Walls beneath the Waves

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Geoarchaeological analyses in New York Harbor reveal intriguing evidence of past oceanic transgression even as we fortify our coasts for the future.

Millions of people live, work, and play along New York City’s 578 miles of beautiful and accessible coastline, some of the most desirable real estate in the world. But in 2012, Superstorm Sandy, the fourth most destructive hurricane in US history, exposed the fragility of our coastal developments and awakened “The City That Never Sleeps” to the existential crisis of climate change. The city’s response has been heavy on coastal barriers, and for good reason: climate experts project global sea levels to steadily rise anywhere from one and a half to three feet by the year 2100, depending in part on how much (or little) we reduce greenhouse gas emissions. The fact that we are now entering a period of intensified global warming may lead us to presume that coastlines were stable in the past and that today’s rising sea levels are an anomaly, but recent archaeological discoveries in New York Harbor show otherwise. A post-Sandy resiliency project in Staten Island reveals not only that humans have always been drawn to the coasts but also tantalizing evidence of long-submerged terrestrial landscapes and the surprisingly dynamic nature of sea level itself.

The living breakwaters project

Staten Island, exposed to the Atlantic Ocean and the region’s unique offshore bathymetry (i.e., the New York Bight), suffered more direct damage from Sandy than any other city borough—23 deaths, thousands of homes damaged or destroyed, and billions of dollars of economic loss. The storm surge that overwhelmed the island’s southeastern shoreline was 12 feet above normal. To help address its vulnerability, the US Department of Housing and Urban Development is sponsoring the Living Breakwaters project in the particularly hard-hit Staten Island community of Tottenville.

Schematic drawing of the Living Breakwaters in New York Harbor. The Tottenville shoreline is off to the right. SCAPE/NYS Governor’s Office of Storm Recovery (GOSR)

Rather than merely “creating a wall between people and water,” this innovative and multi-tiered project calls for both protective and resiliency measures. For protection, the project includes installation of a mile-long chain of highly engineered house-sized concrete structures offshore in Lower New York Harbor (a breakwater) and a fortified earthen berm along the shore. The breakwater will attenuate the power of future storm surges and reverse shoreline erosion, and the berm will protect homes and parkland from flooding. The resiliency measures focus on the community’s understandably wary coastal residents, aiming to re-establish the area’s lost marine habitats and foster “a vibrant water-based culture” through hands-on education and outreach (Scape/Landscape Architecture 2014).

A little context

The Breakwaters project will occur offshore of and within the 265-acre Conference House Park (a National Historic Landmark). The park was named for a diplomatic peace conference held there on September 11, 1776, between John Adams, Edward Rutledge, and Benjamin Franklin, representing the nascent Continental Congress, and Lord Richard Howe, commander of the British Navy, representing King George III. (Spoiler alert, the meeting was unsuccessful in averting the Revolutionary War.) After seeing limited war-time action, the area developed as a center of boat building and an economically important oyster fishery through the late 1800s, when it transitioned to beach-centered recreational activities with the development of an amusement park, summer cottages, and a coastal park. A number of damaging storms and significant coastal erosion during the first half of the twentieth century contributed to a retreat from coastal recreation and the intensive hardscaping of the shoreline with roads, development, and seawalls (Benimoff 2010). Tottenville, once known as “the town the oyster built” (Shepherd 2010), then gradually developed into the densely populated residential community it is today.

The area’s rich coastal resources attracted human habitation long before the historic period. Native Americans occupied Staten Island from the end of the last Ice Age (about 11,000 years ago) through the colonial period, and there are dozens of previously identified prehistoric sites in the vicinity (Meade 2017). The most significant of these, the National Register-listed Ward’s Point Archaeological District, is located within Conference House Park. Ward’s Point was repeatedly occupied over thousands of years (from the Early Archaic through the Contact Period) and includes human burials, shell middens, and evidence of tool manufacturing and use. Unfortunately, erosion, development, looting, and both avocational and vocational investigations have compromised the coastal site’s integrity.

A final aspect of the area’s archaeological sensitivity is offshore. Several years ago the US Army Corps of Engineers sponsored a major geomorphological assessment of New York Harbor for a navigation study. GRA, Incorporated (the Corp’s geoarchaeological consultant) examined a variety of data, including soil borings, to model the harbor’s archaeological potential. They identified the shallow, crescent-shaped offshore portion of the Living Breakwaters project area as the only section of the vast harbor with a high sensitivity for intact, submerged archaeological sites (Schuldenrein et al. 2014). A step back into our glacial past may be necessary to make sense of this.

A glacial interlude

To understand how a prehistoric site could possibly be present out in New York Harbor we must appreciate that global warming and its concomitant rising sea levels didn’t just start in the twenty-first century. When the last Ice Age’s glaciers were at their maximum extent around 26,000 years ago, the volume of water in the world’s oceans was significantly less than it is today. Vast stretches of the now submerged Atlantic coastal plain were exposed to the air and vegetated. When global temperatures started warming during the early Holocene era (beginning c. 11,700 years before present), global sea levels increased by about 390 feet (eustatic sea level change) as approximately 12 million cubic miles of glacial ice melted and entered the oceans as water (Engelhart et al. 2011). (To illustrate the scale of this change, the highest elevation on the eastern seaboard from Florida to Massachusetts’s border with Maine is Staten Island’s Todt Hill, which is only 400 feet above sea level.) Simultaneously, the earth’s compressed crust below the receding ice sheets began to rebound upward and the crust beneath the deepening oceans sank due to the added water’s massive weight. These isostatic forces moderated the absolute sea level.

Relative sea level in a previously glaciated region such as the southern shore of Staten Island is primarily determined by the complex interaction between the processes of eustasy (sea level change) and isostasy (compression or rebound of land masses) as the distribution of weight on the planet changes (ibid.). These forces and others dramatically altered the coastline through submergence, erosion, and changing sedimentation patterns and in the process sometimes buried the terrestrial ground surface beneath protective sediment layers as the oceans rose (Schuldenrein et al. 2014). Were there formerly habitable terrestrial surfaces submerged off the coast of Staten Island? Is there even a practicable way to find out?

The tantalizing boring part

As you can imagine, finding a prehistoric archaeological site that is both deeply buried and under water is exceedingly difficult—sites are hard enough to find on land! Fortunately, the project engineers needed to complete an offshore geotechnical sampling program to design the breakwater and model the subsequent changes to erosion patterns. This program provided an intriguing opportunity for our team’s geoarchaeologist to systematically examine the stratigraphy of the portion of the harbor determined sensitive for submerged sites.

Sampling consisted of 20 widely spaced split spoon borings excavated from on top of a barge to a depth of 60 to 100 feet below mean sea level (bmsl). They were arranged in two transects across several thousand feet of the shallow bay area (2 to 7.5 feet deep) where the breakwaters will be constructed. Most of the borings consisted of reworked and disturbed stratigraphy as a result of significant erosion and/or modern navigational dredging, indicating that much of the area actually has low potential for intact archaeological sites. However, the geoarchaeologist recovered limited but intriguing evidence from two borings at one side of the project area that the shallow muddy bay located just off of Staten Island’s southern shoreline was once a quite different place.

A greatly simplified soil profile of the project area (based solely on those two borings) consists of a layer of reworked silts and sands with shell fragments and gravel at the top; followed by a thick layer of soft dark gray marine “mud” with occasional layers of peat; and laminated light gray clay and fine sand at the bottom (starting at 45 to 80 feet bmsl). In these three layers we have remnants of anywhere from tens of millennia to tens of millions of years of the region’s geological history, depending on the age of the stuff at the bottom. The light gray clay is very old, dating back to the Pleistocene glaciers or even much earlier, and predating any human presence in North America. The upper-most layer is quite recent and has been repeatedly churned by ocean currents and navigational dredging. But the deposition of the soft marine mud coincides with thousands of years of rising post-Ice Age sea levels, with peat forming during periods of stabilization (Lynch 2016). Peering through this muddy lens we can darkly glimpse the dynamic formation of ancient landscapes long ago subsumed by the rising waters.

At the interface of the top layer and the marine mud, about 28 feet bmsl, a soil sample was collected from a very dark grayish brown fine sandy silt layer containing organic material. Flotation of this sample recovered a concentration of marine flora and fauna closely associated with productive, shallow-water marine habitats (Largy 2016). Though not terrestrial, this sample clearly reflects a time period when sea levels were over 20 feet lower than they are today.

A slightly deeper soil sample, this one recovered 35.5 feet bmsl, was taken from a brown silty sand layer just below the marine mud that resembled a B horizon (the lighter soil level typically encountered immediately below the darker, organic topsoil). Flotation of these soils recovered a few seeds and charred oak and conifer remains (wood, bark, pine needles, and a fascicle). Importantly, no marine flora or fauna was present in this sample, unlike in the shallower sample described above. This seemed to be just the evidence we were looking for. Confirmation came through Accelerator Mass Spectrometry (AMS) dating of the charred material, which resulted in a conventional radiocarbon age of 5,500 +/- 30 years before present. Well there you have it, during the Late Archaic period there was a forested terrestrial ground surface in the vicinity of this soil layer well over 30 feet below today’s sea level and 1,000 feet from the current shoreline. The ocean at that point in time may have been quite a distance further to the southeast. This timing corresponds to the settlement at nearby Ward’s Point. It is intriguing to wonder whether the wood was burned as part of a human habitation.

Two other, much deeper soil samples, also recovered immediately below the marine mud at 80.8 and 82.5 feet bmsl (the mud extended deeper in one of the two borings) also contained charred and uncharred terrestrial botanical materials (sedge and knotwood seeds and charred oak and unidentified hardwood) and no associated marine flora or fauna. Unfortunately, the only radiocarbon date recovered for these samples was from a large piece of charred wood from the deeper sample that could not be AMS dated with any confidence beyond >43,500 years before present. The geoarchaeologist hypothesized that this charred wood is intrusive through bioturbation (Lynch 2016).

The last intriguing bit of relevant evidence may be artifactual. Two separate borings, one from the center and one from the eastern end of the study area, each yielded a single possible chert artifact (at 26 and 42 feet bmsl). Chert, a common lithic source for tool manufacturing, was not observed in any of the other borings (although we did recover a few pieces of chert debitage in shovel test pits excavated along the shoreline). These two small nodules were angular and not waterworn and each had evidence of flake scars. However, there was no stratigraphic or botanical evidence that these objects, even if cultural, were recovered from an intact archaeological context. Instead, they were more likely to have been eroded and redeposited from their original depositional context through tidal action and bioturbation (ibid.).

Much tighter interval testing would be necessary to reconstruct this submerged paleoenvironment with any detail, let alone to determine the presence of an actual prehistoric site. However, since the depth and location of the remains made it highly unlikely that construction of the breakwater would pose an adverse effect, those lines of research will remain unaddressed for the time being.

The present, the past, and the future past

Superstorm Sandy was devastating in terms of loss of life, property, and our collective sense of security. But the Living Breakwaters project provided an unexpected opportunity to reflect on cycles of past climate change even as our contemporary fossil fuel-based economy accelerates the rate of that change. Although Staten Island’s prehistoric inhabitants didn’t have the option of constructing walls to slow the sea’s inexorable landward transgression, they could simply move to higher ground. Our high-density urban developments along the coast make such an accommodative strategy impossible. One wonders what archeologists of the distant future will make of the remains of our by then long-submerged and deeply buried coastal walls.

A. Michael Pappalardo, a registered professional archaeologist, is a senior technical director at the NYC-based environmental planning and engineering firm AKRF, Inc. Over his 25-year career Michael has often served as project archaeologist on large complicated development projects to assess impacts to significant resources in compliance with the National Historic Preservation Act and state- and local-level legislation.

References for which there is no online work of record available:

Benimoff, Alan. 2010. “A GIS Study of Urbanization in Hurricane SLOSH Zones on Staten Island, New York.” Presented at the Northeastern Section (45th Annual) and Southeastern Section (59th Annual) Joint Meeting of the Geological Society of America.

Meade, Elizabeth. 2017. “Phase 1A Archaeological Documentary Study, Coastal and Social Resiliency Initiatives for the Tottenville Shoreline: Living Breakwaters and Tottenville Shoreline Protection Projects, Staten Island, Richmond County, New York.” Prepared by AKRF, Inc. for GOSR.

Schuldenrein, Joseph, Ph.D.,Curtis E. Larsen, Michael Aiuvaslasit, and Mark A. Smith. 2014. “Geomorphology/Archaeological Borings and GIS Model of the Submerged Paleoenvironment in the New York and New Jersey Harbor and Bight in Connection with the New York and New Jersey Harbor Navigation Project, Port of New York and New Jersey, Under contract to US Army Corps of Engineers New York District.” Prepared by GRA, Inc. under subcontract to and in conjunction with Hunter Research, Inc., Trenton, New Jersey. Prepared for: Tetra Tech, Portland, Maine; under contract to the US Army Corps of Engineers, New York, New York.

Largy, Tonya. 2016. “Flotation Analysis: Living Breakwaters Archaeological Geo-bore Monitoring Project.”

Lynch, Kerry. 2016. “Archaeological Phase I Geo-bore Monitoring for the Living Breakwaters Project.” Completed by Archaeological Services, University of Massachusetts for AKRF.

Cite as: Pappalardo, A. Michael. 2019. “Walls beneath the Waves.” Anthropology News website, November 15, 2019. DOI: 10.1111/AN.1316