Definition: A depression caused by erosion by water or ice. Low-lying land bordered by higher ground; especially elongate, relatively large gently sloping depressions of the Earth's surface, commonly situated between two mountains or between ranges of hills or mountains, and often containing a stream with an outlet.
Definition: A narrow and shallow incision into soil resulting from erosion by overland flow that has been focused into a thin thread by soil surface roughness. Rilling, the process of rill formation, is common on agricultural land and unvegetated ground.
