The Spectacular Narrows Flood of 13,350 BP

A few bands of Paleo-Indians probably roamed the Georgia coast about 13,350 years ago.  The shoreline then may have been as much as 50 miles east of where it is today because much of earth’s water was locked in glaciers and glacial lakes.  The climate here was pleasant–brief mild winters and long warm summers that didn’t often reach the maximum temperatures of today.  Paleo-Indians living along the coast enjoyed a varied diet of fish, shellfish, turtles, sea birds, sea mammals, as well as big game and wild plant foods.  One summer day, a group of Paleo-Indians spear-fishing, netting, and swimming in the shallow coastal waters may have noticed that the water was much chillier than normal.  The water may have gotten so cold, they decided to stop fishing and swimming.  But they didn’t need to spearfish, the sudden change in temperature stunned and killed millions of fish which suddenly washed ashore, providing more food than they could eat.  The Paleo-Indians living here had no idea why the water suddenly turned cold on a warm summer day, but an Indian standing on a bluff overlooking what is now New York Harbor might have figured it out.

18,000 years ago, the Laurentide Glacier reached its maximum extent, and it towered a mile over what today is the Manhattan skyline.  As it expanded, it pushed dirt and boulders forward forming a long hill known as a moraine.  The glacier buried and froze most of the Hudson River, however, it did not reach the Atlantic Ocean.  Instead, a narrow strip of land existed between the massive glacier and the Atlantic Ocean.  This strip of land likely consisted of a mosaic of grassy steppe, spruce forests, and wetlands and was inhabited by mammoth, horse, bison, and near the coast seal and walrus.  The part of the Hudson River not under ice cut a valley through here.

Canebreaks 008

Map of the submerged Hudson River valley incised when the land here was above sea level during the Ice Age.  The continental shelf was dry land habitat then.  Map from the below referenced paper authored by Robert Theiler and others.

Manhattan skyline.  Note how some buildings are much shorter than others.  This is not perchance.  The tall ones are located on top of hard Paleozoic aged rock.  The short ones are located on softer Pleistocene sediments.  Taller buildings couldn’t stand on the softer, younger sediments without falling over.

Climate warmed and the Laurentide Glacier began to retreat.  But it left behind the Harbor Hill moraine which formed a natural dam across Long Island and Staten Island.  This dam created several glacial lakes.  Lake Passaic covered much of what today is northern New Jersey, and its depth was astounding, ranging from 150-240 feet.

The Narrows–the entrance to New York Harbor.  And the Verazzano Bridge which connects Long Island with Staten Island.  A moraine once blocked this exit acting as a dam which formed several glacial lakes.

Map of the glacial lakes formed by the Harbor Hill Moraine.  The gray area represents dry land that was inundated when the moraine was breached by meltwater.

Further upstream on the Hudson River, ice dams formed 3 major glacial meltwater lakes–Lakes Iroquois, Vermont, and Albany.  Summers kept getting longer and wamer until ~13,350 years BP when the ice dam forming Lake Iroquois collapsed, releasing a torrent of freshwater that carried chunks of ice, whole trees, big boulders, and any wildlife unable to scramble out of the way.  This tremendous flood scoured the Hudson River Valley, exposing rock outcroppings still visible today.  The flood smashed through the Harbor Hill moraine, obliterating the part of the hill that had connected Long Island with Staten Island.  The Hudson River surged through its old valley to the sea where the massive influx of cold freshwater shut down thermohaline circulation–the ocean current that moderated much of Europe’s and eastern North America’s climate.  This led to a sudden drop in average annual temperatures from north Florida to Canada and precipitated the Younger Dryas cold snap which equaled temperatures endured during the Last Glacial Maximum.  (Temperatures from south Florida to the equator rose because tropically warmed ocean water stayed there instead of circulating north.)

Observed from a safe distance, the natural cataclysm that created the narrows entrance to New York Harbor must have been an awe-inspiring sight.  It’s a shame video cameras hadn’t been invented yet.  Nothing like this flood has occurred within recorded history.  Evidence is found strewn all over the bottom of New York Harbor which became the resting spot for tons of sediment, boulders (dropstones), trees, and animal bones that fishermen occasionally dredge up.

Most of the Harbor Hill Moraine is still intact and can be found on Long Island and in New Jersey.  Only the part that went across the narrows was breached and destroyed.

The Paleo-Indians living on the coast of Georgia then were soon forced inland as rising sea levels inundated their favorite beaches.  They had no way of knowing the rising sea was tied to the unusually cold water they felt on a summer day a few months earlier.

References:

Merguerian, Charles

“The Narrows Flood–Post Woodfordian Meltwater Breach of the Narrows Channel, NYC”

http://www.geo.sunysb.edu/lig/Conferences/abstracts-03/merguerian-03.pdf

Thieler, Robert; et. al.

“A Catastrophic Meltwater Flood Event and the Formation of the Hudson Shelf Valley”

Paleogeography, Paleoclimatology, and Paleoecology (241) 2007

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One Response to “The Spectacular Narrows Flood of 13,350 BP”

  1. James Robert Smith Says:

    Those glacial lakes must have been amazing sights before the dams burst and cut them loose.

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