I have always thought of sand dunes forming on beaches, where the waves and wind pile the sand up just beyond the edge of the water. But how do inland sand dunes form? Based on the two I have seen, Great Sand Dunes National Park in southern Colorado and Bruneau Sand Dune State Park in southern Idaho, they form in a basin where the wind piles sand up near a mountain or rise. The Great Sand Dunes formed in a valley from sand left behind by ancient lakes, which the prevailing wind piles up in front of the mountains; regular opposing winds keep the sand out of the mountains and pile the dunes higher. The Bruneau Dunes are formed by sand-laden wind blowing over a smaller basin and dropping sand; again, regular opposing winds help keep the sand in place and push the dunes higher.
Different wind patterns create different dune shapes. When there is a strong prevailing wind and occasional crosswinds, it creates seif dunes, long and swordlike. Moderate one-way winds produce transverse dunes, long, wavy dunes perdendicular to the wind direction. Strong one-way winds create parabolic dunes where sand is plentiful and barcham dunes where sand is scarce; they are both crescent-shaped, but face opposite directions. The Bruneau Sand Dune is a stellate, or star, dune, created by winds coming from different directions and forming several arms to the dune.