SEATTLE -- An archaeological survey to clear the way for construction near a mall has unearthed thousands of stone tools crafted at least 10,000 years ago.

"We were pretty amazed," archaeologist Robert Kopperl, who led the field investigation, told The Seattle Times. "This is the oldest archaeological site in the Puget Sound lowland with stone tools."

The rare find is shedding light on a time when prehistoric bison and mammoths still roamed what is now western Washington. Only a handful of archaeological sites dating back 10,000 years or more have been discovered in the region.

Chemical analysis of one of the tools revealed traces of the food they were eating, including bison, deer, bear, sheep and salmon. The dig also uncovered a fragment of salmon bone, evidence that the fish made its way up local streams for at least 10,000 years. It also revealed other unusual tools, including the bottoms of two spear points that have concave bases.

The site near Redmond Town Center mall in Redmond, Washington, was initially surveyed in 2009, as the city embarked on a project to restore salmon habitat in Bear Creek, a tributary of the Sammamish River. The creek had been confined to a rock-lined channel decades before.

The Washington State Department of Transportation largely paid for the salmon-restoration project as a way to mitigate some of the environmental impacts of building the new Highway 520 floating bridge over Lake Washington and widening the roadway.

The site appears to have been occupied by small groups of people who were making and repairing stone tools, said Kopperl, of SWCA Environmental Consultants. He and his colleagues published their initial analysis earlier this year in the journal PaleoAmerica.

"This was a very good place to have a camp," Kopperl said. "They could use it as a centralized location to go out and fish and hunt and gather and make stone tools."

Crews initially found unremarkable artifacts. But when they dug deeper, they found a foot-thick layer of peat - remains of a bog at least 10,000 years old. Below the peat, they later discovered a wealth of tools and fragments.

"We knew right away that it was a pretty significant find," Washington State Historic Preservation Officer Allyson Brooks told The Times.

Kopperl said that because of where the artifacts were located below the peat, which had not been disturbed, it's clear they predate the formation of the peat. Radiocarbon analysis conducted on charcoal fragments found with the tools confirmed the age.

"It's hard to find this kind of site west of the Cascades, because it's so heavily vegetated and the Puget Lobe of the big ice sheet really affected the landscape," Kopperl told the newspaper.

A handful of sites have been discovered east of the mountains with tools dating back between 12,000 and 14,000 years.

So it's clear that humans have lived in the area since soon after the glaciers retreated, but a lot of mystery still surrounds the region's earliest occupants and their origins, The Times reported. When Kopperl and his team are done analyzing the artifacts, they will hand them over to the Muckleshoot Tribe for curation. There are no immediate plans to display the artifacts publicly.