Sometimes nature can come to the aid of archaeologists when dating finds. There is an area in the western United States, in Harney County (Oregon), called the Rimrock Draw Rockshelter. A team of archaeologists from the University of Oregon Archaeological Field School and the Bureau of Land Management have been researching the area for the last 4 years. The area is fairly adjacent to the active volcano known as Mount St Helens.
     Recently they had excavated down to an undisturbed level of volcanic ash thrown up by a documented eruption in approximately 13,800 BCE. Located underneath this layer of ash they found a tool which had been used to scrape animal hide (confirmed by analysis of blood residue which belonged to Bison Antiquus, an extinct forebear of the modern buffalo). The excitement of this find lies in the fact that the previous oldest known human discovery I  the area (dried faeces) only dated back to approximately 12,000 BCE.
     Scott Thomas, of the Bureau of Land Management, is quoted as saying "The discovery of this tool below a layer of undisturbed ash that dates to 15,800 years old means that this tool is likely older, which suggests the oldest human occupation west of the Rockies". Dr Patrick O'Grady of University of Oregon also added "While we need more evidence before we can make an irrefutable claim, we plan to expand out excavation this summer and hopefully provide further evidence of artefacts found consistently underneath that layer of volcanic ash".

Edited from Lake County Examiner (25 March 2015)