How Was Long Island Formed?
The surface of Long Island was formed at the end of the last glacial maximum, or the time of the furthest extent of the ice sheets during the last ice age, around 21,000 years ago. As the ice sheets advanced from the Arctic, they entrained, or carried along, sand, gravel, and other material which was continually supplied to and accumulated at the melting margin of the ice front, similar to a conveyor belt. As the climate warmed, they began to retreat northwardly leaving piles and layers of sediment. The loose gravel and sand carried here as the ice sheets scraped and ground down rock completely cover the bedrock; therefore, no significant rock formations are located on Long Island. Ridges formed by the glacier stretch across the island as a terminal moraine. The Ronkonkoma Moraine forms the backbone through the center of the island. South of this begins the glacial outwash plain along the south shore. During the irregular retreat of the glacier, it left behind a second moraine: the northernmost, named the Harbor Hill Moraine, which runs along the north shore. The glacial history of Long Island explains the difference between the two shorelines, with bluffs and rocky beaches along the Sound and sandy beaches along the Atlantic Ocean. Other glacial features can be found on Long Island, such as Lake Ronkonkoma, which is a kettle lake that formed when ice was buried by sediment, melted, and left a depression that eventually filled with water.
The melting and retreat of the glacier caused by warming of the climate, led to increased sea level. While the surface of Long Island may have existed 21,000 years ago, the island as we know it was not there; our shorelines have a dynamic history as they have moved drastically along the continental shelf. As more meltwater entered the ocean, sea levels continued to rise, Long Island became an actual island and seawater filling in the low areas eventually gave it the distinctive shaped shoreline that we see today. The story of sea level rise continues as climate change is still shifting Long Island’s shoreline.
