Cape Cod's Shape Constantly Changing, for Good and Bad

By Heather Goldstone, WGBH.org

The Cape Cod National Seashore's Marconi Beach in Wellfleet, which has seen the effects of erosion on the beach cliffs. Credit: Cape Cod Times/Steve Heaslip

This story was originally published in the Cape Cod Times as partof a four-day series of stories, photos, videos and interactives that examine the Cape Cod National Seashore as it turns 50-years-old. The Cape Cod Times worked in conjunction with WCAI, the Cape and Islands NPR radio station, on the series.

Henry Marindin may not have the same name recognition as Henry David Thoreau, but he commands a dedicated following among local geologists.

Between 1887 and 1889, Marindin led a team of surveyors who traversed the length of what Thoreau called Cape Cod's “great beach,” now the Cape Cod National Seashore. He recorded the details of the expeditions in 13 leather-bound journals.

Marindin's work is more than a historical curiosity. The information he gathered is helping scientists predict how Cape Cod's coastline will change in the future.

Using old but accurate surveying techniques, Marindin's team generated beach profiles every 1,500 feet between Chatham to Provincetown.

“They came on the train,” explained Mark Adams, the geographic information specialist at Cape Cod National Seashore. “Then they hired a horse, and then they rode out to these stations on the coast and built tripods out of wood and sent rowers out in dories and sighted from their coastal tripod to the dory. They were able to triangulate using these old methods.”

Each of the 229 profiles the team created started at the top of a bluff or dune along the Outer Cape and extended approximately a mile offshore. Marindin compiled his topographic data and estimates of erosion rates in a scientific treatise with a title that resonates today: “Encroachment of the sea upon the coast of Cape Cod.”

Marindin's survey — which found an erosion rate of about 3 feet per year — along with two more recent surveys conducted by Adams and Graham Giese of the Provincetown Center for Coastal Studies, has provided unparalleled insights into the evolution of the Cape's coastline, particularly the rates and processes of change.

“You can't understand the change of today without having this foundation in how the process works over centuries,” Adams said.

Cape Cod is often described as a flexed arm, but that shape is relatively new in geological terms. Since its creation nearly 15,000 years ago, the forces of erosion have reworked Cape Cod's shoreline ceaselessly.

Five thousand years ago, the Outer Cape looked more like a triangle with the tip somewhere around High Head in Truro. Wind, waves and rising seas gradually wore away both the inner and outer coasts, leaving a narrow “arm.”

The “fist” of Provincetown is a direct product of that erosion, growing by the accumulation of sand washed off the Atlantic-facing beaches to the south. The narrowing arm and growing fist, as well as a highly changeable elbow, have been the hallmarks of erosion in recent times.

But Adams said we're entering a new era of change. Human-caused global warming is expected to ratchet up the intensity of coastal storms, making dramatic erosion more likely. In addition, the rate of sea level rise has tripled in the past century, from 1 to 3 millimeters per year.

“That's a huge increase,” he said, “even if it's in millimeters.”

The pace of sea level rise continues to accelerate. Many scientists now expect the sea level to rise at least 3 more feet by 2100. That increase could spell disaster for low-lying homes and roads, but it won't affect all areas of the coast equally. Understanding which areas will be most impacted is crucial for effective planning and adaptation.

But a single snapshot of erosion rates isn't sufficient to make those predictions. Rather, scientists need to understand the processes that have shaped Cape Cod over centuries and millennia, and how they will respond to rising water levels.

For the past 15,000 years, Georges Bank has served as a protective barrier for Cape Cod, blocking large ocean waves coming from the southeast. Now, the water over Georges Bank is deep enough to allow those waves to make landfall. That's accelerating erosion at southerly beaches, pushing more sand northward and slowing erosion at beaches up the coast.

“Basically it means that Cape Cod is rotating clockwise about half a degree per millennium,” said Adams. “Put in those terms, it seems like ‘who cares,' but it actually has some really noticeable effects.”

Whether or not those effects are negative is a matter of perspective. Adams said homeowners and town planners have good reason for concern, but stresses that rising sea levels rise and erosion don't actually harm coastal ecosystems. Without hard structures there to block its movement, the beach simply retreats while retaining its natural shape and character.

“In fact,” Adams said, “if you're on the beach now, it might appear to be the same beach that Henry Marindin saw 120 years ago. It's just in a different place.”

With its long stretches of undeveloped coastline, Cape Cod National Seashore is an ideal place to study coastal changes without the added stress of worrying about homes and infrastructure.

“We're in a fantastic position,” Adams said. “We can watch it erode, we can measure it, we have this exciting laboratory for change here – a chance to see nature in action.”

Read more about the Cape Cod National Seashore at Heather Goldstone's blog, Climatide. Republished with permission.
Editor's Note: This content is not available for syndication under Climate Central's republication guidelines.