Here's a breakdown of why:

* Barrier Islands: These are elongated, narrow islands of sand that run parallel to the mainland coast. They are separated from the mainland by a lagoon or bay.

* Formation: Barrier islands form due to the deposition of sand by ocean currents and waves. They act as natural barriers, protecting the mainland from the full force of storms and erosion.

* Long Island's Characteristics: Long Island fits the definition of a barrier island. It's a long, narrow strip of land separated from the mainland by Long Island Sound. It also has a distinctive sandy shoreline and is subject to ongoing erosion and deposition.

While Long Island has some hilly areas, its overall shape and formation as a barrier island are its defining characteristics.