Erosion is visible on these cliffs   at Caumsett State Park.

Erosion is visible on these cliffs  at Caumsett State Park. Credit: Newsday / Bill Davis

On a summer afternoon, the sun-dappled view of azure sky and turquoise sea stretches uninterrupted to an impossibly distant horizon.

But the view south from Montauk Point is a portal to the past for the scientists who study Long Island’s natural history. They see something that isn’t there anymore: a great ridge of sand and rock that constitutes what may have been the vanished third fork of the East End.

Local geologists have conflicting ideas about when and how a glacier built that ridge, but there’s no argument about what obliterated it. Ocean erosion has been an inescapable fact of life on Long Island ever since the slowly rising sea began pounding away at that now-vanished ridge, probably about 12,000 years ago.

The relentless surf has not only chewed up cliffs and thinned beaches, it has also drastically reshaped Long Island over the centuries. And while Long Islanders remain focused on fighting the short-term effects of erosion, sometimes in costly and controversial ways, the Island’s history teaches the humbling lesson that over the long run the sea will not be denied.

The missing ridge south of Montauk is perhaps the most dramatic example of erosion’s power on Long Island. It is probably part of the terminal moraine, the line that marks the place where, 22,000 years ago, a glacier finally stopped expanding southward. As the great ice sheet retreated, it left a ridge of bulldozed rubble in its wake. Most of that moraine is still visible today as the elevated central spine of Long Island that roughly parallels the Long Island Expressway. But farther east, geologists believe, the moraine dipped south and left the present-day Island at Amagansett before curving northeast and continuing on to Martha’s Vineyard.

As it receded back toward Canada, the glacier paused several times and deposited several recessional moraines that were so tall and wide that they became the Montauk peninsula and the North Fork. Whether there were three forks on the East End is uncertain, since no one knows if water was ever in between the Montauk peninsula and the now-vanished moraine farther south. But what is certain is that a bouldery ridge on the ocean floor is all that remains of the missing moraine.

”That’s an enormous amount of land. The fact that the entire terminal moraine is gone is a very impressive thing,’’ said Les Sirkin, a research professor of earth science at Adelphi University. “In 12,000 years, several miles of terminal moraine have been totally eroded away.’’

No section of Long Island’s 1,180-mile coastline is immune from erosion’s effects, but the Montauk peninsula has been hit hardest because it is unprotected by barrier islands and lies squarely on the historic pathways of nor’easters and hurricanes. Over thousands of years, the area has been narrowed so severely that Long Island’s axis has shifted like a weathervane from east-west to northeast-southwest. Massive amounts of sand have been taken from the vanished moraine and from the cliffs at Montauk and pushed west, blown by the prevailing northeast winds.

On a placid summer day at Jones Beach or Fire Island, it’s hard to envision so much sand moving so quickly from east to west. But, in fact, at various spots along the South Shore, anywhere from 100,000 to 600,000 cubic yards of sand are pushed westward every year, according to studies by New York State. Storms blowing in from the west can briefly reverse the flow, but the dominant trend is for the sand to move from Montauk toward the Rockaways.

The North Shore isn’t as volatile. Calmer seas and the shore’s many harbors all serve to keep more sand in place, so the sand transport rate anywhere along the shoreline is never greater than 100,000 cubic yards per year.

But the natural drift of sand along its North and South Shores can’t be the only reason Long Island is slowly wasting away. It can’t explain why the calmer North Shore is losing an average of one or two feet of shoreline every year, and so are parts of the South Shore that should be benefiting from the river of sand flowing west from Montauk.

What is happening, experts say, is that Long Island’s coasts are also being hammered by a double whammy of rising sea level and inept erosion-control efforts by humans.

The ocean is slowly rising by about one foot per century, part of a 20,000-year trend that began when the glacier melted as it retreated toward Canada. What Long Islanders perceive as a vanishing beachfront is actually, in part, a gradually rising water line. The trend has worsened erosion by slowly raising the platform for the devastating storms that periodically strike the Island. During the very worst storms, waves can crest as much as eight feet higher than normal — enough to wash away more than 100 feet of beach in a few hours.

But even the nastiest winter storms cannot equal the havoc humans have wreaked through misguided erosion control efforts. On the South Shore, especially, the east-to-west flow of sand has been badly disrupted by the 69 major groins and jetties built to protect favored stretches of beach and keep six inlets navigable for boats. The most infamous example is in Westhampton, where the construction during the 1960s of 11 rocky groins projecting into the ocean has led to devastating erosion along Dune Road, just west of the sand-grabbing groins.

“We’ve made a lot of mistakes in the past,’’ said Jay Tanski of the New York Sea Grant Institute at the State University at Stony Brook. “The interruption of the natural sand flow has caused very severe problems and the most spectacular erosion that we see.’’

Even the U.S. Army Corps of Engineers, which built many of those groins and jetties, now favors ``soft’’ erosion-control methods such as building up protective dunes and pumping sand across inlets to avoid interrupting the natural sand flow. “I certainly don’t like structures,’’ said Joseph Vietri, deputy planning director of the corps’ New York office. “It’s true that most of the structures that have been built over the years to control erosion would never be built today.’’

But even those “softer’’ solutions won’t be able to hold off coastal erosion indefinitely, Vietri acknowledged. Beach renourishment projects may be economically justifiable if they can hold the beach in place for 30 or even 50 years, he said. But over the longer term, if the sea continues to rise and current erosion patterns continue, controlling erosion will become so expensive that the only feasible option will be a strategic retreat.

“It’s questionable whether there will ever be enough money to keep the shoreline from retreating over the long term,’’ Vietri said. “If you look beyond 50 years, it becomes very hard to justify any of these projects.’’

Newsday LogoSUBSCRIBEUnlimited Digital AccessOnly 25¢for 5 months