Millions of years of Earth time and Earth water create a natural arch, rib bone of Earth—red sandstone, pale limestone, dark basalt—by ﬂaking small pieces of rock off a slender wall until a hole ﬁnally forms. Water, the agent of erosion, dissolves the rock and gathers in its small cracks and fractures, freezing and expanding, loosening rock grains sometimes too small to see. Arches are Earth clean to the bone. A person walking through one walks through Time. Land arches are most common in the Southwest, particularly in the Utah canyonlands and the Four Corners area. Sea arches occur in coastal bluffs, where it’s the constant pounding of ocean waves that wears a hole through a promontory wall. Natural bridges are a type of arch, but they are created in a different way. Instead of rainwater and snowmelt, it’s the current of a stream or river that eventually cuts a hole in the rock. A century ago it was believed that, along with water, wind was a major force in the erosion of arches and natural bridges, but—strange as it may sound—wind plays no role.