Why do some coastlines have cliffs while others are flat and sandy?

Coastline shape depends on the balance between erosion and deposition. Hard rocks like granite or basalt can stand as cliffs because waves undercut them but cannot remove them quickly. In contrast, sandy coasts are built by sediment delivered by rivers, longshore drift, or offshore sources, which allows beaches and low barrier systems to persist. Sea level, storm energy, and rock type all influence the result. This is why dramatic cliffs and broad beaches can exist in very different settings along the same ocean.