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Contractors removing dirt at Lincoln’s Bluff Road Landfill have run into a deposit of 600,000-year-old volcanic ash.

The ash likely came from a big eruption around Yellowstone, more than half a million years ago.

The deposit is large, about 250 feet long, five to 15 feet wide and at least four to five feet deep, said Greg Westphal, a geotechnical engineer with Alfred Benesch & Co.

Finding volcanic ash in this area is not unusual. But it is unusual to find a deposit this large, Westphal said.

The ash was uncovered below the glacial till, sitting in what would have been a glacial lake that formed in front of ice sheets hundreds of thousands of years ago.

Westphal is hoping to send samples to the state geologist, who will officially date it. But he's 90 percent sure this is from the Yellowstone eruption.

Westphall said he has has seen ash in the landfill in previous excavations, but nothing as large as this deposit.

He believes it might be a drift, pushed up against a stream.