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Natural Areas

Rocky Outcrop

  • Location: Funston Street & 14th Avenue
  • Established: 1988
  • Size: Park: 1.6 acres/Natural Area: 1.6 acres
  • Park Type: Dune

This steep, rocky park has beautiful examples of Franciscan chert. The rocks are home to a variety of plants, including two native succulents, dudleya and stonecrop. Stonecrop is the host plant for the San Bruno elfin butterfly, a federally listed endangered species. Rock Outcrop is a small park, but it offers habitat for a variety of wildlife.

Geology

The folds and fractures found here are evidence of the powerful forces of plate tectonics. This rock formation began to develop about 200 million years ago in the middle of the Pacific Ocean. Magma emerging through the earth's crust at the spreading center between the Pacific and Farallon plates contacted sea water and cooled to form pillow basalt, the bedrock of oceans everywhere. Over the next 75 million years, radiolaria—amoebae-like single-celled organisms with silica-rich exoskeletons—lived, died, and drifted down through the deep ocean waters to the ocean floor. Eventually the combination of pressure and time turned these microscopic organisms into the rock we call Franciscan radiolarian chert.

The movements of the tectonic plates eventually brought the rocks close to shore. Sand eroding from the Sierra Nevada washed into the ocean and turned to sandstone layered atop the chert. As the Farallon plate subducted under the Continental plate, the rocks of the outcrop were scraped off, scrambled up, and pitched upon the land.

Park Features: