Wind rarely carves mushroom rocks; it mostly shifts sand along the ground
Desert mushroom rocks and wind caves look like the work of sandblasting gales, but geologists now blame other causes, such as uneven weathering and rainwash. Windborne sand mostly stays within half a metre of the ground and almost never rises above two metres, so it cannot sculpt tall rock very far up.
Geologists call wind-driven change aeolian, after Aeolus, the Greek keeper of the winds. Worldwide, flowing water erodes far more than moving air, but wind matters wherever plants are sparse, soil is dry and loose sediment is plentiful. That means deserts above all, yet also shorelines, glacial outwash and river valleys in semiarid country; the downwind sides of the Platte, Arkansas and Missouri valleys are blanketed with dunes. Off-road 4x4 driving speeds wind erosion further.
Deflation, the lifting of loose grains, works three ways. Bouncing hops, called saltation, account for perhaps half to 70 percent of it; fine particles carried aloft in suspension make up 30 to 40 percent; and heavier grains rolling or sliding along the surface supply 5 to 25 percent. Where fines are stripped away, a sheet of stones called desert pavement is left behind and shields the ground beneath. Such stony plains make up the regs of the Sahara, divided into rocky hamadas and gravelly serirs.
Deflation also scoops out hollows known as blowouts. The smallest are dimples about 30 centimetres deep, while Mongolian examples reach 8 kilometres across and up to 100 metres deep. Wyoming's Big Hollow stretches 14 by 9.7 kilometres and plunges as deep as 90 metres. Wind also delivers much of the sediment on deep ocean floors. Loess, windblown silt, is common in humid regions, and sand and loess from Ice Age glacial outwash underlie much of North America and Europe.
Abrasion does leave some marks. Yardangs, streamlined ridges up to tens of metres high and kilometres long, are grooved along the prevailing wind, usually in soft silts. Polished, faceted stones called ventifacts are rare, needing abundant sand, strong winds and bare ground. In parts of Antarctica, blowing snowflakes wear down exposed rock. As grains collide in flight they round each other and gain a frosted finish, and chipping off their clay coatings produces much of the world's dust between 2 and 5 microns across.
Source: Aeolian processes
