Geological Side Trips from Interstate 80: Griffith Quarry in Penryn
As the young state of California began to prosper, it came time to make substantial buildings. Thus the opportunity arose for the difficult art of quarrying and finishing granite to supply the need for good building stone. The High Sierra is made of granite, but in the 1800s it was remote and challenging country. (Note: By "granite" I mean what geologists refer to as granitoid, a wide range of rock types that qualify as commercial granite.)
Sierra granites represent the deep-seated bodies of magma that supplied the lava for a long chain of volcanic islands, rather like Japan or Indonesia today, during the Mesozoic Era some 80 to 160 million years ago. However, an accident of California geology placed an outlier of the High Sierra granite near Sacramento at the edge of the foothills.
Quarrying of this granite began early in the 1850s, but the industry really burgeoned when a Welsh immigrant experienced in stonework, named Griffith Griffith, picked a good outcrop and started a quarry near Rocklin in 1864. He named his company town Penrhyn after the old Welsh slate district. (Today's it's Penryn.) Here's where it fits on the state geologic map.
The Griffith Quarry soon landed contracts from governments and was off to the races. Parts of the Capitol building in Sacramento, the foundation of the San Francisco Mint and many old unsung railroad beds consist of Griffith stone.
Today the pit is preserved in a Placer County park. The old quarry office, made of the finest Penryn granite, is a museum that's open weekend afternoons. Don't miss it if you're there.
Some old quarries become destinations while some become eyesores. This one is something between the extremes in a state of genteel decrepitude, with interpretive trails and safety railings plus some benches and picnic tables. It's just the kind of place that kids should explore.
The stone itself is still widely exposed in waste piles as well as the pit walls. Don't bring a rock hammer—you won't need it and the stones are protected anyway.
Close up, the stone is revealed as a relatively dark fine-grained granitoid with abundant biotite (black mica) plus black hornblende, white feldspar and a little gray quartz.
I might call it a quartz diorite or even a tonalite, but you don't have to. Just enjoy its rugged good looks.
To visit the quarry park, take the Penryn Road exit off I-80 and head north, then turn right on Taylor Road. Total distance, just over a mile. For more geologic and historical detail, visit the Penryn page at Quarries and Beyond.