"Rock of the Month # 265, posted for July 2023" ---

What are Ventifacts (?)

Ventifacts are stones sculpted by the abrasive power of the wind, which carries particles of sand, snow and ice in suitable environments, notably in hot and cold deserts (e.g., Namibia and Antarctica, respectively). Ventifacts are commonly found on the cm and dm scales, but may be larger. Large (metre-scale) ventifacts may be formed in settings such as Aiyuittuq national park (north of Pangnirting, Baffin Island, Nunavut, Canada), shaped by katabatic winds that blow down-fjord in winter, up-fjord in summer (McKenna Neuman, 2022).

The study of the nature and evolution of terrestrial landforms is called geomorphology. This includes processes of physical erosion and chemical weathering (e.g., Birkeland and Larson, 1989; Tarbuck et al., 2015). Estimates of the erosion rates involved in the formation of these surfaces can be made by analysis of radionuclides generated by cosmic rays in the exposed surfaces (Biermann and Caffee, 2001). On occasion, ventifacts may be seen in museums (e.g., Carnegie Museum, Pittsburgh, 2017; Yifu Museum, Beijing, 2019). However, given that any material can be abraded by the wind, a natural form of sandblasting, samples may only be found in displays that focus on mechanisms of landscape evolution. Not all rounded, sculpted forms in a desert setting are necessarily formed by the action of wind (Meek and Dorn, 2000).

The chosen examples were amongst many lying upon the surface of a black sand (outwash of volcanic ash and lava) devoid of trees or shrubs: constant access to the wind favours ventifact generation, and in this environment the most abundant macroscopic life forms are lichens and transient low-lying plants, which offer little shelter from flying sand grains. The lithologies were not analysed in detail, but are evidently all of volcanic origin, with variable development of feldspar phenocrysts and vesicles (gas bubbles trapped in the magma as it cooled). The pale grey tints suggest relatively felsic (siliceous) compositions. Quite probably the ventifact material is rhyolite from the nearby, ice-shrouded volcanic edifice of Oraefajökull, which unleashed dangerous historical eruptions in 1362 and 1727 (Guttormsson, 2011, p.50).

Ventifacts are generally encountered in hot deserts such as the Mojave of California (Howard et al., 2001) and in cold deserts, such as the Canadian Arctic (Mackay and Burn, 2005). No exposed rocks are spared, not even exotics such as lunar meteorites in Oman (Korotev, 2012)! Ventifacts and the weathering rind known as desert varnish may also be encountered at archaeological sites (Waters, 1992). The typical waxy-lustred, medium brown smooth desert varnish is often only a fraction of one mm thick, but is also seen upon thousands of the "NWA" meteorite finds recovered from the Sahara, in the broad region referred to in the meteorite context as North West Africa.

The last Ice Age would have brought cold desert conditions to much of the northern hemisphere, generating ventifacts in regions of milder climate today, such as upstate New York (Mills and Wells, 1974; Van Diver, 1976). Ventifacts are generally noted on today's landscapes, but they may occasionally be recognized in ancient strata. An example is the Bunter (sandstone facies) of the Triassic in the British Isles. Characteristic angular, three-faced pebbles known as "dreikanter" may be seen in the Bunter, as in modern Namibia (Wells and Kirkaldy, 1966, pp.323-327). Based on this analogy, the angular pebbles presumably arose in sand dunes in Triassic deserts. Evidence of wind ablation extends back to Archean times. It has been suggested that some 5% of the pebbles in the gold-bearing Witwatersrand conglomerates in South Africa may be ventifacts (Minter, 1995), including some pebbles in the Carbon Leader unit (McCarthy, 1995).

Figs. 5-6: Further examples of both smooth and larger, deeply scoured, pitted and fluted ventifacts.