Purpose of the activity: To provide a fun activity, using a common item, to demonstrate the geologic history of the Marin Headlands (and the SF Bay Area east of the San Andreas Fault) from approximately 180mya to about 30mya.

Geologic Concepts: Plate tectonics, plate boundaries, subduction, accretionary wedge, Franciscan Complex.

Background: It often seems that geologists use familiar food to describe rocks and landforms when mere hand-waving can’t do the job. Maybe it’s the hunger pangs we feel after tramping around landscapes better suited to mountain goats than bipeds. Whatever the reason, I still recall my geology field course outings in the early 1970s—eating bag lunches in rugged and remote mountains, commiserating with fellow students from the University of Montana. Invariably, our lunches became source material for geologic modeling. The humble PB&J sandwich could be bent and folded into tortured strata. The frosting slab of a sandwich cookie could become the Lewis Overthrust Belt. Trail mix was the stuff of a good conglomerate.

California geologists continued the edible trend. When I started my career as a park ranger in the Golden Gate National Parks in the mid-1980s, Clyde Wahrhaftig described the chaotic Franciscan Complex as a “tectonic stew,” or Jell-o salad. Today, Doris Sloan uses the phrase “melted ice cream landscape” to describe our Bay Area hills.

When I began developing ranger-led geology programs in the Marin Headlands in 1992, co-worker Diane Dobos-Bubno mentioned that she liked to use Oreo cookies for the geologic story of the Marin Headlands. After much research and field testing, I fashioned a script to accompany a sandwich-cookie demonstration to explain the formation, migration, and accretion of the Franciscan rocks onto the North American plate.