The Geology of Cape Lookout National Seashore: Barrier Islands on the Move

Cape Lookout National Seashore protects an intact barrier island system on North Carolina’s Outer Banks, part of the Atlantic Coastal Plain, where physical, geological and ecological processes still run the show.

The seashore spans three long, narrow islands: North and South Core Banks, stretching northeast of Cape Lookout, and Shackleford Banks, running at right angles to them west of the cape. The Atlantic lies to the east and south; Pamlico, Core and Back Sounds lie behind; Beaufort Inlet and Ocracoke Inlet bound the ends. The islands hold estuaries, mudflats, salt and freshwater marshes, ponds, maritime forest, grassland, dunes and beaches — habitat for federally protected species and a great variety of birds.

Young islands on an old foundation

Geologically, the Outer Banks are young. Their foundation was built by repeated swings in sea level:

• About 5.3 million years ago, the sea stood much higher and laid down marine sediment across today’s coastal plain.
• From 2.6 million years ago, sea level rose and fell many times as ice sheets grew and melted, and rivers cut channels into the older marine layers — a buried landscape still reflected in the shape of the sounds and the position of the islands.
• Over the last 10,000 years, rising seas filled and buried those old channels with new sediment.

The ground beneath the islands is therefore a complex mix: compact peat and mud, loose to partly cemented sand, gravel and shell beds. Storms, waves, tides and shifting inlets keep reworking it, so much of the present Outer Banks is less than 3,000 years old.

Two kinds of barrier island

Forces that shape the islands

• Sediment transport is much higher along Core Banks. Waves, wind and storm surge move sand through inlets and along and across the islands. Overwash raises the islands, extends marshes and creates early-succession habitat. New sand comes from ancient river channels, the Cape Lookout shoals and Pleistocene deposits offshore.
• Inlets open when storm surge breaks through from ocean or sound. Tidal currents build flood and ebb tidal deltas, vital to the islands’ sand budget and marsh growth.
• The sounds differ: Pamlico Sound provides fish nurseries, feeding grounds and seagrass; Core Sound is very shallow, with water levels set by wind and tide; Back Sound is well flushed by tides near Beaufort Inlet.
• Groundwater is a lens of rain-fed fresh water floating on denser salt water, shielded by dunes from small storms. Freshwater ponds occur on North Core Banks and western Shackleford Banks.

Geologic map of Cape Lookout National Seashore. Image from NPS’s page.

Rising seas

As sea level keeps rising, low-lying parts of the park face more erosion, overwash, new inlets and shifting wetlands, with effects that vary with the geology beneath. A large rise could push the islands past a tipping point — migrating landward faster, breaking up, shrinking or even going under water. Warmer temperatures and more variable rainfall could bring both more frequent droughts and worse flooding and erosion.

Fossils on the beach

Pleistocene shells are common on North Carolina beaches, and telling fossils from modern shells can be tricky. Stained shells are considered fossils: brown ones are generally younger than black ones, though shells can blacken in oxygen-poor mud in as little as three weeks. The clam Mercenaria turns from chalky white (modern) to yellow-orange (Holocene) to gray-black (Pleistocene). Boring by other animals, wear and chemistry also help.

Fossils also wash in from offshore: a late Pleistocene terrace in 9–12 m (30–40 ft) of water yields sponges, bryozoans, worm tubes, barnacles, echinoderms and mollusks, and offshore ridges of coralline algae up to 26,250 years old add to the sand. Nearby Middle Marsh and Back Sound hold estuarine shells and echinoids dated from 2850 BCE to 380 CE. All fossils in national parks are protected by the Paleontological Resources Preservation Act of 2009.

A shell-strewn beach. Image from NPS’s page.

Sources

Based on "NPS Geodiversity Atlas—Cape Lookout National Seashore, North Carolina," National Park Service; a work of the United States government in the public domain.