What shapes the coastlines?

Waves

Explanation:

Coastlines are formed due to the continuous battering of the coast by waves. These waves cause soil erosion when they take some coastline material back to the water as they recede. They also deposit some material on the coast when they make contact.

This causes the coastlines to take different shapes as the process goes on and on. Some coasts will be harder to shape however when the waves will be unable to erode the coast so easily because those coasts are made of harder material like rocks. That's why rocky coastlines stay more of less the same for a long period of time.