Archaeologists Find Prehistoric Communities Lost to Sea-level Rise in Denmark and Florida

The discovery of vast shell middens, aka garbage from seafood feasts, attests that the rising seas as the Ice Age waned didn't wash away all trace of prehistoric societies living by the sea

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One of the underwater shell midden sites.
One of the underwater shell midden sites.Credit: Associate Professor Jonathan Benjamin / Flinders University

She sells seashells by the seashore, and she always did. Humankind has always been so enamored of the briny that some suspected our ancestors’ exit from Africa hugged the coastlines, a hypothesis somewhat marred by inland hominin finds.

Also, for all that we can’t survive in water unless we keep our heads out of it, a great many of us dote on seafood and always have. Neanderthals in Italy are even thought to have dived to get clams.

Now, two separate studies have found rare evidence of prehistoric communities by the seashore, in the form of now-submerged shell middens – vast piles of shells resulting from seafood consumption thousands of years ago.

One of these middens lies underwater in the Gulf of Mexico. The other was found in the sea of Eastern Jutland, Denmark. Both papers were published this week in Quaternary Science Reviews. No question, these mounds of mollusks were anthropogenic in origin, which we know because, among other things, they show evidence of burning – i.e., they were cooked – explains the international team of archaeologists from Moesgaard Museum in Denmark, the University of Georgia, the University of York in the Ukraine, and Flinders University and James Cook University of Australia.

These are neither the first prehistoric shell middens ever found on paleo-shorelines, or underwater ones at that. But finding submerged evidence of prehistoric society is rare. The papers show that wherever the marine animals were eaten, a shell midden is a shell midden – they have “similar sedimentological profiles,” and also at least some they could survive the gradual inundation of sea level rise, though it is assumed that most remnants of past communities on the shores have washed away. This is key to the future search and study of ancient coastal communities lost to rising seas.

A tape measure showing the height of the prehistoric shell midden.Credit: Associate Professor Jonathan Benjamin / Flinders University

In the case of the Danish shell deposits, they had been covered by sand and overgrown by eel grass, the authors explain. Then a combination of climate change and pollution in recent decades gradually destroyed that shield, exposing the deposits.

These middens were not small: the archaeologists found two separate piles of shells about 50 meters (164 feet) apart, each about 20 by 30 meters in extent and about 80 centimeters thick. That is a lot of shells. There is a suspicion that the middens had been bigger but some was, after all, lost to erosion.

Subsequent to the discovery of the shell deposits, micro-study – detecting, for instance, the signs of burning, i.e., cooking – demonstrated that these were man-made middens that accumulated on the beach, not natural deposits of shells over time, the researchers explain. Nature doesn't barbecue mollusks.

Gradual inundation

We tend to think of our planet as immutable, that the coastlines today were always the coastlines (leaving continental drift out of it). In fact, the coastlines today have only been the coastlines we know for more or less the last 6,000 years. During ice ages, sea levels are significantly lower because of water locked up in ice sheets. But that was before the rise of modern civilization, let alone writing, and we don’t seem to have a distinguishable collective memory of that.

Exploring a shell midden site.Credit: Associate Professor Jonathan Benjamin / Flinders University

How much significantly lower was the average sea level? During the Last Glacial Maximum 21,000 years ago, the average sea level was about 130 meters or 400-plus feet lower than today. 

“For a large portion of humanity’s existence, sea levels have been significantly lower, up to 130 meters than what they are today, exposing millions of square kilometers of land. And the archaeological record clearly demonstrates that people in the past lived on these coastal plains before they were drowned by past sea-level rise,” says Jonathan Benjamin, associate professor at Flinders.

The midden found on the island of Hjarno Sund in the Straits of Denmark dates to around 7,300 years ago. That was during the Mesolithic period – the transitional time between the Stone Age and the Neolithic revolution that brought settlement and agriculture.

Moving onto the midden found in the Econfina Channel, now a few meters beneath the Floridan waters, it dates to the Middle to Late Archaic period. Occupation of the Econfina site dates to at least 7,000 years ago and the midden has been dated to about 5,500 to 3,000 years ago.

Despite the gaps in distance and possibly in time, the two piles of shells evince similar sedimentological profiles – where people ate clams and tossed shells, they ate clams and tossed shells much the same way, it seems. Which is a lesson for ye litter bugs on the seashore: the shells are still there. They had accumulated on the shore in the open air, and the piles were subsequently inundated but not destroyed by sea-level rise.

Shell midden.Credit: Peter Moe Astrup

“Shell middens are a classic, worldwide marker for the intensive use of marine resources, but archaeologists have always assumed that these sites would have been destroyed by sea-level rise,” says Prof. Geoff Bailey of the University of York and visiting professor at Flinders University. Well, these two sites weren’t.

Since we found them and it is all but unthinkable that we found the only two middens that existed at the time, the discoveries could indicate that inundated prehistoric coastal sites are more common than had been assumed, write Jessica Cook Hale of the University of Georgia and colleagues in their paper “Submerged landscapes, marine transgression and underwater shell middens.”

Among other things, the research proved inhabitation of the Gulf of Mexico coast in Florida (specifically) somewhat earlier than thought, the authors claim. It bears adding that the Floridan gulf coast is ideal for seeking ancient, now underwater, remains because the continental slope is a nice gentle one.

In Denmark, it was clear that prehistoric peoples thronged the coasts, but actual finds of middens are rare in the south and may hint that this type of site was more common than assumed.  

To be clear, some believe Florida was occupied over 14,500 years ago, based on claims of discovering stone tools with mastodon bones at a site called Page-Ladson. As for the coasts, we know the shores of what is now the southeastern United States were densely peopled by at least 4,500 years ago and perhaps even as early as 6,000 years ago. However, earlier evidence is now submerged, which is a snag.

Divers exploring the shell midden site.Credit: Associate Professor Jonathan Benjamin / Flinders University

As for the underwater evidence, middens aren’t hallmarks of prehistoric obsessive-compulsiveness about discarding shells. They were general trash piles that in some cases contain other food remains, discarded tools and gewgaws, and in some cultures, even human burials.

In this case, the Danish pile contained tools made of stone, bone and antler, wooden artifacts including decorated paddles and bows, ax shafts, and vertical wooden stakes that the archaeologists say are the remains of a stationary weir (a trap for fish) built out from the shore.

The Floridan midden contained remains of a stone tool manufacturing industry, among other things.

In fact, archaeological investigation has found ample evidence of ancient coastal communities, including, as said, evidence that Neanderthals were diving for clams – as deep as 4 meters – 106,000 to 74,000 years ago in Italy. Though bizarrely, the Neanderthals may have been clamming less for the nutritive value of slimy mollusk flesh and more for shells suitable for scraping hide, the archaeologists postulated in that case.

The United Kingdom is also famed for the archaeology of lost coastal peoples, discernible for instance by submerged walls off the Isles of Scilly.

This is what ultimately happens when you don't take your trash with you at the beach.Credit: Associate Professor Jonathan Benjamin/Flinders University

In another case, Israeli archaeologists identified what they believe are the remains of a seawall built over 7,000 years ago to protect a community from the rising Mediterranean. Well, it didn’t work, and the seawall and village are now underwater.

And we also note that just last year, archaeologists reported on ancient fish farms discovered at Mound Key, Florida, where a Native American group known as the Calusa had been – it seems – catching fish and storing them in artificial pools built on an artificial island. Not just any artificial island: it had been built as much as 1,500 years ago from a midden of oyster and other shells.

In the case of the Floridan midden, a political issue has raised its head: A potential conflict between development such as offshore wind farms versus preservation of ancestral remains – which we now realize may be more common than thought.

“Here in North America, the big push for offshore wind farms is underway, but Indigenous voices must remain foremost,” stated Cook Hale. “These new findings support ongoing work to ensure that Indigenous and First Nations have a critical seat at the table, so to speak, in managing the offshore cultural heritage of their ancestral lands by documenting these relationships into the deep past.”

Indeed, the new papers are useful to the cause of preserving ancient remains for posterity, by demonstrating that sea-level rise thousands of years ago didn’t necessarily erase all signs of past human occupation. We ate seashells by the seashore, we always did and the more drowned landscapes from yore we locate, the better we can understand the evolution of our relationship with the sea.