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 tundra is a terrestrial biome which includes, across its spatial extent, only low-growing vegetation such as dwarf shrubs, sedges and grasses, mosses, and lichens. Tundra biomes rarely have monthly average temperatures above 10 degrees Celsius and have low evapotranspiration ratios.
