the most dangerous boundary in the world.
The metamorphic rocks of the Mother Lode consist of three (actually more, but let's keep things a bit uncomplicated) terranes which become progressively older to the east. They include the Foothills terrane (with mostly Mesozoic rocks), the Calaveras Complex (late Paleozoic and early Mesozoic rocks), and the Shoofly Complex (the oldest, containing Paleozoic rocks). One of the best ways to see them is to follow Hornitos Road to Highway 140 through Mariposa, following the Merced River to Yosemite. The town of Mariposa and the Melones Fault are the boundary between the Foothills rocks and the Calaveras Complex. The boundary with the Shoofly is less obvious, but lies a short distance upstream from the "Geological Exhibit" near Savage's Trading Post on the Merced River.
the chert of the Marin Headlands.
Up next: the culmination of our journey through the most dangerous plate boundary in the world!