FAYETTEVILLE, Ark. — Beneath the sandy shore of Nags Head, N.C. lies a river system that flowed across the continental shelf to the ocean during the last ice age, according to a University of Arkansas researcher. Such geology underlying North Carolina’s Outer Banks still influences the present and future of the barrier islands.
The researchers examined a 15-by-25 mile area off the shores of Nags Head and Kitty Hawk, N.C. They cruised back and forth across the site in a boat towing geophysical equipment, creating a grid of survey lines about two miles apart. As they moved across the water, they used sound waves to produce "images" of the seafloor and the layers beneath it. They also used side-scan sonar, which produces images that look almost like an aerial photograph.
When processing the images, the researchers identified features that indicated a series of channels buried below the present-day sea floor. They carefully located the boundaries of the channel system and created a map showing the extent of an ancient river system that once flowed across the now-submerged continental shelf to the sea. Core samples taken at dozens of locations, were used to verify the boundaries of the river system. The layers that looked like channels show clear evidence of sand, mud and shell deposits, indicating a former river system that became an estuary as sea-level rose over the continental shelf 10,000 years ago.
Boss and his co-workers were able to correlate offshore strata with layers onshore and in the modern estuarine system that other researchers studied.
"We wanted to bracket the time when this channel might have been active," Boss said. The initiation of the channel system appears to correspond to layers that are 75-80,000 years old – dates that coincide with the beginning of the last ice age, and a time when sea level began to fall, ultimately dropping 125 meters lower than it is today, Boss said.
Boss likens the ancient channel system to a flood plain like that of the Mississippi River. This plain would have drained into the Atlantic Ocean farther off the current coast, but because of the higher sea levels today similar rivers end in estuaries on the continental side of North Carolina’s barrier islands.
Even though the evidence of the ancient river system lies buried under smoothed-over layers of mud and sand, evidence of its influence on the coastline can be seen by looking at the current geology of areas such as Kitty Hawk Bay.
"Where islands are narrow, often a river channel has cut through there in ancient times," Boss said. "The underlying geology still controls the present-day look of the islands."
Source: University Of Arkansas. September 2001