system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why in sand dunes is there no gravel or small pebbles? Only sand for miles? Wouldn't there be small pebbles at least?
gpt
Wind pushes the grains around.  The heavy stuff doesnt move as well as the light stuff, so it gets left behind.  The too light stuff gets blown away completely.  What is left is sand, too heavy to fly away completely but light enough to move alot.