Nature Notes – An infinite variety of patterns, colours and textures

by Amy Schoeman

Ripples in the Namib by Amy Schoeman

One of the most photographed topics in the Namib Desert, if not in deserts all over the world, is the ripple patterns formed by wind on dune backs and slipfaces and flat desert surfaces such as dune streets. Because there are such infinite varieties of patterns, colours and textures, it is also an evergreen topic to photograph.

As with dune formations, there is nothing simple, incidental or haphazard about the way ripples form, and complicated rather than simple patterns are the rule. Other than variations in wind strength and flow patterns, the main factors that determine the degree of rippling seem to be the amounts of sand available, the variation in size and specific gravity of the grains and the combination of types of sand and gravel. Rippling also seems more likely to occur where the sand is a mixture of fine and coarser grains, in which case the patterns become particularly complex and striking.

Especially surreal are the patterns seen on the backs of hump or shrub-coppice dunes in the Skeleton Coast, where the sand has a high mineral content. Fine blood-red garnet crystals that originate in diamond pipes occur sporadically along the beaches, covering long stretches in washes of dark maroon. Once dry this ‘garnet sand’ blows into the dune areas to form a fine coating on the backs of dunes, creating especially intriguing patterns down their slipfaces because of the garnet crystals’ higher specific gravity in relation to the quartz grains of the main dune bodies. When there are strong winds and plenty of moisture, the patterns on the windward sides become especially pronounced and graphic.

Two black minerals that are frequently seen in association with garnet sand are magnetite and ilmenite. The tiny particles of magnetite, which adhere to a magnet when brought close enough, are a form of iron ore. Ilmenite consists of iron oxide and titanium. In combination with light-yellow quartz granules, magnetite and ilmenite form striking zebra-like stripes on the backs of shrub-coppice dunes. These minerals are often seen in association with garnet crystals as coatings, and form particularly complex and colourful patterns on dune slipfaces.

This article appeared in the April ‘05 edition of Travel News Namibia.