The Harbor Hill Moraine, in the

geography of Long Island Long Island is in southeastern New York (state), New York, United States, separated from the rest of the state by the East River and from Connecticut by Long Island Sound. Long Island contains four counties, of which the western two are The fi ...

, forms the northern of two ridges along the "backbone" of

Long Island Long Island is a densely populated continental island in southeastern New York (state), New York state, extending into the Atlantic Ocean. It constitutes a significant share of the New York metropolitan area in both population and land are ...

Description

The Harbor Hill Moraine, skirting the North Shore, represents the

terminal moraine A terminal moraine, also called an end moraine, is a type of moraine that forms at the terminal (edge) of a glacier, marking its maximum advance. At this point, debris that has accumulated by plucking and abrasion, has been pushed by the front e ...

of the most recent advance of the Wisconsinian glaciation, which reached its most southward advance about 18,000 years ago; the earlier Ronkonkoma Moraine, much cut through by outwash streams from the Harbor Hill Moraine, lies to the southeast. The Harbor Hill moraine is represented by the North Fork of eastern Long Island and in three disjunct sections farther east, Plum Island, Great Gull Island, and Fisher's Island. The western end is the Heights of Guwan in Brooklyn. The Harbor Hill Moraine, named for its prominence at Harbor Hill, Roslyn, New York, the highest point in Nassau County, resulted from a lingering equilibrium stage in the glacier's episodic retreat, creating a stationary melting front; the Long Island area became permanently free of glacial ice in the range of 13,000 to 12,000 years ago. The Harbor Hill Moraine intersects with the earlier Ronkonkoma Moraine at Lake Success. East of the lake, the two moraines may be seen, but west of Lake Success the Harbor Hill moraine overrode and effaced the Ronkonkoma moraine.Garvies Point Museum and Reserve: the Geology of Long Island
/ref> South of the moraines is a broad

outwash plain An outwash plain, also called a sandur (plural: ''sandurs''), sandr or sandar, is a plain formed of glaciofluvial deposits due to meltwater outwash at the glacier terminus, terminus of a glacier. As it flows, the glacier grinds the underlying r ...

sloping towards the

Atlantic Ocean The Atlantic Ocean is the second largest of the world's five borders of the oceans, oceanic divisions, with an area of about . It covers approximately 17% of Earth#Surface, Earth's surface and about 24% of its water surface area. During the ...

, much of which has been submerged as ocean levels rose in the post-glacial period. The Harbor Hill Moraine forms the major drainage divide in Nassau County. Streams to the northwest of the moraine flow into

Long Island Sound Long Island Sound is a sound (geography), marine sound and tidal estuary of the Atlantic Ocean. It lies predominantly between the U.S. state of Connecticut to the north and Long Island in New York (state), New York to the south. From west to east, ...

, whereas those to the south empty into the series of bays or sounds behind the barrier islands and so into the Atlantic.

References

{{Commons, Harbor Hill and Ronkonkoma Moraines, Harbor Hill and Ronkonkoma Moraines Geography of Long Island Moraines of the United States Landforms of Suffolk County, New York

Description

See also

References