Brookhaven Town as we know it today, is the result of glacier deposits and erosion that began about fifteen million years ago. The place where a glacier stops moving south and begins to melt is called a "terminal moraine," and is usually hilly and rocky. Brookhaven has two such moraines: running east to west along the center of the Town is the Ronkonkoma Moraine. Bald Hill is a part of this area. Along the north shore, the hills, rocky areas and cliffs that face the Long Island Sound are part of the Harbor Hills Moraine.
The glaciers also created large holes, known as "kettleholes,” which filled with water, forming Lake Ronkonkoma, the largest lake in Suffolk County, Artist Lake, Spring Lake (both in Middle Island), Lake Panamoka in Ridge, and others. The outwash plain that formed the South Shore of Brookhaven, resulted when the water from the melting glacier carried sand and gravel from the moraines, building up a wide sandy plain. Many streams and ponds formed in this area, including two of Suffolk's rivers, the Peconic and Connecticut (or Carmans), both of which are located within the Town.
The north and south shores are tall timber areas, covered mostly with oak and maple. The middle section is called the "Pine Barrens," with stands of scrub oak and scrub pine. This is an important area for replenishing our water supply through a process called "water recharge." Long Island is one of the few areas in the United States that obtains its water solely from the underground aquifer. The vegetation of the Pine Barrens filters the water which percolates down through layers of cleansing sand.
The barrier beaches, situated between the various bays and the Atlantic Ocean, from Brooklyn to Southampton, are constantly changing, due to the action of winds and waves. During severe storms new inlets may be opened and old inlets to the bays may be closed. These barrier beaches or islands provide protection for the south shore of Long Island.