A mud lion is a geographic feature which forms in a desert area. Mud lions are created when winds packed with abrasive sand repeatedly scour rock, creating outcroppings of material which can often develop into into fanciful shapes, especially when people look at them with imagination in mind. The development of mud lions is often a sign of desertification, as they are associated with hot, dry areas with steady winds; in areas with abundant natural moisture, mud lions cannot form, since the wind cannot pick up dried dust and dirt.
As a general rule, a mud lion forms from soft rock, like limestone or sandstone, although mud lions can also be formed from harder materials. It can take centuries for a mud lion to appear, and often they appear in clusters, like ridges along the desert's surface. Mud lions are also known as yardangs, from a Turkic word which is used to refer to steep banks. “Yardang” entered the English language in the early 1900s.
In order for a yardang to form, conditions must be hot and dry, and the wind must be steady. The wind must also come from a constant direction, because otherwise the rock would be equally worn down, simply melting away rather than forming a distinctive outcropping. These conditions are common in many parts of Africa, especially in the Egyptian desert, where mud lions are quite abundant.
As the slang term “mud lion” suggests, yardangs often erode into shapes which look like people, animals, or structures. Some historians have suggested that the Sphinx may have started out as a mud lion before being enhanced by Egyptian stoneworkers. Some people refer to mud lions as natural Sphinxes, since many of them do end up looking rather Sphinx-like, thanks to the fact that they tend to develop a tapered shape, with the windward end being much bulkier than the lee end.
In addition to being found in Africa, it is also possible to see mud lions in North America, where they abound in national parks which include desert areas. These yardangs can vary widely in size and shape, from monolithic and relatively new outcroppings to small, old nubbins. On occasion, conditions are right for a hole to be bored through a mud lion, creating a tunnel through the outcropping which can make quite a remarkable sight.