Gobs of oil continued to wash ashore in Washington and British Columbia two weeks after a spill from a tugboat-tanker collision that one official said could scar the coast for "months if not years."

As experts assessed the ecological damage to the Pacific Northwest, Shell Oil Co. estimated Thursday that a ruptured pipeline in Missouri spilled 840,000 gallons of oil Dec. 24 near a tributary of the Missouri River in one of the biggest inland spills in U.S. history.Shell has recovered about 441,000 gallons, company spokesman Tony Canino said.

The state of Missouri is "probably headed for legal action" against Shell for not notifying officials immediately after discovering the rupture, a spokesman for the state Department of Natural Resources said.

In northwest Washington, experts hiked along the shoreline of Olympic National Park Thursday to assess damage to fragile tidepools, kelp beds, rocky shores and wildlife from an estimated 231,000 gallons of bunker oil. The oil spilled after the Dec. 22 collision of a tugboat and a tanker in heavy seas two miles off Grays Harbor on the central Washington coast.

Olympic Chief Ranger Chuck Janda said a damage assessment should be ready by Saturday.

Evidence of the spill, which has washed up along 300 miles of coast from Newport, Ore., to Vancouver Island, British Columbia, will remain "a long, long time - months if not years," he said.

Tides and heavy surf appear to be gradually shifting the oil northward, Janda said.

State wildlife officials say about 3,700 sea birds have died in Washington.