The Namib Desert environment is for the most part so dry that it supports little permanent vegetation, other than sparse thornscrub and scattered trees lining dry river courses. Artesian springs are found at widely separated intervals, and are usually brackish, while runoff from occasional rainfall lies briefly in open rocky pools, or disappears beneath the sand of ephemeral rivers. Sometimes this water is accessible in shallow, hand-dug wells known colloquially as gorra, the most basic essential of life in the Namib before the arrival, in colonial times, of the borehole and wind-pump. In these conditions, it is relatively easy to predict the location of Holocene archaeological sites, especially those dating to within the last 5000 years, during which human responses to the aridity of the Namib developed certain distinctive features.