Deserts & Sand Dunes.

The Igneous section's Coconino Sandstone entry mentions "ancient desert sand dunes" as its origin story, but nothing on the site actually explains how wind builds a dune, or why deserts look the way they do. Worth completing that thought — and it's the natural third entry alongside water and ice as the major forces that erode and reshape Earth's surface.

Wind is a much weaker erosional force than water or ice, but deserts give it room to work uncontested: without much vegetation or moisture to hold loose material in place, wind can pick up and carry sand more freely than almost anywhere else on Earth. Wind erosion works two ways: deflation, where wind simply lifts and removes loose, fine material, leaving coarser rock and gravel behind (this is how a "desert pavement" of packed gravel forms, once wind has stripped away everything smaller); and abrasion, where windblown sand grains themselves act like sandpaper, scouring and shaping exposed rock — a slower-motion, drier cousin of the same abrasion mechanism a glacier uses.

Dunes form once that windblown sand has somewhere to accumulate — typically behind any small obstacle that slows the wind down. A dune grows as sand blows up its gentler windward slope and cascades down the steeper leeward (downwind) side, which is why dunes are asymmetric, and why an entire dune field can slowly migrate across a landscape over time as this process repeats, grain by grain, dune by dune. The layers left behind inside a dune — thin, angled bands called cross-bedding, tilted in the direction the wind was blowing — are exactly the "fossilized" wind-blown structure still visible today in ancient wind-formed sandstone like Coconino Sandstone, millions of years after the actual sand stopped moving and became rock.