (gravel and sand) dated to a minor pluvial phase at the end of the Late Pleistocene (~12 kyr). In contrast, younger (~9 kyr) sulfate-capped ridges formed during a minor arid phase with evaporite deposition in a pre-existing channel that armored the underlying deposits. Collectively, inverted channels at Salar de Llamara resulted from multiple episodes of surface overland flow and standing water spanning several thousand years. Based on ridge relief and age, the minimum long-term deflation rate is 0.1–0.2 m/kyr, driven primarily by wind erosion. This case study is an example of the equifinality concept whereby different processes lead to similar landforms. The complex history of the two ridge types can only be generally constrained in remotely sensed data. In situ observations are required to discern the specifics of the aqueous history, including the flow type, magnitude, sequence, and paleoenvironment. These findings have relevance for interpreting similar landforms on Mars.