system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do some stretches of coastline have beaches while others do not?
gpt
(oversimplified) Some beaches(or coastlines) are erosional and some are depositional. The erososional rocky beaches have cliffs, no or little sand, while depositional beaches (where sand and sediment are deposited at a rate that exceeds erosion) have barrier islands shores lagoons and such.