Formation
Several processes can cause the floor of a depression to become more resistant to erosion than its surrounding slopes and uplands:
- First, coarse-grained sediment, such as gravel, accumulates in the depression, i.e. a stream valley or lake basin. Next, wind erosion removes fine-grained sediments in areas adjacent to the depression, but leaves behind the more resistant coarse-grained sediments as a hill or ridge.
- A fluvial valley might fill with volcanic material such as lava or welded tuff pouring into it. This would resist erosion while the surrounding surface is eroded away to create a ridge.
- Cementation of underlying sediments by minerals dissolved in water may occur in a depression. On Earth, this often happens in stream valleys as the result of the formation of duricrusts, i.e. silcrete or ferricrete, by pedogenesis. 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. Again, the cemented sediments would resist erosion while the surrounding countryside is eroded away to create a ridge or hill.
Read more about this topic: Inverted Topography
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