Definition: Gravel is an environmental material which is composed of pieces of rock that are at least two millimeters (2mm) in its largest dimension and no more than 75 millimeters.
Definition: A point where groundwater or steam flows out of the ground, and is thus where the aquifer surface meets the ground surface or where there is a fissure.
Definition: The gnawing and biting-off of small fragments of tissue, whether from plants or dead invertebrates. [ https://www.worldcat.org/search?q=bn%3A0-412-40180-0 ]