Posted on January 12, 2021
The Ghosts of Glaciers Past
Cape Cod, Mass., is famous for being a summer beach paradise but owes its entire existence to ice. During the Wisconsin Glaciation, the final period of the last ice age, the Laurentide Ice Sheet covered all of what is now eastern Canada and New England in ice up to 3 kilometers thick. The southern reaches of this ice sheet extended down the Atlantic coast to an archipelago of glacial landforms collectively called the Outer Lands: Long Island, Staten Island, Cape Cod, Martha’s Vineyard, and Nantucket Island.
“At the peak of the last glacial, where I am sitting was under ice,” said Robert Thieler, director of the U.S. Geological Survey’s Woods Hole Coastal and Marine Science Center in Woods Hole, Mass. As it advanced and retreated along the coast, this mass of ice plowed enormous piles of sediment and glacial fill along its front. “These terminal moraines make up the long east-to-west spine of Cape Cod, as well as the north-to-south oriented moraine that runs under Woods Hole,” Thieler said.
Around 23,000 years ago, the Laurentide Ice Sheet reached its maximum extent and then started retreating, leaving a highly malleable landscape in its wake. Radiocarbon dating indicates that the bulbous landmass that would become Cape Cod was likely ice free by around 18,000 years ago.
The pockmarked scars of this retreat can still be seen on the cape in the form of hundreds of kettle ponds. Kettle ponds form when chunks of ice break off from a retreating glacier and get packed in sediment, which insulates the ice. As the ice slowly melts, it creates a round, water-filled depression. On the cape, kettle ponds often host cranberry bogs, where wild cranberry vines take root in the organic matter that collects in the pot holes. Archaeological evidence indicates that the Wampanoag people harvested wild cranberries from kettle ponds on the cape as far back as 12,000 years ago.
Drowning George’s Bank
Even after the ice sheet retreated from the coast, so much water was still locked up in ice that global sea levels remained much lower than they are today. Several huge lobes of land were exposed along North America’s eastern seaboard, including two lobes south and east of what is now Cape Cod. At times, these lobes were forested and occupied by megafauna and Indigenous peoples; mammoth bones and ancient artifacts are habitually dredged up far off the current coastline.
As the ice age continued to wane, sea level rose in pulses. During some of these pulses, sea levels rose so rapidly that geologists don’t have a clear picture of what the processes of coastal erosion and inundation would have looked like. “It’s a fascinating problem for geologists because the process we are trying to understand is destroying the record that we need to study to learn about it,” Thieler said. “It’s a very fragmentary record out there on the continental shelf. Finding those fragments and stitching them together into a compelling story is a big challenge.”
Models show that one of the major turning points in the shaping of Cape Cod was the inundation of Georges Bank, said Graham Giese, an oceanographer and sedimentologist at the Provincetown, Mass., Center for Coastal Studies. Now a famous offshore commercial fishing ground, Georges Bank was once the outermost lobe of the Outer Lands. “As sea levels rose and waves started washing over Georges Bank, the increasing wave energy reversed the direction of sediment transport on the cape,” Giese said. Previously, sediment had mainly moved along the outer shore of the cape from north to south, driven by northeast storms coming down from the Gulf of Maine.
“Back then, there was no hook at the end of the Cape,” Giese said, referring to the upraised fist of Cape Cod, where Provincetown is located. However, as more wave energy started crossing over George’s Bank, sediment started moving from south to north, and the Provincetown hook began growing.
From around 6,000 years ago, when both climate and the existing ice caps stabilized, the planet has experienced a period of fairly stable sea levels. Radiocarbon dating of the marshes on Cape Cod indicates that the cape’s distinctive shape likely emerged between 5,000 and 4,000 years ago. “A few thousand years is a blink of an eye, geologically speaking,” Thieler said.
A Shapeshifting Spit of Sand
Cape Cod’s shapeshifting continues in the present day. “Even as a kid, I understood that Cape Cod is not a permanent place. You see big changes happening all the time,” said Tim Famulare, an environmental planner for Provincetown. These changes are most obvious during storm surges, which can flood the cape and shift huge quantities of sand in a matter of hours, but nuisance flooding during high tides is also becoming more common.
In the winter, Provincetown is a quiet fishing village home to around 3,000 year-round residents. In the summer, however, the community at the hooked end of the cape turns into a completely different beast, hosting as many as 100,000 revelers on weekends. “Provincetown is a very dynamic place that withstands a lot of change between seasons,” said Richard Waldo, the town’s director of public works.
But just how much change the town can withstand remains to be seen. In 2016, Provincetown conducted a coastal resiliency risk assessment. “The most alarming findings were the projections for future sea level rise,” Famulare said. Since 1922, sea levels on the cape have risen by 28 centimeters, and projections for 2100 range up to 3 meters. “Provincetown is very densely developed, and as sea level rises and flooding events become more frequent, we really don’t have anywhere to retreat to,” he said. “Some of our most important assets, like our airport, are located in low-lying areas.”
On 4 January 2018, a higher-than-predicted storm surge delivered by a nor’easter during high tide inundated parts of Provincetown. “The last time we experienced flooding that severe was in 1978, so that kind of flooding was out of the memory of most of our residents,” Famulare said. “It was really a wake-up call for what will happen as sea level rises.” The upshot of that storm, however, is that “our grant work has since been very successful and we’ve been able to undertake several adaptation and mitigation projects to lessen the extent and impact of flooding on some of the areas that were hit during that storm.”
For example, town planners secured a grant for a dune enhancement project to elevate the beach west of MacMillan Pier, a low-lying area near the historic downtown that was a major inundation pathway for flood waters during the January 2018 storm event. But although the Provincetown hook is still growing, the incoming sand isn’t easily accessed for beach nourishment projects, Famulare said. “We have more sand coming in, and that’s a wonderful problem to have, but it comes into the wrong spot, and because of state and environmental regulations we can’t just move it to where it needs to go. Once it starts getting shellfish and beach grass and eel grass growing in it, that sand becomes a sensitive aquatic habitat, and we can’t just dig it up.”
Shipwreck Shacks on the Outer Cape
Provincetown is taking progressive steps to protect itself from future flooding, but the Cape Cod National Seashore, located on the other side of the peninsula on the Outer Cape, subscribes to a very different, hands-off approach, Waldo said. “The philosophy of the national seashore is to leave it alone and let nature take its course.”
This is the wilder side of Cape Cod that I am most familiar with, having spent time in a historic dune shack on the national seashore. In the late 1800s, when shipwrecks were still common on the shoals and sandbars off the coast of Cape Cod, a series of shacks was built along the Outer Cape to provide shelter and supplies to shipwrecked survivors. With better mapping and navigation, shipwrecks became less common, and the shacks began attracting writers and artists, including Henry David Thoreau, Jack Kerouac, and Jackson Pollock.
In 1961, when the Outer Cape became the Cape Cod National Seashore, the dune shacks, many in disrepair, were slated to be destroyed in an effort to return the seashore to its natural state. But the Massachusetts Historical Commission stepped in and recommended that the shacks be listed on the National Register of Historic Places. Today, the National Park Service owns 18 out of the 19 surviving dune shacks, several of which are available for artist residencies and long-term leases.
My dune shack was one of the smallest, a one-room shed with an outhouse and a hand-pumped well. Other shacks are more elaborate, but they all fit in with the dunes’ remote and wild character; you can easily imagine drenched and bedraggled shipwreck survivors dragging themselves to your doorstep.
The year I was in the dunes, a seldom-seen and ghostly relic from the shipwreck era reappeared near Race Point, the northernmost point of the cape. The wreck of the HMS Somerset, a British warship that ran aground on 2 November 1778, resurfaced as it had in in 1886 and 1973. The jagged, waterlogged timbers of the ship’s hull always emerge on the beach in exactly the same spot, a reminder that although change is endemic to the cape, some places are actually quite stable, Giese said.
The spot where the wreck of the Somerset lies in state actually represents the Outer Cape’s tipping point between erosion and accretion, Giese explained. Although the sandy beaches of the central arm of the cape are rapidly eroding, the dunes on the back of the hand and fist are still growing, as is the Provincetown hook.
The ever-growing dunes will keep Provincetown from becoming an island, cut off from the mainland, for as long as there is beach sand available to feed the dunes, Giese said. But although the wild dunes will endure, the fate of the road that connects Provincetown to the mainland is in doubt. “I do get nervous when I see sand washed up on the highway after a storm,” Famulare said. “As sea level rises and flooding events become more frequent, it won’t take as much of a storm surge to flood the road and cut us off from the rest of the cape.”
Keep Your Dukes Up
How long will Cape Cod keep its pugnacious fist raised against the rising seas of the Atlantic Ocean? “Cape Cod will cease to exist when sea level reaches the highest elevation on the landform,” Giese said. The highest point on the arm of the cape is Scargo Hill at almost 49 meters. Provincetown tops out at 28 meters, and parts of Truro dip as low as 8 meters.
However, because of uncertainties in climate models and the rates of sea level rise, predicting when the Atlantic Ocean will overtop those high points is anybody’s guess. “Current projections for sea level rise are in a range that the cape has not experienced since about 9,000 years ago,” Thieler said. “We simply don’t have good modern analogs for what that rate of change will look like.”
In his 1896 book The Outline of Cape Cod, the father of American geography, William Morris Davis, ventured a guess: “The Truro mainland will soon be destroyed and the sands of Provinceland will be swept away as the oceanic curtain falls on this little one-act geographical drama…10,000 years hence.” Modern projections indicate the curtain may fall even faster, Giese said. “William Morris Davis didn’t know about sea level rise, which is, of course, now the main player in this little geologic drama.”
—Mary Caperton Morton (@theblondecoyote), Science Writer
Living in Geologic Time is a series of personal accounts that highlight the past, present, and future of famous landmarks on geologic timescales.