Bays What is a Bay made of? Hard and soft rock. How are Bays formed? - There are various ways that bays can be formed.  The largest bays have developed as a result of continental drift (movement of the earth’s continents). Largest bay in the world is the Bay of Bengal. Another way bays are formed is via glacial and river erosion. Fjord = bays formed by glaciers. Rias are created by rivers and are characterised by more gradual slopes. Most gulf and bays are formed by the folding of the earth’s crust as well as coastal erosion due to waves and currents. Currents can make waves more constant, while soft rocks will allow erosion to have a stronger effect. Where would a bay be found in relation to other landforms? Near a headland. Gulf of California is an example of a bay created by the geological process of folding.