SHIFTING BY ROGER SHEW
WHAT IS OUR COASTLINE MADE UP OF? If you scoop up a handful of beach sand, you will be mostly looking at quartz-rich,
clear-to-milky sand grains that are often 90-plus percent of the composition with some whole shells but mostly shell
fragments broken by wave energy. If you could go back in time, the island of Wrightsville Beach that you
are enjoying today would look very different. At times, the beach would have been seaward or landward
of its current location and you might have been in a forest or in ocean waters tens of feet deep.
SO WHAT DO THE GEOLOGY AND SEDIMENT TYPES REVEAL ABOUT WRIGHTSVILLE BEACH?
Let’s “dig” a hole to approximately 200 feet and describe what we see from base to top.
GROUND LEVEL
At 20-50 feet is the Castle Hayne confining unit separating the Castle Hayne
aquifer from the overly surficial deposits that include the suite of sand and mud
deposits that comprise the barrier island.
50 FEET BELOW GROUND LEVEL
At 50-100 feet is the Castle Hayne Limestone, comprised of variable amounts of
shells of mollusks, bryozoans, sponges and other organisms that lived in a normal
marine environment. Yes, around 35 million years ago our current beach would
have been in water depths greater than 30 feet. The Castle Hayne has excellent
aquifer characteristics but is subject to high chloride contents here and in north-eastern
New Hanover County.
100 FEET BELOW GROUND LEVEL
At about 100-150 feet is the Peedee confining unit, a low-permeability zone,
rich in clay, separating the underlying Peedee aquifer from the overlying Castle
Hayne aquifer.
150 FEET BELOW GROUND LEVEL
At more than 150 feet below the surface, you would encounter the Peedee Formation,
a late Cretaceous deposit — meaning it formed about 68 million years ago — made up
of gray, calcareous sands to clay-rich sands with some beds of molluscan shells. This
would have been an oceanic setting. The Peedee is a confined aquifer and is a good
aquifer at this location. It provides groundwater for Wrightsville Beach.
200 FEET BELOW GROUND LEVEL
WBM july 2017 42
A large shift in the beach
location has occurred dur-ing
the last 18,000 years as
there has been a change
from maximum continen-tal
glaciation to a warmer,
much less glaciated world.
Seawater was contained
in mile-thick ice sheets on
land. At this time, the beach
would have been more than
40 miles seaward of present-day
Wrightsville Beach. Our
current beach was a forest at
the time and rivers extended
much farther east. Since this
time, sea level has been ris-ing
with the melting of the
glaciers and establishment
of the current sea level, bar-rier
islands and coastline.