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: An alpine condition is an environmental condition in which the monthly mean temperature is less than 10 degrees Celsius and which occurs at altitudes above the tree line and below the snowline.
Definition: National Museum of Natural History, Smithsonian Institution (USNM), Washington, District of Columbia, USA.\r\nNMNH and USNM both refer to the National Museum of Natural History. Collections are associated with one or the other acronym. US, the US National Herbarium, is a collection within the National Museum of Natural History. URL for main institutional website, http://www.mnh.si.edu/rc/\r\nURL for institutional specimen catalog, http://collections.mnh.si.edu/