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A dense deposit of shells found within tunnel machine Bertha’s repair vault likely wasn’t left by Native tribes, but by settlers who worked shucking oysters, officials say.

Allyson Brooks, state historic-preservation officer, said Wednesday the shells are mostly small Olympia oysters that grew far from Elliott Bay and later were harvested to near-extinction. Oysters were eaten in saloons along the early Seattle waterfront and also exported by ship to San Francisco, she said. Nowadays, efforts are under way to re-establish Olympia oysters in Puget Sound’s southern inlets. They have “an unmistakable sweet, metallic, celery-salt flavor,” says oysterguide.com.

At this time, it seems the archaeological investigation will take days, rather than weeks or months, she said.

Brooks visited the pit Wednesday morning, where the shells were about 18 feet below street level, and an east-west trench was dug to see a stratigraphic cross-section of the deposits. Shell concentrations are densest in the middle of the circular pit, and in some places butter-clam shells were found, as well as remnants of wooden pilings, she said.

“So far, we’re not seeing any Native-American cultural material, and it’s looking to be early, historic Euro-American settlement,” Brooks said.

Still, the state needs to continue with caution, in case there are tribal artifacts or human remains. It’s entirely plausible that items from settlers and Natives could coexist in the busy area, she said. It’s also possible tribal traders brought Olympia oysters to Elliott Bay on canoe trips, but Brooks said they wouldn’t have left such dense piles of shells in Seattle.

“We want to make sure we’re not carving through anybody’s remains, to make sure we’re not going through any burial grounds,” Brooks said earlier, on KIRO radio’s Dori Monson Show.

The investigation began Tuesday. Crews will keep digging trenches or taking cylindrical samples, through layers of loose fill and former tideflats, until they reach untouched earth, she said. At about 50 feet below street level will be intact glacial till. The top of Bertha’s cutting head rests about 60 feet deep, and beyond the reach of the 19th century.

“They’re not done with the investigation. There’s more to do,” cautioned Laura Newborn, Washington State Department of Transportation (WSDOT) spokeswoman. She emphasized that excavation of the repair vault, to be used in retrieving and rebuilding the Highway 99 tunnel machine’s front-end drive parts, won’t necessarily resume immediately after archaeologists leave.

The search for buried artifacts wouldn’t have happened, except that a December stall of the giant tunnel machine led Seattle Tunnel Partners to create a vertical access shaft, 120-feet deep, that passes near Seattle’s original seaport.

But if the team continues to find just shells, there won’t be a need for time-consuming preservation measures, Brooks said.

However, state archaeologist Rob Whitlam said a sample of these oyster shells might be valuable to biologists — to study what shells were like before their chemistry was altered by pollution.

Mike Lindblom: 206-515-5631 or mlindblom@seattletimes.com.