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Ancient Indigenous clam gardens off northern Quadra Island have been dated by researchers to be at least 3,500 years old – 2,000 years older than previously thought.

The study by archeologists from Simon Fraser University (SFU) and the Hakai Institute, a B.C. scientific research centre, provides valuable data on the human-built rock walls known as clam gardens. The walls, used to farm clams along tidal zones, are hidden under the high tide and provide an ideal habitat for the mollusks.

“Some of these walls were built 3,500 years ago and continue to be used.” said Dana Lepofsky, lead author of the study and professor at Simon Fraser’s department of archeology.

Using a combination of radiocarbon dating and historic sea-level mapping, researchers have been able to date the clam gardens, Prof. Lepofsky said.

She said the work, published in the journal PLOS ONE, puts to rest any doubt whether these walls were built before colonial contact.

“First Nations have been saying that it’s an age-old practice because it’s in origin stories, it’s in their place names, language and song. Now we have Western scientific knowledge to support that,” she said.

The structures have a unique shape and are built of rock from different areas. First Nations used bedrock boulders and fit them together like a dry stone wall, rather than constructing them from a base with rounded rocks.

For local Laich-Kwil-Tach and northern Coast Salish peoples, the harvesting of clams that gather and grow on these ancient walls is an integral part of life and culture for their communities.

Knowledge of the rock gardens has been passed down from generation to generation for Indigenous communities ranging from Washington State to Alaska. This vital knowledge has been linked to resource management, livelihoods, and identity.

“It proves that we have had technology,” said Christine Roberts, co-author and community member from the Wei Wai Kum First Nation.

With help from elders and locals, researchers were able to identify nine clam gardens to conduct their study. Having access only during a tidal range that occurs between the months of May and August, researchers had just a few hours a day during a five-day window of each month when the gardens were exposed enough to complete their field work.

Other forms of ancient Indigenous resource management practices such as pruning and tilling have been harder to date because no physical structure is left behind. That’s why archeologists have turned to marine harvesting practices such as clam gardens for research. Other marine structures such as fish traps have been dated to be at least 5,000 years old – evidence of some of the oldest recorded resource management techniques in British Columbia.

For local First Nations such as Ms. Roberts’s, clam harvesting is a practice that continues in her community. Her sons have also taken on the tradition in order to keep it alive, passing it on from generation to generation.

“If you don’t use them they overgrow themselves. When you dig them you are stirring the pot.”