Inverted relief is a landscape that is part of a planet's surface, e.g. Mars, that contains positive landforms, i.e. hills and ridges, that were once depressions in its surface. On Earth, inversion of relief can occur whenever either the sediments or other strata underlying a depression, typically the floor of a floodplain or fluvial valley, becomes more resistant to erosion than the strata in which the valley has been cut. Then differential erosion preferentially removes the less resistant strata underlying the walls of the depressions and adjacent uplands, leaving the floor of the depression as a topographic high.[1]
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Multiple processes can cause the floor of a depression to become more resistant to erosion than its surrounding slopes and uplands. First, coarse-grained sediments, i.e. gravel, accumulated within depressions, i.e. stream valleys and lake basins. Later, erosion — possibly by wind, which can't move coarse-grained sediments — removed fine-grained sediments comprising the surround surface layers, but left behind the more resistant coarse-grained sediments as a ridge. Second, another process by which inverted relief might be created would be the filling of a fluvial valley with either lava or welded tuff. A layer composed of either of these volcanic rocks would resist erosion while the surrounding surface is eroded away to create a ridge out of what was formally a valley. Finally, inverted topography might be formed by the cementation by minerals dissolved in water of sediments that once either filled or underlay a topographic depression such as a lake or fluvial valley. In terrestrial environments, such cementation often occurs within stream valleys as the result of the formation of duricrusts, i.e. silcrete or ferricrete, by pedogenic processes. Minerals for cementation can come from groundwater. It is thought that a low point, like a valley focuses groundflow, so more water and cements move into it, and this results in a greater degree of cementation.[2]
As in case of lava, the cemented sediments would resist erosion while the surrounding countryside is eroded away to create a ridge or hill where a depression once existed.[1][3]
A classic example of terrestrial inverted relief is Table Mountain, Tuolumne County, California. In case of Table Mountain, multiple lavas flows filled an ancient fluvial valley that cut westward through the across the central Sierra Nevada to the Central Valley about 10.5 million years ago. These Miocene lava flows filled this ancient river valley with a thick sequence of high-K, trachyandesite lavas that are significantly more resistant to erosion than the Mesozoic siltstone and other rock in which the valley was cut. As a result, subsequent differential erosion left these volcanic rocks as sinuous ridge, which now stands well above landscape underlain by more deeply eroded Mesozoic rocks.[4]
Inverted relief in the form of sinuous and meandering ridges, which are indicative of ancient, inverted fluvial channels, is argued to be evidence of water flowing on the Martian surface in the past.[3][5][6] Examples of such ridges, which might represent ancient fluvial channels, are shown below from various parts of Mars. Because of sinuous ridges, which might be ancient fluvial channels, and minerals that indicated the past presence of water, Miyamoto Crater was proposed in 2010 as a potential location to be searched for evidence of life on Mars.[7]