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Julie asks her grandfather if she is right that she is 1/16th or 1/32nd Native American. The actors, Suzanne Marie Doyon (Julie) and her sister Janine Doyon (Susan) are actually about 1/16th (or more, the lineage is not 100% clear) Penobscott and Miq'Maq Indian (tribes from the Northeast US and Southeast Canada).
The treasure legend is a real Oregon Coast legend. There are various versions of the legend. Only a few versions talk about the black slave. Spielberg's film, The Goonies, is based on the same legend but tells a whole different story.
The story begins with a mystical elk on the beach running at Julie and jumping over her. The scene's genesis was in a real event. One summer, when Suzanne Marie and Janine Doyon were just toddlers, they were on the Manzanita beach with their mother, director Jane Beaumont Hall. The children were a hundred feet away playing in the sand. Suddenly, from out of nowhere a huge Roosevelt Elk came running down the beach straight for the girls. "There was nothing I could do," says Hall. "Time stood still as I saw the Elk run towards the girls and then suddenly swerve around them. The elk ran into the ocean and swam out past the breakers. It was an amazing experience. I don't think I had time to be scared for the girls and the elk was such a magnificent creature." Hall remembered that scene when she and Richard Doyon wrote the screenplay. Both the elk on the beach and the elk swimming in the ocean were written into the script.
The films star, Suzanne Marie Doyon, plays Julie in the film. Julie's sister, Susan, is played by Suzanne's real life sister, Janine Doyon. When the film was shot, Suzanne was 14 the age of her character. Janine was 16 but played an 18 year old.
The two bad guys are digging under a hump of sand. The floor of the tunnel has dry sand that they are scooping out. If the tunnel were made of dry sand, the roof would have collapsed before they got six inches in.
Grandpa Jack (played by Max Gail, TV's Wojo from Barney Miller) and Standing Elk (Floyd Red Crow Westerman) play close friends in the film. In real life, Floyd and Max have been friends for several decades. Max introduced Floyd to the producers for the role of Standing Elk.