Formation of a Spit

Here is an example of an explanation of the formation of a spit.

1. A spit is formed by longshore drift (LSD), when the prevailing wind direction causes swash to push material up the beach at an angle, but backwash moves material back down the beach at 90 degrees.
2. This causes material to move in a zig-zag fashion.
3. Where there is a change in direction of the coastline (e.g. a headland or the mouth of a river) LSD continues in the same direction, leading to the deposition of a thin stretch of material extending away from the coastline. This is a spit.
4. If the wind direction changes, the direction of LSD can change, resulting in a spit with a recurved end.
5. Salt marshes and mudflats build up in the calm, shallow water behind the spit, and sometimes a lagoon forms.

Note, the formation of a tombolo and a bar can be explained in a similar way, but for a bar, the spit extends until it rejoins the coastline (across a bay or river mouth) and a tombolo develops until it joins an island.