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.
