San Francisco is instantly recognizable by its dizzying topography, a landscape defined by steep streets and prominent peaks like Nob Hill and Twin Peaks. This distinctive geography is a direct consequence of ancient geological forces that have shaped the California coastline for millions of years. The city’s unique profile illustrates the power of earth-shaping processes, a story told through the compression of tectonic plates and the resilience of a singular type of rock.
The Engine of Uplift: Plate Tectonics and Compression
San Francisco’s hilliness results from its position along the boundary between the Pacific Plate and the North American Plate. This boundary is the San Andreas Fault system, where the plates slide horizontally past one another in a right-lateral transform motion. The Pacific Plate is grinding northward relative to the North American Plate, a motion that has progressed for millions of years.
This sliding motion is not perfectly smooth or straight along the fault trace. Where the main fault line develops slight bends, the horizontal movement translates into localized areas of intense squeezing, known as transpression. This lateral compression forces the crust to buckle and fold, pushing the land mass upward to create the characteristic steep ridges and hills of the Coast Ranges.
The uplift began between four and seven million years ago. The force of transpression causes reverse and thrust faulting, where one block of crust is shoved up and over another. This slow-motion process built the elevations seen throughout the San Francisco Bay Area.
The resulting topography is a series of fault-bounded blocks that have responded differently to the pressure. The Santa Cruz Mountains block, which includes the San Francisco Peninsula, is currently experiencing uplift at a restraining bend in the San Andreas Fault. This ongoing tectonic deformation has created the rugged terrain that forms the foundation of the city.
The Foundation of the Hills: The Franciscan Complex
The resilience and steepness of the hills are tied to the material they are made of, a unique geological formation called the Franciscan Complex. This complex is a jumbled mixture of rocks, or mélange, that was scraped off the subducting oceanic plate—the Farallon Plate—as it plunged beneath the North American continent millions of years ago. This scraping process created an accretionary wedge.
The Franciscan Complex contains durable rock types that resist weathering and erosion. These include tough, dark sandstones called greywacke, which make up a large portion of the material. Also present are radiolarian chert, a glassy, dense rock composed of microscopic silica shells, and basalt, a volcanic rock often metamorphosed into greenstone.
A particularly important component is serpentine, California’s state rock, which originated as ultramafic rock deep within the oceanic plate. The durability of these rock types is the reason the hills remain steep and prominent after being uplifted. The hard, tightly compressed Franciscan material allowed the hills to maintain their sharp angles and height against erosion.
The composition of this accretionary wedge, with its diverse and resilient rocks, is a key factor that differentiates San Francisco’s hills from other fault zones. The strength of the Franciscan Complex meant that when it was subjected to the intense folding and thrusting of the San Andreas Fault system, it maintained its structural integrity.
Sculpting the Terrain: Erosion and Sea Level Changes
Once the Franciscan Complex was thrust upward by tectonic compression, secondary forces of erosion and water refined the landscape. The long-term effects of wind and water carved away the softer, less-resistant materials. Differential weathering left the harder, more resilient cores of the Franciscan rock as the steep, prominent peaks, such as Twin Peaks and Nob Hill, that stand today.
Fluctuations in global sea level during glacial and interglacial periods also played a significant role in shaping the terrain. During the ice ages, vast amounts of ocean water were locked up in continental ice sheets, causing the sea level to drop drastically, sometimes by as much as 400 feet. The current San Francisco Bay floor was repeatedly exposed and became a deep, river-carved valley.
This low sea level stand allowed ancient rivers, primarily the Sacramento and San Joaquin, to erode and deepen the channel that is now the Golden Gate. When the glaciers melted, the sea level rose rapidly, flooding these valleys and creating the San Francisco Bay. This cycle of deep erosion followed by inundation further isolated and highlighted the uplifted, resistant Franciscan hills.