What primarily forms sand dunes?

• A. Water deposition
• B. Wind action
• C. Glacial grinding
• D. Volcanic ashfall

Answer: (B)

Sand dunes are shaped by wind-driven transport of loose sand. When the sand is dry and plentiful, the wind lifts grains and moves them by saltation (bouncing) and surface creep (rolling). As wind speed decreases or encounters obstacles, grains settle out and accumulate, building dune mounds. Repeated winds from roughly the same direction create a characteristic dune shape with a gentle windward slope and a steeper leeward face. Water deposition tends to make beaches and deltas, not inland dunes; glacial grinding yields various sediments without forming dunes in the same wind-driven way; volcanic ashfall can deposit layers but doesn’t primarily create the classic wind-formed dunes. This wind-driven process is why sand dunes form.