Around 20,000 years ago, the massive Laurentide ice sheet halted its southerly advance just past Cape Cod and, shedding its load of till, and began to form a terminal moraine still discernible in the series of hills that stretch from Tuckernuck to […]
By Sara E. Pratt
Aug 19 2008
Around 20,000 years ago, the massive Laurentide ice sheet halted its southerly advance just past Cape Cod and, shedding its load of till, and began to form a terminal moraine still discernible in the series of hills that stretch from Tuckernuck to Siasconset. Back then, sea level was nearly 400 feet lower than today, placing the east coast of North America more than 100 miles offshore, near a prodigious pile of dry gravel and sand. By 11,500 years ago, rising seas had inundated the mound to form Georges Island, and, a few thousand years later, Georges Bank — where, 6,000 years hence, the occasional waterlogged tree trunk is disgorged from this ancient coastline and lands in a fishing net.
Much like the tides that help shape them, shorelines ebb and flow. Geologists tend to take the long view on this process: Sand scoured from one location supplies the material to build a spit, a sandbar, a shoal, or a barrier beach in another. But determining the rate of this movement can be tricky, depending on how long a view you take.
In 1978, the shoreline at Low Beach, south of Siasconset, was two feet seaward of its 1846 location, producing a seemingly minuscule long-term rate of change indicative of a stable beach. A house built at Low Beach in the late 1800s, however, would have been anything but stable. Although 238 feet of sand accreted between 1846 and 1887, by 1955 the beach had eroded 32 feet. Twenty-three years later, in 1978, another 204 feet had disappeared, for a short-term rate approaching nine feet per year.
Erosion rates depend on interactions among wind, waves, currents, sea level, seafloor shape, and the intensity and frequency of storms. But on Nantucket, and on other low-lying islands around the globe, it all comes down to one factor.
“Sea-level rise is the primary driver of erosion on Nantucket,” says coastal geologist Les Smith of Epsilon Associates, a Maynard, Massachusetts, engineering and environmental consulting firm that has studied Siasconset’s erosion. Sea level has been rising since the Pleistocene, Smith explains, and it’s still rising, at the rate of about a foot per century.
That’s relative sea level, Smith explains. It’s a combination of global sea-level rise (due to melting ice and the thermal expansion of warming waters) and local sinking of the crust as it continues to adjust to the removal of burdensome glaciers, which reached their thickest and heaviest in central Canada. As the northern crust rebounds, the crust under the ice’s edge near Nantucket and Martha’s Vineyard, which was previously warped up into a “peripheral bulge,” flexes back down, taking the islands with it.
Each year, 65 acres of Massachusetts disappears below the waves owing to sea-level rise alone — not including active erosion. In many locales, land lost to submergence exceeds the amount eaten away by active erosion, but the two are inextricably linked, as rising seas expose more land to erosional forces, including storms.
Off southeast Nantucket, the Shoals, a notoriously dynamic submerged sand-and-gravel ridge, used to act as a breakwater, dissipating wave energy from storms. “But the shoals have shifted,” notes Rebecca Haney, geologist with the Massachusetts Department of Coastal Zone Management. Newly formed gaps in the shoals now focus wave energy on the southern and eastern shores. “So although there may not necessarily be more storms,” she adds, “more storm energy is reaching the shore.”
Smith and Haney say the erosional trend on Nantucket, and in certain other coastal New England areas, doesn’t look as though it will reverse anytime soon. Boston’s sea level, for example, is predicted to rise 22 inches by the year 2100. The projected cumulative cost of sand replenishment to protect the state’s coastline from that rise has been estimated at more than $2.5 billion. But even replenishment is, at best, a temporary fix. When it comes to the coast, the sea, and the sand, a lesson could perhaps be taken from Galileo’s defiant declaration as he signed his recantation: Eppure si muove. “But still it moves.”
Read Ian Aldrich’s article, Nantucket: A Disappearing Island.