Clay Film from Ground Water

This is an example of a natural clay iridescent film that may be found on the surface of ground water at a construction site. The film may be mistaken as an oil film but the fractured nature of the film is typical of natural clay films. This site was in the Everett Clay Formation in Western Washington. The film was dried and in air on the slide for this photograph. The interference colors are more evident here in this colorless film.