Granite Blocks - Menlo Park, California - Rock and Mineral Displays on Waymarking.com

These granite blocks are one example of a series of large blocks of rocks from various locations, which are on display around the USGS Campus in Menlo Park. There is a self-guided tour of the campus available at the visitor center.
Text of Sign: “Granodiorite monoliths from near Sonora Pass in the Sierra Nevada that crystallized about 86 million years ago during the Cretaceous Period.
These three blocks of granodiorite (weighing 9, 3, and 2.5 tons) were collected from a large intrusive mass or pluton about 150 square miles in size. It intruded as nearly molten rock under a cover of a few miles of overlying country rock. This pluton is similar to several othes that are among the largest and youngest of the granitic intrusions that make up the Sierra Nevada Batholith.
Not the large crystals (phenocrysts) of orthoclase feldspar, several inches in size, that are set in a groundmass of smaller quartz, plagloclase, orthoclase, biotite, and hornblende. Such phenocrysts formed during a long period of cooling and occur only in the largest plutons that cooled the slowest.
On the polished surfaces, the phenocrysts reveal many concentric zones or layers that mark the outer surface of the crystals at various stages during their growth. The prominent dark zones in many represents a time when crystalization was interrupted long enough for small crystals of black biotite and hornblende to stick onto the surface of the large growing orthoclase crystals.”