Clues to the travels of ancient hunters can be found in the glassy debris from volcanoes that they used to make sharp tools and arrowheads. Obsidian is abundant in Oregon (USA) and was relatively easy to craft into knives, spear tips and other prehistoric tools. "They're razor sharp, sharper than any steel could be ground," said Dennis Jenkins, senior staff archaeologist at the University of Oregon.
     Jenkins said archaeologists can use an X-ray procedure to match the mineral composition of the obsidian to a specific volcanic source across the West. "There will be these rare minerals in there that can be chemically identified, and they are unique to each volcano," Jenkins said. "And that allows us to track each individual artifact back to the exact location where it came from," he said. "And it's not uncommon in a particular site to find up to 30 or 40 obsidian sources represented there."
     Archaeologists also can trace the movement of obsidian artifacts by the amount of grinding and chipping done by those who made the tools. They would usually begin with large, often fist-sized cores of obsidian and break them down, leaving millions of tiny flake pieces. But the tools themselves would get smaller and smaller through time as their owners traveled and sharpened them, leaving further trails of flakes. As a general rule, Jenkins said the smaller the artifact, the farther it is from the original source. "They were leaving a trail of evidence behind them," he said.
     There also various ways to date the obsidian artifacts including a process called obsidian hydration. As obsidian naturally absorbs water from the atmosphere, archaeologists can measure the amount of absorption and determine the time it took to occur. They first, however, must calibrate that measurement to the average soil temperature of the location where it was found. Another method of dating obsidian is a visual estimate of the general date of a tool based on its appearance.
     "Technology improved through time," Jenkins said, noting the tools became increasingly sophisticated. Ancient hunters began crafting relatively crude oval-shaped tools around 13,500 years ago before slowly advancing to extremely sharp and aerodynamic arrow points. In a study he conducted, Jenkins examined the source and age of about 5,600 obsidian artifacts in Oregon. He found a few general patterns for obsidian artifacts that traveled with their owners north and south, with little east to west movement. The ancient cultures may have migrated north and south as old storm patterns moved west to east, and carried their resources along the way, he said. "Maybe this is a product of just normal movement through the environment," Jenkins said.

Sources: Associated Press, East Oregonian, KGW (16 March 2008)