Exhumed river channel

An exhumed river channel is a ridge of sandstone that remains standing once the softer mudstone of an old flood plain has eroded away around it. It begins as sand deposited within a river channel while mud settles on the adjacent floodplain, and once the channel is abandoned it becomes buried under flood deposits from other channels nearby. Because the sand is more porous than the surrounding mudstone, groundwater moves through it more easily, and once erosion strips away the softer rock the sand filled channel is left standing as a ridge.