On the Origin of Parabolic Dunes 
Near Friar's Head, Long Island, New York

Introduction

An analysis of the Grandifolia Sandhills

(Long Island, New York) reveals that a series of dune forms and an associated endemic forest dominated by American Beech (Fagus grandifolia) located on and adjacent to Long Island Sound bluffs exceeding 100 feet in height are probable Late Pleistocene climatic relics. Information derived from both historic U.S. Coast and Geodetic Survey charts and U.S. Geological Survey digital elevation maps demonstrates that the Sandhills are parabolic dunes that have been truncated by long term erosional recession of the Long Island north shore.

Audrey Watson Wigley

Dwarf American Beech (Fagus grandifolia) as found at Baiting Hollow, Long Island

Extrapolation of original dunefield structure indicates that many individual dunes spanned a dunefield that was at least one mile in width and that all originated from a northwesterly sand source that appears to have been drowned by the post-glacial eustatic rise of sea level. Shore face retreat has removed more than half of the initial length of many surviving Grandifolia dunes, a process that could only have occurred over thousands of years.

Large parabolic dunes similar to these occur in contemporary periglacial regions. The Grandifolia Sandhills apparently began to form approximately 18,000 years before present when the last periglacial climate affected coastal New York

Many of these dunes ascended the east-west trending spine of the Harbor Hill Moraine (Fuller, 1914) and were stranded in their present position when winds powerful and persistent enough to move them dissipated as the climate ameliorated.

Although tundra vegetation probably helped trap and bind loose sand to create these parabolic dunes, when the climate warmed enough to support trees Fagus grandifolia--possibly in a dwarf form living at the edge of the range of the species--may have been the earliest tree to colonize. Once established, this species gradually modified surficial dune sand into a soil that promoted its domination and tall form.

As shore face retreat undercut dune and forest over thousands of years the parent Fagus grandifolia population tenaciously evolved--or reverted to-- a unique dwarf morph that may have been the pioneer.

Although dwarfism today occurs on these dunes other than where the forest is undercut, the pygmy form appears to grow in particularly stressed micro-environments that are distal from water or where soil may be thin such as dune crests. Dwarf trees are most abundant where the fabric of the forest runs down the steep Sound shore bluff to be continually cut back by slumping and other effects of storm waves. This biogeologic phenomenon appears to be globally singular. Preservation for scientific research, study, and education is recommended.

History

Previous reports on the possible geological origins of a series of sandy hills overlooking Long Island Sound near Friar's Head, Riverhead, New York have not provided interpretations that simultaneously explain both these hills and the apparently unique presence of a forest dominated by dwarf and other growth forms of the American beech (Fagus grandifolia).

Most workers, including Lewis (1876), Fuller (1914), and Black (1998), have concluded that these features are dunes.

Recent analysis by Terchunian and Sirkin (1999) suggest that the Grandifolia Sandhills have glacio-fluvial cores that are merely veneered by locally derived wind-blown sediment which became mobile as a result of mid-19th century land clearing activities. This explanation does not reconcile with clearly observable geomorphic and bio-geological phenomena.

USGS map circa 1838

USGS map circa 1885

Note that on each of these historic maps mature trees were recorded for the dune area.

Discussion

The idea that the Grandifolia Sandhills is no more than a thin veneer of sand resulting from wood harvesting activities south of the eroding bluffs during the time interval of the last hundred or so years (Terchunian and Sirken, 1999) is not supported by any evidence. Historic maps show cultivated fields adjacent to sand hills that appear to have been continuously vegetated. With the exception of laborious clearings for farm fields, the wood cutting activities of the nineteenth century typically did not injure or remove tree roots. Without root removal vigorous re-growth, not soil erosion or abundant loose sand, would have been the expected result of wood cutting. This conforms with the observations of Good and Good (1970) who concluded that "None of it (the Grandifolia Sandhills) was ever farmed, although timber was cut to supply firewood for New York City in the late 1800's".

The oblique "shadow effect" on a colored Digital Elevation Map (DEM) based upon a U.S. Geological Survey 71/2^o quadrangle shows numerous northwest-southeast trending parabolic dunes both east and west of Friar's Head. Because the curvilinear shapes of these structures also define the boundaries of the Grandifolia Sandhills, it appears that these relief features of aeolian origin. The DEM clearly shows that this field of dunes is truncated by the erosional scarp that characterizes the eastern half of Long Island's north shore. This truncation not only demonstrates the pre-modern origin of the Grandifolia Sandhills, but suggests that the north fork of Long Island was a once much more extensive land surface across which these dunes migrated.

The slip faces of upwards of ten large parabolic dunes are preserved where they collided with the north side of the Harbor Hill Moraine and each other. It appears that many climbed to its height before stranding occurred. Before encountering the moraine most of these dunes were apparently thirty to more than fifty feet high. Ground penetrating radar, coring, examination of primary bedding structures, detailed mineralogic analysis, and other research should determine the full extent to which the Grandifolia dunes are comparable to modern cold climate parabolic dunes of the high latitudes.

Retreat of the Late Wisconsinan glacier from southern New England is believed to have occurred between 20,000 to 15,000 years before present. This occurence inevitably caused a dramatic climate change; the region's previous wind and weather patterns adjusted. As the climate warmed, the dunes were trapped and bound by the subsequent colonization of Fagus grandifolia.

The prevailing wind that created these dunes came from the northwest and must have been both powerful and persistent. While the prevailing wind during Long Island's stormy winter season is today from the northwest, typical modern storms are relatively weak and have not created dunes  like these anywhere along the Long Island Sound shore. Modern hurricanes, while potentially powerful enough to create parabolic dunes forms on Long Island, typically track from the south and are of limited frequency and duration. The Grandifolia Sandhills are, therefore, relics of a dramatically different glacial fringe regional climate. Within this context, the Grandifolia Sandhills exist despite, rather than because of, modern wind patterns.

Close up of dune sand from the Grandifolia Sandhills. Note the high degree of sorting. Preliminary examination of this sand including its narrow range of grain sizes and shapes, color, mineralogic composition, and other characteristics suggests that it may be a secondary deposit of Cretaceous sand. Based upon dune orientation it is inferred that the primary Cretaceous deposit or source was in the north-central portion of what is now the Long Island Sound when sea level was lower and portions of this basin were sub-aerially exposed.

Glaciofluvial deposits typical of the study area. Exposure is in an excavation immediately south of the Harbor Hill moraine near the DeFriest Blowout. Note the wide range of particle sizes.

Dunes that are similar to the Grandifolia Sandhills in size, shape, relief, and orientation are found at Hither Hills State Park near the eastern edge of Napeague Bay in East Hampton Town on Long Island's south fork. As with the Grandifolia Sandhills, no modern source of sediment has been convincingly identified for the Napeague Dunes. Expansive growth of Fagus grandifolia is not evident at the Napeague Dunes where an oak-pine forest performs a similar function. While this and other vegetational differences distinguish these two dune fields, the comparable size and shape of the dunes, their northwest-southeast orientation and other geomorphic similarities are striking. Further investigation is needed to determine whether the Napeague Dunes are of mixed ages or if they may all have been created when sea level was low and a common tundra landscape connected what is now the North and South Forks of eastern Long Island to sand sources in the Long Island Sound Basin.

Conclusion

The relic vegetation and parabolic dunes at and near Friar's Head are unique. They appear to be among the rarest of landforms still extant from a now vanished climate and time when Long Island's landscape was both exceedingly cold and vegetationally primitive. The globally singular association of Fagus grandifolia with this ancient climatic relic duneform offers an exceptional opportunity for scientific research and study. Preservation of the Grandifolia Sandhills would be consistent with the wise decision previously made that created preservation of the Napeague Dunes. Such preservation would provide opportunity for the public to appreciate and enjoy an important and remarkable part of their natural history heritage. Link here for a proposed sequence of events for the development of the Grandifolia Sand Hills during the late Wisconsin to the present. 

Eastern Long Island showing the location of all known parabolic dunes in coastal New York. Note that these dunes, found only at Riverhead and East Hampton, are similarly oriented. The Riverhead dunes appear to have been climbing the north side of the Harbor Hill Moraine when a changing climate stranded them. Still undetermined is whether the dunes at Napeague also originated from a now submerged sediment source in the Long Island Sound Basin.If so, they could have advanced to their present position after moving through one of the gaps in the easternmost Harbor Hill Moraine.The subsequent rise of sea level has flooded both the Long Island Sound and Peconic Bay estuaries. On each fork it has also constructed connecting beaches across morainal gaps to form tombolo complexes. On the South Fork, rising sea level has elevated the water table, created extensive freshwater wetlands, and apparently begun to drown the Napeague Dunes.

Area of Baiting Hollow, Town of Riverhead, showing large parabolic dunes that have been significantly truncated by erosion associated with long term shoreface retreat.

Historical Accounts of the Grandifolia Sandhills

References

Addendum

 Direction of prevailing wind            

Typical Parabolic Dune

Parabolic dunes are formed when wind causes loose sand to coalesce into a mobile, crescentic-shaped landform as above. Vegetation typically anchors the out lying perimeter causing "trailing tails".

Parabolic dunes are unique among the known range of dune forms in that the relationship between vegetation and dune appears to be causal. The importance of plants to the formation of parabolic dunes can be inferred by their complete absence on the otherwise dune-rich Desert Planet, Mars--where there are no plants.

Aerial photo of parabolic dunes at White Sands, NM. Wind direction is from lower left to upper right. Note how parabolic symmetry is dependant upon presence of vegetation. Conversely, see how other dune forms are present where vegetation is absent.