system
Given the following post, respond as if you're commenting on Reddit:
human
What causes bays to form?

So, the overall shape of a continent is decided by tectonic movement, but what causes the shapes of the shorelines you see when you look closely at a map?
gpt
Usually the formation of bays is due to different types of rock. Where the rock type is more easily weathered away by the sea then a bay will form. Conversely, headlands will form where the rock types are less easily weathered.