New Jersey’s geological ‘layer cake’
The State We’re In column by Michele S. Byers, Executive Director,
New Jersey Conservation Foundation.
The fictional character Forrest Gump famously remarked that “life is like a box of chocolates” … you never know what you’ll get.
New Jersey is more like a slice of layer cake. This state we’re in is composed of five distinct bands running diagonally across the state – natural regions formed by geological events millions of years ago.
In the state’s far northwestern corner is the top “layer” of our cake, the steep Ridge & Valley region, a result of waves of Appalachian mountain building in the the Paleozoic Era approximately 360 to 500 million years ago.
Moving southeasterly, the next layer is the oldest: the rugged Highlands, today only tiny remnants of giant mountains formed during the Precambrian Era more than 545 million years ago. The rocks of the Highlands region formed before any plant or animal life on land, and the first vertebrates were still about 25 million years short of swimming in the oceans!
The third layer is the Piedmont, whose shales, siltstones, and volcanic basalts are much younger. They date from the Triassic and Jurassic periods of the Mesozoic Era, between 145 and 250 million years ago, when a shallow sea expanded after Africa broke off and slowly sailed away from North America.
Southeast of the line between Trenton and New Brunswick is the Inner Coastal Plain, built primarily of sands of the seacoast during the Cretaceous Period that ended 65 million years ago.
The very “new” Outer Coastal Plain, east of a rough line from Freehold to Salem, is pure ocean bottom sand from the Cenozoic Era of the last approximately 60 million years.
Such diverse geography in a small state is incredible! Draw a straight line from the Delaware Water Gap to the beaches of Asbury Park, and that 80-mile line passes through over half a billion years of earth history, all recorded in the rocks!
The outermost reaches of the Outer Coastal Plain includes New Jersey’s iconic barrier islands: narrow, moving waves of sand that are among the state’s most popular summer destinations.
But did you know that barrier islands don’t stay in one place? These waves of sand constantly shift upward and westward.
If you dig straight into the sand at the north pavilion at Island Beach State Park, eventually you’ll hit the organic remains of a maritime forest that once perched on the ancient shoreline of Barnegat Bay. This buried forest existed only a few thousand years ago, when sea level was lower and Barnegat Bay’s barrier island was about three-tenths of a mile east of today’s location.
How do barrier islands move up and west? Sand is constantly blown and thrown onto the dunes by wind and storms, and trapped by beach grass. Beach grass is tolerant of salt, and it survives being buried alive by sprouting new roots from the base of its stems. Beach grass seeds are also excellent at colonizing newly deposited sand.
As storms blow from the sea, sand collects on the sheltered, westward side of dunes. New beach grass sprouts to stabilize these areas, while the eastern sides erode.
Barrier islands have always been on the move. It’s only recently that humans have tried to stop them. But whatever we do, the geophysical foundation of our barrier islands continues to change.
These changes are easily seen from above. If you look at an aerial photo of Holgate on Long Beach Island, you’ll see a striking contrast between the beach and dunes of the Holgate wilderness in Forsythe National Wildlife Refuge, which has naturally migrated westward, and the built portion of Holgate beach to the north.
Sea level is much higher now than when shore towns were built in the late 1800s, and it is becoming more expensive to replace those beaches with each passing year.
In places like Island Beach State Park and Forsythe National Wildlife Refuge, no damage was done by the recent January nor’easter because it doesn’t matter much if the beach moves inland. Habitat for rare shorebirds is actually improved by dynamic storm events at Sandy Hook, Island Beach State Park, and the Holgate wilderness beaches.
But in most other places, where permanent towns have arrested the westward beach migration, each big storm takes away more of the remaining natural beach. These days, man-made interventions like seawalls and sand replenishment projects are required to keep the Atlantic Ocean from meeting Barnegat Bay.
In a handful of decades, though, these projects will likely become impossible. Time marches on, and so do barrier islands, one of New Jersey’s fascinating geological features.
To help you visualize New Jersey’s five geological regions, I’ve attached a NJ Department of Environmental Protection map and also one from Wikipedia Commons (credit: JimIrwin at English Wikipedia).
Sandy Stuart Perry
New Jersey Conservation Foundation