Geology of the Golden Gate Headlands William P. Elder, National Park Service, Golden Gate National Recreation Area Fort Mason, Building 201, San FRancisco, CA 94123 e-mail
[email protected] Introduction Wahrhaftig wrote on the rocks of the San Francisco area, both for the professional geologist This field trip focuses on the rocks of the penin- and the general public (e.g., Wahrhaftig, 1984a, sular headlands found just north and south of the 1984b; Wahrhaftig and Murchey, 1987; Golden Gate, on lands of the Golden Gate Wahrhaftig and Sloan, 1989). In this paper, I will National Recreation Area. Exposed in dramatic describe the characteristics of the rocks and geo- sea cliffs, these rocks not only form a spectacular logic processes observed at the six field trip stops, backdrop for the Golden Gate Bridge, but also interpret their geologic story, and place them into provide a detailed geologic record of Pacific basin a regional geologic context. The primary focus and active continental margin processes spanning will be on geology of the Franciscan Complex, back 200 million years. This is arguably one of but other aspects, such as the Quaternary geology the longest records of its type in the world. The and the relationship between the geology and the significance of these rocks, however, goes beyond plant communities, will be discussed. the geologic history that they tell, for they, and others of the Franciscan Complex associated with During the field trip, please keep in mind that the them, played a critical role in developing our cur- sites visited all lie within a national park rent understanding of subduction zone mechanics and that sample collecting of any kind is prohibit- and processes.