When it comes to archaeology, rock and sediment often outlast softer materials like cloth or wood. People have used rock walls for thousands of years to create structures like agricultural terraces and fish traps. But finding out when these structures were built can be a challenge. From Alaska south to Washington, people living along the coast built rock walls near the shoreline creating beach terraces known as clam gardens. But archaeologists never knew how old these features were—until now.
Archaeologists frequently use organic material to date an artifact or a structure with a technique called radiocarbon dating, which looks at how a radioactive isotope of carbon, carbon-14, decays over time. This works with things like bits of charcoal, bone, or shell. If the object is inorganic, like a stone tool, archaeologists date neighboring bits of organic material. Unfortunately, that material isn’t always available. Luckily, there’s another way to date rocks and sediment. And it’s a very bright idea.
To date sediments and rock surfaces, scientists turn to optical dating. Optical dating uses flecks of quartz or feldspar minerals to establish when they were last exposed to sunlight and subsequently buried.
“[Optical dating] has become an essential arrow in the quiver of scientists worldwide, enabling geological, biological, and archaeological events to be placed on a timescale extending from the present to half a million years ago or earlier,” said Richard Roberts and Olav Lian in their 2015 paper in Nature that reviewed the last 30 years of optical dating.
However, no one had ever used optical dating to deduce when a clam garden was built, or how long it took for the terrace behind it to fill in with sediment. Northern Quadra Island was a logical place to test the method, since the density of clam gardens here is among the highest in British Columbia.
Optical dating revealed that the three clam garden sites tested on northern Quadra Island were between 1,000 and 1,700 years old. Christina Neudorf, a post-doctoral researcher and Hakai scholar, and her colleagues from the University of the Fraser Valley and Simon Fraser University published these results in a new study in the journal PLOS ONE.
“But our results also showed that construction of the walls didn’t occur all at once,” says Neudorf. “It was built over several generations rather than one short-term event.” She added that this reflects a future-focussed, continuous investment in these landscapes, which is consistent with the stories of coastal First Nations today.
The rock walls work to increase the area of ideal clam habitat. Sloped beaches are infilled behind the wall by rising and fall tides and flatten out. Optical ages suggest that clam garden wall construction caused these beaches to fill up with clam-friendly sediments four times faster than they would have without a wall.
Optical dating isn’t new. The technique was invented at Simon Fraser University in 1985 and has since been used by scientists all over the world to date events as far back as 500,000 years. For example, archaeologists have used optical dating to show that early modern humans made ornaments in southern Africa 70,000 years ago, and that humans arrived in Australia by 50,000 years ago.
Optical dating could be used in the future to figure out when rock-walled root gardens and stone fish traps in the same region were built. No organic material? No longer a problem.