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: movement of an organism from one location to another using an inchworm like motion. This motion consists of extending the front part of the body and pulling the rear along the surface to meet the front.
Definition: A wavelike sequence of muscular contraction and relaxation that passes along a tubelike structure, such as a vertebrate intestine, or the body of an earthworm
