Analysis of the fossils and the rock’s minerals [of the Leschi Park erratic] shows it may have come from the Nooksack Group near Mount Baker, or from the Harrison Lake area of southern British Columbia.
Around 5,000 years ago, a large chunk of the volcano slid away and that debris avalanche helped to produce the massive Osceola Mudflow, which went all the way to the site of present-day Tacoma and south Seattle.