Geologically speaking, Long Island is quite young. This 22,000 year old mound of sediment was created when the last southern most ice sheet retreated leaving behind all the rocks, sand and clay it pushed ahead of it. The sea level began to rise around this mound of glacial till resulting in the elongate island you see today.

Evidence of the terminal moraine are the hills that are oriented in an east-west fasion beginning in Bay Ridge, Brooklyn and run through central Long Island and into the Atlantic Ocean near Montauk Point. North of these hills are more east-west trending ones showing where recessional moraines formed while the ice sheets slowly retreated.