A tombolo is formed when a spit connects the mainland coast to an island. A spit is a feature that is formed through deposition of material at coastlines. The process of longshore drift occurs and this moves material along the coastline.
Material is pushed up onto beaches at an angle when the swash brings it onto the coastline at a 45 degree angle. The backwash takes it back out towards the sea at a right angle to the coast.
Through this process material is constantly moved along the coastline. When the coastline changes direction or there is a river estuary the process of longshore drift continues. This causes material to be deposited in a long thin strip that is not attached to the coast and is known as a spit. If this feature moves in the direction of island and connects it to the mainland then it becomes a tombolo.