The Mystery of the Silver Bluff Shoreline

Scientists give names to prehistoric sea shorelines along the Atlantic Coast.  Logically, we know the oldest shorelines are the ones located farthest to the west because otherwise subsequent high sea stands would have obliterated them.  The location of the Georgia shoreline during part of the mid-Wisconsinian Interstadial (~46,000 calender years BP-~30,000 calender years BP) is called the Silver Bluff Shoreline, and it roughly corresponds with our modern day shoreline.  This correspondence puzzles scientists.  During the mid-Wisconsinian Interstadial, glaciers covered most of Canada about as far south as the Great Lakes, and therefore sea level should have fallen many miles to the east as it eventually did between ~29,000 BP-~7,000 BP.  The carbon dating of Silver Bluff organic material is consistent, eliminating the possibility that scientists have been misled by contaminated dates.  Some scientists suggest a tectonic explanation for why sea level was as high as today when it should have been much lower due to the glacial expansion.  There is some evidence of this–buried oyster beds in the St. Mary’s River have been found in sediment that dates to a time period when the ocean should have regressed far to the east.  However, I have a different hypothesis.

I believe sea level regression lagged behind glacial expansion.  Though more and more of earth’s moisture was becoming locked in glacial ice, there was still enough atmospheric precipitation and groundwater to keep the ocean filled with water comparable to modern levels until some critical threshold was reached.  Perhaps this threshold was the Great Lakes themselves.  As long as the Laurentide Glacier remained north of the Great Lakes, there was enough atmospheric moisture to keep sea levels high, but once this glacier advanced over the Great Lakes and froze all that water, sea levels dropped dramatically.  Sea levels remained low until 7,000 BP.  Then, enough water was released from melting glaciers for the sea  to rise to modern levels.

The Silver Bluff Shoreline is colored red.  The oldest shoreline is colored yellow and dates to the Pliocene (~2 million BP).

Scientists have taken numerous cores of sediment from several areas of St. Catherines Island, Georgia that date to this mid-Wisconsinian Interstadial when sea levels were as high as they are today despite cooler average annual temperatures.  The northern part of St. Catherines Island was part of the Pleistocene-aged Silver Bluff shoreline but currents are eroding into this part of the island, and this sediment is accreting on the southern side of the island.  (Most of Georgia’s barrier islands are of Pleistocene age on the north side and of Holocene age on the south side.  The Holocene side is built from eroded Pleistocene sediment.)  The pollen record from these cores reveal some interesting and unique environments that existed on St. Catherines Island during the interstadial.  The most unusual core sample was taken from Yellow Bank Bluff.  The pollen composition consisted of 41% composites (flowers from the aster family), 17% alder (a shrub related to birch), 10% grass, 5% ragweed, and with the presence of pine and hickory.  There is no natural community in the modern day south that even closely resembles this.  The closest modern analogue would be a midwestern tag alder wetland.  Just visualize an open grassy meadow covered in a variety of colorful wildflowers and thickets of alder shrubs and widely spaced trees.  Megafauna foraging likely had a great influence on this landscape.

Yellow Bank Bluff on St. Catherines Island.  This was a much larger hill well above sea level between ~29,000 BP-~7000 BP.  It was connected to Guale Island which didn’t become submerged below rising sea levels until as recently as 4000 years ago.  The most unusual core sample of ancient Pleistocene pollen was found here.

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A tag alder wetland in Wisconsin.  An environment like this existed on St. Catherines Island, Georgia about 40,000 years ago.

Cores taken from other sites on St. Catherines Island reveal less unexpected environments.  The St. Catherines Shell Ring core shows that pine and oak were equally common in a diverse forest growing next to a salt marsh.  Pollen from this core also included hickory, sweetgum, tupelo, birch, elm, cypress, grass, ferns, and salt marsh chenopods.  Beech was a surprisingly common component here.  Beech no longer occurs on the Georgia coast.  Today, the nearest population of beech grows on north facing bluffs along the Savannah River.  An abundance of grape and aquatic plant pollen suggests a cypress swamp covered with grapevines.  The ample fern and grass pollen is evidence of a fire-influenced environment such as a freshwater “prairie”, not unlike those found in the modern day Okefenokee Swamp.  The core from the Cracker Tom Hammock indicated an environment similar to that from the Shell Ring Core.

Several sites from Skidaway Island have also been cored and analyzed for pollen composition.  They also show that southeastern coastal plain environments have long hosted a stable diverse composition of pine, oak, hickory, cypress, and sweetgum, but during the cooler interstadial northern species such as beech, alder, spruce, and even hemlock were present.

Scientists have recently investigated the Central Depression Savannah on St. Catherines Island.  This was a beautiful 5 square mile wet meadow with many freshwater springs.  This site is considered to be of unusual and unknown origin.  Unfortunately, it was drained during the 1930s.  Pollen analysis of Pleistocene aged sediment here revealed a forest consisting of myrtle, heaths (blueberry or rhododendron), pine, oak, sweetgum, and a significant presence of hemlock, though the latter pollen may have been windblown from the coastal plain.  All of these studies suggest the floral composition of this region has remained stable for tens of thousands of years, but northern species did colonize this area during cooler climatic stages.

See also https://markgelbart.wordpress.com/2013/03/25/pleistocene-fossils-found-on-the-georgia-bight/

Reference:

Rich, Fred; and Robert Booth

“Quaternary Vegetation and Depositional History of St. Catherines Island”

From Geoarcheology of St. Catherines Island, Georgia: Proceedings of 4th Caldwell conference 2009

Anthropological Papers of AMNH #94

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One Response to “The Mystery of the Silver Bluff Shoreline”

  1. The Paleoenvironment of the Georgia Bight when it was Above Sea Level (~80,000 BP-~7,000 BP) | GeorgiaBeforePeople Says:

    […] the geology of this region, might have an answer to the mystery of the Silver Bluff Shoreline (See: https://markgelbart.wordpress.com/2014/06/01/the-mystery-of-the-silver-bluff-shoreline/).  This shoreline is thought by some to date to about 38,000 BP, a time when most of Canada was […]

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