The Republic of South Yemen lies on the edge of one of the world's great sand seas, the Rubh-al-Khali, but even this dry desert region bears the unmistakable imprint of flowing streams and rivers. The branching pattern in the photograph could only have been produced by running water, draining off the surrounding land. These filigree patterns are termed "dendritic drainages" because of their similarity to the way in which trees branch out into progressively finer twigs. The term comes from the Greek dendrites, meaning tree-like.
The dry gullies or waids appear to pose something of a paradox in an area that is apparently exceptionally arid desert, with no vestige of plant life. Freak rainstorms and flash flooding might deepen and extend the gullies, but they are far too infrequent at the present day to have produced the pattern seen here. The drainage pattern is clearly a fossil. When the Earth emerged from the last Ice Age, the Sahara and the Rubh-al-Khali were savanna grasslands with a more temperate climate and much higher rainfall than they experience today. Runoff from the coastal mountains carved the dendritic drainage pattern, which was then "fossilized" when the climate became more arid.