Glen Park BART to Twin Peaks: Lots of chert
About the event
Five miles, 800 feet of climbing, and lots of chert! This route showcases some of San Francisco’s best exposures of Franciscan Complex ribbon chert, a deep‑ocean sedimentary rock later folded and uplifted by plate‑tectonic forces. If you’ve driven past the red‑and‑gray layers along O’Shaughnessy Boulevard, this hike lets you examine them up close.
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🧭 Route Overview
We’ll walk \~2.5 miles uphill on a mix of city streets and natural trails to Christmas Tree Point on Twin Peaks, then descend \~2.5 miles through Glen Canyon Park. ([AllTrails map](https://www.alltrails.com/explore/custom-routes/twin-peaks-from-glen-park-bart-915cded?u=i&sh=pqelwt))
• Car‑Free Summit Loop: The east side of Twin Peaks Blvd is closed to cars, giving us quiet, wide views of the city and Bay. • Chert Exposures: Along O’Shaughnessy Blvd we’ll stop at folded ribbon chert—thin layers of silica deposited on the deep ocean floor, later compressed and bent during subduction. • Glen Canyon Geology: Fewer rock outcrops, but excellent examples of canyon incision, slope processes, and Franciscan mélange.
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🪨 Geology Focus
• What is ribbon chert? Silica‑rich mud that settled on the ocean floor in rhythmic layers, later hardened into rock. • Why is it folded? The Pacific Plate pushed these rocks into California, crumpling them as they were scraped off into the Franciscan Complex. • What to look for: Color bands, tight folds, fractures, and how vegetation interacts with different rock types.
⏱️ Approximate schedule
• 10:00–10:15 AM: Welcome + short geology intro • Hat passed for $2 requested donation to help with Meetup fees. • 10:15 AM: Depart • \~2:00 PM: Return (depends on geology stops and this group's pace) • Pace: \~2 mph when you include observation breaks
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🥪 Food, Water & Restrooms
• Bring lunch for the summit; no food is sold there. • Canyon Market (Diamond St) is a good pre‑hike stop if you want to purchase lunch. • Carry water; summit water is unreliable. • Restrooms at Christmas Tree Point are automated and may require a short wait. • Optional social gathering at the end at Cuppa for Boba and/or froyo.
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👥 Group Policies
• Kids: Welcome if they can handle the distance and elevation. • Dogs: Not permitted on our hikes. • Attendance: SF hikes often have high RSVPs but actual turnout is typically 50–70%.
[Our last hike here](https://www.meetup.com/sf-bay-area-geology-hiking/events/226487303/).
