Thousands of stone tools crafted more than 10,000 years ago have been discovered during an archaeological survey to clear way for construction near a mall.

Archaeologist Robert Kopperl, who led the field investigation, told The Seattle Times that they were pretty amazed. He said it is the oldest archaeological site in the Puget Sound lowland having stone tools.

The strange discovery has thrown light on a period when prehistoric bison and mammoths used to roam on what is presently western Washington. So far, just a handful of archaeological sites dating back 10,000 years or more have been found in the region.

Traces of the food they used to eat, including bison, deer, bear, sheep and salmon have been revealed in the chemical analysis of one of the tools. In the digging a fragment of salmon bone has also been discovered. It is proof that the fish made its way up local streams for around 10,000 years. It has also revealed other rare tools such as the bottoms of two spear points that have concave bases.

The location near Redmond Town Center mall in Redmond, Washington, was at first surveyed in 2009, when the city embarked on a project for restoring salmon habitat in Bear Creek, a tributary of the Sammamish River. Decades ago, the creek had been confined to a rock-lined channel.

The Washington State Department of Transportation has been paid huge amount of money for the salmon-restoration project. This project is a way to mitigate some of the environmental effects of construction of the new Highway 520 floating bridge over Lake Washington and widening the roadway.