<span>Earthquake waves that vibrate from side to side and up and down only through solids are known as secondary waves, or simply S-waves. As you can see in the question, they pass through solids - to be more precise, they flow through rocks and cause them to change their shape. They are quite strong, but not as strong as primary waves, as they cannot pass through liquids or gases. </span>
Wind erosion is the main cause. And since it is the desert that wind can pick up tiny sand rock particles that slowly grind against the rock. Over time the erosion will give a jagged rock the smooth features as if it had been submerged in a river for the roughly same period of time.
