The natural agent of erosion mainly responsible for the formation of barrier islands along the southern coast of Long Island, New York is wave action.

Here's why:

* Wave action: Waves are the primary force shaping coastal landscapes. They erode the shoreline, transport sand, and deposit it in new locations. The powerful waves along the Long Island coast have played a crucial role in:

* Creating the sand: Waves break down existing rocks and cliffs, creating the sand that forms the islands.

* Transporting the sand: Waves carry the sand along the coastline, eventually depositing it in offshore bars.

* Building the islands: Over time, these offshore bars grow and merge, forming the barrier islands that we see today.

While other natural agents like wind and tides also play a role in shaping coastal environments, wave action is the dominant force in barrier island formation.