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: Built by fragments (called ejecta) thrown up (ejected) from a volcanic vent, piling up around the vent in the shape of a cone with a central crater.
