3.1.2.3 - Process, time and contrasting desert landscapes

Landscapes preserve several timescales

Processes operate together at different rates. Saltating grains move in seconds, a storm can change a channel in hours, repeated floods build a fan over much longer intervals, and tectonic relief or inherited weathering surfaces persist over geological time. A short event can rearrange material within a much older landform. Present climate and one active process rarely explain the entire visible landscape.

Distinguish activity from inheritance. A dry valley may have been enlarged during wetter conditions and now experience occasional floods. A former lake floor may now be a dust source. A dune may migrate within a sand sea whose sediment supply has a far longer history. To explain a landscape, connect source rock, transport pathways, storage and environmental change; to establish change rate, use repeat observations or dated deposits.

The Mega-Chad record illustrates a large lake falling around 5 thousand years ago, while final Bodélé drying occurred much later. The staggered history warns against applying one regional date to every arid landform. Different combinations of erosion, deposition and persistence create spatial contrasts within the same climatic zone.

Death Valley and the Namib compared

Death Valley is a mid-latitude inland basin in the Mojave of California; the Namib Sand Sea lies on Namibia's lower-latitude Atlantic coast. Both are hot-desert settings, but their location, relief and supply pathways differ. Use these named geographical units consistently: Death Valley basin is not all of the Mojave, and Namib Sand Sea is not the whole of Namibia.