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.

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

Advertisement

The Beauty of Pleistocene Swans (Cygnus buccinator)

Photo from google images of a trumpetor swan.

No animal symbolizes the beauty of the Pleistocene more than the trumpetor swan (Cygnus buccinator).  I suppose we can consider it a stroke of good fortune that this species didn’t become extinct with the spectacular megafauna of that bygone era.  Contemporary efforts to protect the bird and help re-expand its range have even been moderately successful.

Modern range map of the trumpetor swan.  It occurred as far south as South Carolina during colonial times.  During the Pleistocene there was likely a sizeable population of this species in the southeast where it is completely absent today.  Fossils of this species have been recovered from northern Alabama and Florida.  Overhunting by men extirpated this species from much of its former range.

Before European settlement of North America trumpetor swans were more common and widespread, migrating as far south as South Carolina during severe winters.  But during the Pleistocene they ranged even further south.  Bell Cave in northern Alabama and several sites in Florida have yielded fossils of this bird.  Ice Ages provided ideal habitat for this species.

Audubon mentions that trumpetor swans prefer a moderate climate.  Ice Age summers in the south were generally cooler and winters were still moderate, so swans would have had a favorable climate in this region then.  It seems likely that Pleistocene trumpetor swans bred and nested on the abundant glacial lakes near the boundaries of the great ice sheets. Then during winter they didn’t have far to migrate because the distance between glacial lakes and favorable wintering habitat in the south was much less than the distances they have to travel today.  Perhaps a segment of the population remained and nested in the south year round, much like modern day sandhill cranes of which some migrate and some are permanent residents.

Map of the Laurentide glacier.   During the LGM swans didn’t have far to migrate between summer nesting grounds near glacial lakes and winter habitat in the south.  Some segments of the population probably lived year round in the south.

Swans inhabit ponds and small lakes with aquatic plants growing on the bottom upon which they feed. Extensive beaver ponds and marshes, and oxbow lakes were the kinds of abundant habitat in the south available to the big birds then.  Swans nest and take cover on beaver dams, muskrat lodges, and islands where they’re relatively safe from mammalian predators. If hungry enough, mammalian carnivores will expend the energy to swim and search for food in wetlands, but it’s not their first choice when looking for an easy meal.  During stadials, islands on braided rivers were common, giving swans lots of favorable habitat.  This wouldn’t have kept swans safe from eagles, however.

Photo from google images of a bald eagle killing a swan.  Eagles of several kinds were common during the Pleistocene.

Grinnell’s crested eagles, golden eagles, and bald eagles were capable of hunting and killing swans during the Pleistocene.  But swan defense mechanisms were adequate enough to maintain substantial populations.  Swans weigh up to 30 pounds and a blow from their wing is powerful enough to break human bone.   They can also flee by submerging and swimming for some distance.  They’re most vulnerable to eagles and human hunters when in flight.

Swans have an interesting method of feeding.  While they swim on the water, they lower their long necks to reach the aquatic plants growing on the bottom.  Because they have longer necks than geese, they can outcompete them for food by eliminating all the fodder within a goose’s reach.  Swans also graze grass on land; and they eat snails, reptiles, and small mammals.

If I could live during the Pleistocene (Part one)

For many years now I’ve often fantasized what I would see, if I could really travel in time back to the Pleistocene.  A quick trip of a few hours wouldn’t give me enough time to study the plants, animals, and climate of that time.  Instead, it would be necessary to spend many years there.  In fact I would like to spend a lifetime living in that long gone unspoiled wilderness.  Nevertheless, I’m not a big fan of roughing it, and I don’t want to give up the creature comforts of modern civilization.  I just don’t want to live without such things as college football, good books and movies, the internet, certain foods, and modern dental care.  So let’s suppose there’s a kind of time portal or wormhole that connects the modern world in 2010 with a time period, say about 38,000 radiocarbon (or about 41,000 calender) years before present.  I would live in a dwelling that I would construct 41,000 BP, but for example if I get a painful cavity, I would travel through the wormhole to the dentist’s office.  Moreover, I’d have wires running through the wormhole so I could communicate with the present.  I could sit in my 41,000 year BP home and watch modern television and communicate through phone lines and the internet with the modern world of 2010.

Why I would choose 41,000 BP

I would choose a place in what’s now Georgia before people lived in the region.  I don’t fear the megafauna–I can avoid rampaging mammoths and packs of dire wolves–but primitive men do worry me.  They could be cannibalistic or maybe they’d choose to kill me just for the hell of it–who knows what a primitive man’s motivation might be?  Besides, I want to study the ecology before humans had any impact on it at all.  Therefore, 41,000 years ago is probably a safe bet for avoiding humans in southeastern North America.  According to archaeologists, concrete evidence of humans in the region prior to 14,000 calender years ago is scarce.  Before 19,000 calender years ago, it’s nonexistent.   If there were people on the continent 41,000 years ago, which is highly doubtful, they were so few in number I’d doubt we’d cross paths.

Another reason why I’d choose 41,000 calender years ago is climate.  This time period was an interstadial–a somewhat warmer and rainier phase of the Wisconsinian Ice Age than a stadial or glacial phase.  Though temperatures are still cooler than those of today, for about 2,000 years they were warmer than the coldest time of the Ice Age.  When I originally conceived of this fantasy, I thought I might like to live during the last Interglacial, about 120,000 years ago.  However, the heat of the current summers in Georgia are uncomfortable enough, and the Sangamonian interglacial was even warmer.  I decided I’d rather live during a time when Georgia summers were much cooler than those of today.  The following is a link with a graph depicting climate change over the past 40,000 radiocarbon years.  Scientists are able to estimate past average temperatures based on the gas content found in air bubbles encased in a glacier in Greenland.  Scientists take cores of this 100,000 year old glacier and can count the years which correspond to the rings in the ice created by melting in summer and addition of ice in the winter, much like a tree’s age can be calculated by counting the number of rings in the trunk.  The ratio of heavy to light oxygen isotopes correlates to exact average temperatures.  Using this method, scientists know what temperatures were thousands of years ago.  Analysis of carbon dioxide and the amount of dust particles also give evidence of past climate.  Here’s the link:

http://www.mos.org/soti/icecore/studies.html

See the first upward spike in this graph.  That’s the time I would choose to live.  Note the precipitous drop in temperatures after the interstadial ended.  Average annual temperatures must have dropped drastically within a few decades.  Note also how steady average annual temperatures have been over the last 11,000 years (the Holocene) compared to the Pleistocene.  These sudden changes in climate from moderate and rainy to cold and dry must have had a profound effect on the composition of plant species and the distribution of animal species.  In recorded history climate fluctuations that are merely a blip compared to the huge shifts during the Pleistocene have had devastating effects on agriculture.  A period known as the little Ice Age lasted from 1320-1870.  Cold rainy summers ruined crops and caused mass starvation.  Imagine what would happen to modern agriculture, if average temperatures suddenly dropped by more than ten degrees F as happened frequently during the Pleistocene. 

Fluctuations in Pleistocene temperatures correlate with the waxing and waning of the Laurentide glacier which covered most of Canada throughout the last Ice Age.  Interstadials resulted from when warm temps melted the glacier which put more moisture in the atmosphere; stadials occurred when this melting ice caused massive amounts of cold fresh water and icebergs to flood the north Atlantic which shut down the gulf stream, causing a sudden reversal in climate.  More moisture became locked in ice and the atmosphere became more arid and sea level dropped.

Georgia’s summer temperatures were much cooler and more comfortable during the interstadial than thay are in the present, but winter temperatures in the southeast were probably only a little cooler than they are today, thanks to the gulf stream off the Atlantic coast, which scientist believe created a warm thermal enclave in the region, especially near the coast.

During the interstadial I’d expect to find a mix of 75% forest and 25% meadow or small prairie in what’s now east central Georgia.  I’ll have more on the  ecology and landscapes of my chosen place of habitation next week along with a discussion of the geographical location of my Pleistocene homestead, and the adobe house I would build.