When rock strata in the Earth’s brittle crustal layers fracture under stress, the blocks slip past each other to a point of equilibrium along a fault line that may be inches or hundreds of miles in length. These sudden breaks trigger earthquakes, and faults of three different sorts are the result. When the crust is pulled apart, a normal or tension fault occurs, with one side of the fractured strata dropping vertically, sometimes leaving the other block exposed as a prominent cliff, or fault scarp. When the Earth’s crust is compressed, one side of the fracture rises abruptly above the other—a reverse fault. (A thrust fault, when one side overrides the other at a low angle, is a type of reverse fault. When two fault blocks rotate past each other by pivoting at the same point, the result is a scissor fault.) When movement along a fault line is horizontal rather than vertical, the break is called a strike-slip or transcurrent fault. California’s six-hundred-mile-long San Andreas fault system is perhaps the best-known North American strike-slip fault. Lateral shearing, in contrast to the cliffs that vertical shearing produces, creates rift valleys and offsets, sometimes spectacular ones. In Assembling California, John McPhee compares the dissimilarity of the two sides of Tomales Bay, a ﬂooded San Andrean rift valley thirty miles north of San Francisco, writing: “A tan cotton sock on one foot and a green wool sock on the other could not represent a greater mismatch. . . . The granite on the west side of the Tomales broke away from the southern Sierra Nevada and has traveled north along the fault at least three hundred miles, an earthquake at a time.” A river may be offset by strike-slip faulting where it’s forced to follow the fault line, sometimes for miles, in an abrupt dogleg until it reconnects with its displaced channel. In some places—California, the Rocky Mountains, Alaska’s Prince William Sound—the history of thousands of slip-faults is starkly visible. Many such breaks were violent responses to stress, but McPhee writes that blocks along some faults “assume an almost steady creep,” leaving behind a seismic record of barely detectable tremors. Faulting leaves its signature in many features—scarps, offset rivers, sag ponds, desert oases, and the smooth friction polish on a rock surface known as slickenside.