Rapid Sea Level Rise on the Georgia Coast

Glaciers expanded and sea levels fell during Ice Ages.  20,000 years ago, the Georgia coast was located 90 miles to the east of the present day shoreline.  The continental shelf off the Georgia coast (known as the Georgia bight) was above sea level between 80,000 BP-7,000 BP.  Near the end of the Ice Age, glacial ice melted and sea level rose rapidly but in stages.  The partial dissolution of glacial Lake Agassiz in Canada 12,900 years ago caused a sudden rise in sea level.  The Atlantic Ocean likely advanced toward the present day shoreline many miles in a few short years.  But an ice dam reformed, containing the rest of Lake Agassiz’s water until 8200 BP when it completely collapsed, releasing the rest of this great lake’s water.  (Lake Agassiz contained more water than all of the modern Great Lakes combined.)  This caused another sudden rise in sea level along the Georgia coast.  By 4500 BP the Georgia coast consisted of just 2 barrier islands separated by the southeasterly flowing Altamaha River.  The Ogeechee, Satilla, and St. Mary’s Rivers were tributaries of the Altamaha and didn’t directly drain into the Atlantic Ocean as they do today.  The Altamaha drained into a huge sound located to the east of where Little Cumberland Island stands today.  Sea level rose again, flooding and eroding the areas where the Ogeechee, Satilla, and St. Mary’s flowed into the Altamaha and splitting the 2 barrier islands into 10 smaller ones.  All of these rivers formerly flowed southeasterly, but the high marine transgression caused them to straighten to a more easterly direction.

trail map

Map of the  modern day Georgia coast.  About 4500 years ago, there were only 2 barrier islands between the Savannah River and the Florida border, and they were bisected by the Altamaha River which then emptied into a huge sound located at the same latitude as modern day Little Cumberland Island.  The other rivers in this region that today flow into the ocean were tributaries of the Altamaha.

Aerial view of Georgia salt marsh.

Aerial view of salt marsh in Glynn County, Georgia.  There are 4500 year old logs from a rapidly flooded forest underneath the surface.

Wood storks (Mycteria Americana) on a salt marsh behind Sapelo Island.  There are now 10 major barrier islands off the Georgia coast, but before 4500 years ago, there were just 2, separated by the Altamaha River.  A person using an excavator to dig here would probably find 4500-2000 year old logs in less than an hour.

The Holocene Climatic Optimum between 9000 BP-5000 BP was a warm climatic phase, especially at higher latitudes, resulting from both the northern hemispheric tilt of 24 degrees and the timing of the perihelion.  The earth was closest to the sun during the boreal summer.  Summers were much warmer but winters were cooler.  The effects of the Holocene Climatic Optimum were felt in different regions at different times.  The sea level rise on the Georgia coast occurred toward the end of this climatic phase.  The ocean’s response to the Holocene Climatic Optimum may be a factor explaining this marine transgression.  Tectonic processes (the shifting of the earth’s crust) and isostatic adjustment (the rebound and subsidence of the earth’s crust in response to the weight of glaciers) also probably played a role in this sea level rise.

Scientists studying the rapid rise of sea level on the Georgia coast find fossil trees beneath salt marshes, and they date to between 4400 BP-2000 BP.  St. Simon’s and Jekyll Islands were connected until 2000 BP.  The youngest fossil trees are found here.  This was one of the last areas of the coast to get inundated, though Ossabaw Sound was breached about the same time. There are abandoned river channels beneath salt marshes as well. Sea level is still rising but not at an unprecedented rate as global warming alarmists and political pundits claim.  Sea levels haven’t even reached the marine transgression high stand of the last interglacial known as the Sangamonian (~132,000 BP-~118,000 BP).

References:

Chowns, Timothy

“Drainage Changes at Ossabaw, St. Catherine’s, and Sapelo Islands and their Influence on Island Morphology and Spit building on St. Catherine’s Island”

American Museum of Natural History Anthropology Papers 94

Chowns, Timothy; and S. Hannah Hill

“Mid-Holocene Sea Level Rise on the Georgia Coast”

Georgia Journal of Science Abstracts 2014

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