Long Island’s formation: North Fork Outdoors

Glacial erratic on Long Island. (Credit: Chris Paparo/Fish Guy Photos)

Walk any of the North Fork’s beaches and you will notice they vary greatly from those found on our neighboring South Fork. Instead of fine sands, we have many stones, cobbles and huge boulders. Why is there such a difference, especially when there is only a short distance between the two?

To understand this change in topography, we need to go back in time to the Pleistocene Epoch (~2 million years ago). It was during this period of the Earth’s history when we experienced the last ice age. Large glaciers covered much of the planet’s continents. As the climate cooled the glaciers would advance, bulldozing the landscape as they grew. Large amounts of sediment (fine clays to huge boulders) were trapped in the ice flows and carried southward as they advanced. Approximately 55,000  years ago, our local glacier reached its southernmost advancement and started to slowly retreat. As it melted, runoff streams carried finer sediments such as sand and clay to the Atlantic Ocean which helped to create our sandy South Shore beaches.

There were two advances/retreats during this ice age. The first stopping point formed the Ronkonkoma Moraine, which is the hilly backbone section of Long Island that runs from Queens to Montauk and re-appears in the hills of Block Island, Martha’s Vineyard and Nantucket. The second stop created the Harbor Hill Moraine, which can be seen in the steep bluffs and boulder-strewn beaches of Long Island’s North Shore, Plum, Great Gull and Fisher’s Islands.

It is believed that the Long Island/NYC area was completely free of glacial ice approximately 12,000 years ago. Since then, Mother Nature has continued to change the face of the island that she created. Many miles of the Ronkonkoma Moraine that originally ran off of Montauk has eroded through coastal processes. Erected in 1796, the Montauk Lighthouse originally stood 300 feet from the ocean, today it stands a mere 80 feet from the water’s edge.

Duck Pond Point. (Credit: Chris Paparo/Fish Guy Photos)

Over thousands of years, the ocean has pummeled the shoreline of Long Island. As waves crash against the shore, boulders and rocks are tossed continuously and broken down into smaller and smaller particles, eventually creating fine sand. This same process is responsible for creating the sand found on a majority of the world’s beaches. Variations in the materials making up a beach will cause the color of the sand to vary greatly from location to location. For example, beaches along the tropics tend to be pink in color as they are made from crushed coral and seashells, while some Hawaiian beaches can be completely black, consisting of sand made from volcanic rock.  This wave action also erodes the bluffs of the North Shore, revealing large boulders known as glacial erratics. These huge rocks were carried great distances as the ice sheet grew and were left behind as the glacier melted.

In addition to creating sand and exposing rocks, wave action creates a current of water that moves parallel to the beach that is known as the longshore current. On Long Island’s South Shore, this current flows westward carrying with it fine sediments. Over time, as the sand is deposited, a network of long, narrow islands known as barrier islands were created. These islands protect the main land from the full force of the ocean’s power. Barrier islands are not exclusive to Long Island. They are found along most of the east coast of the U.S. and make up about 10 percent of coastlines worldwide.

Glacial erratic. (Credit: Chris Paparo/Fish Guy Photos)

Once established, barrier islands continue to grow in the direction of the longshore current. An example of this growth can be seen with Fire Island Inlet and the Fire Island Lighthouse that was built to aid mariners to the inlet’s whereabouts. When first built in 1826, the Fire Island Lighthouse stood on the east bank of Fire Island Inlet. Today, thanks to the longshore current, the inlet has “migrated” six miles to the west of its original location. To prevent this migration, many inlets have been stabilized with rock jetties. Even though jetties stabilize the inlet’s location, they can speed up erosion, especially on the down current side of the inlet. As the up current jetty traps sand, the beach widens. The down current side of the jetty becomes narrower as it is starved of sand replenishment. Over time, the beach down current from the inlet can become so narrow that the ocean can easily breach it.

Sediments were not the only objects left behind by the glaciers. As they withdrew, large chunks of ice broke free from the glacier and formed large, deep depressions in the landscape known as kettle holes. Overtime they filled with rain and ground water to form kettle lakes. The best known and largest of Long Island’s kettle lakes is Lake Ronkonkoma. There are several kettle lakes on the North Fork, with the largest being Laurel Lake in Southold.

It has taken great forces of nature to shape Long Island to how it looks today. Even though there are no signs of an impending ice age, these powerful forces will continue to shape the face of our shorelines for eons to come.

With a degree in marine biology from LIU/Southampton, Chris Paparo is the manager of Stony Brook Southampton’s Marine Sciences Center. Additionally, he is a member of the Outdoor Writers Association of America and the NYS Outdoor Writers Association. You can follow Paparo on Facebook and Instagram @fishguyphotos.