San Francisco is known for its hills, but before that touristic meme caught on, stone quarries were on course to remove some of them entirely. Twin Peaks had quarries; so did Bernal Heights; so did Diamond Heights; so did Point Lobos. There was a quarry in Golden Gate Park. But it all began at the nearest bedrock to the city's original waterfrontTelegraph Hill.
A modern city needs rock as much as it needs water. Roadbeds and foundations are important uses that may come to mind today. As the Gold Rush began, San Francisco needed stone to build wharves and piers, stone to fill in the marshland and build more piers, and stone to use for ballastrocks to weigh down the holds of ships after they were emptied of cargo. The pace of business was frantic, and regulations were few. Every possible hillside was pressed into service, and Telegraph Hill was the one that served the most.
The Vulcan Quarry was located on Francisco Street near Kearny Street. The George P. Wetmore Quarry was at the corner of Lombard and Montgomery streets. Other quarry faces are found just north of Broadway on the south side of the hill. But the biggest was the Gray Brothers quarry operation, headquartered on Sansome Street at Green Street, where a great vertical cut almost 200 feet high still stands, barely covered with vegetation.
By 1900 the ongoing damage alarmed not just the hill's modest homeowners, but civic groups who wanted it preserved as a landmark. A member of the California Club told the Call on 3 January 1900 that the hill was "one of the natural and beautiful sites of San Francisco in addition to being closely identified with the early history of the city." But quarry owner George Gray called it "very unsightly" and argued, among other things, that "if Telegraph Hill were cut down, residents on the east side of Russian Hill would have a magnificent marine view they do not now enjoy." A decade later, the quarry was shut down and the city crept in. In 1927, Philo Farnsworth invented television in his Green Street lab on the quarry site.
A walk around the south, east and north sides of Telegraph Hill will give you many opportunities to peek at its innards. On the whole, it's holding up well.
The sandstone is so massively bedded here that it crumbles less than your average former quarry. I think it's fair to say, without offering a professional geologist's opinion, that the next big earthquake will leave the hill mostly intact.
Telegraph Hill is one of seven outcrops of the same Franciscan sandstone in this part of the city. On the geologic map below, it's the one on the upper right; going clockwise from it the others make up Rincon Hill, Nob Hill, Pacific Heights/Cathedral Hill, Presidio Hill, Black Point/Fort Mason and Russian Hill. Alcatraz and Yerba Buena Islands also have this stone, and all are mapped as part of the Alcatraz terrane.
The other important thing to note on the map is the amount of artificial fill (Qaf). That's where much of the early quarry output went.
The quarried material of Telegraph Hill was always called blue sandstone, but what you'll see today is brown from weathering. Geologists of the U.S. Geological Survey have described it as dark gray "with rare tinges of blue." It is classified as graywacke, an "immature" sandstone with abundant fine-grained matrix and rich in rock fragments as well as sharp-edged mineral grains.
Under the microscope the sandstone displays metamorphic minerals such as pumpellyite, attesting a moderate degree of heat and pressure. That happened as this former seafloor was turned to rock, squeezed up and torn to ribbons in California's dynamic tectonic environment during the last few dozen million years.