Telegraph Hill has a colorful history extending back more than 150 years, and the geology and legacy of this promontory currently haunt the urban corridors that closely line its slopes. Telegraph Hill is underlain by resistant Franciscan Complex greywacke sandstone, largely unsheared, and grossly stable. The high quality rock in close proximity to the burgeoning shipping industry was a valuable commodity in the mid- to late-1800s, and the sandstone was mined extensively for seawalls, jetties, roads, and for ship ballast. The quarrying resulted in near-vertical rock faces up to 50m in height along the eastern slopes. The resistant sandstone exposed along many of the old quarried slopes remains standing to this day at near-vertical angles, with very slow retreat rates that resulted in builders gaining confidence in ‘snuggling’ close to these old slopes. The high, steep walls experienced blast fracturing and relaxation jointing from the removal of large volumes of the hillside, resulting in periodic, hazardous rockslides that serve as a persistent reminder of the quarrying legacy.
Telegraph hill received its name in the mid-1800s from the signal flags, or semaphore, located at the top of the hill that were raised by the lookout to inform residents of incoming ships . Prior to the gold rush era, Telegraph Hill was a uniformly sloped, stable hillside, as illustrated in artist George H. Burgess rendering of the hill in 1849 (Fig. 1) . However, this gently to moderately inclined, rolling hillside topography was dramatically and forever altered when the eastern slope experienced approximately 50 years of intense quarrying activity. The geology of this hillside, with resistant sandstone outcrops along the eastern slope, proved to be an invaluable and irresistible commodity for San Francisco’s gold rush and post-gold rush boom. The resistant and hard rock qualities that made this rock so valuable for a variety of construction related activities are also the qualities that have preserved these steep quarried faces, some virtually unchanged, for nearly 150 years. The resistant rock lured builders into placing structures dangerously close to the tops and toes of these slopes, leaving little margin for error. Telegraph Hill continues to experience hazardous rockslides and rockfalls along an approximate 1 km (0.7-mile) stretch of the southeastern, eastern and northeastern portions of the hill.