ABSTRACT
At Green River, Wyoming, the road goes under a remnant of the bed of a good-sized Cenozoic lake. The tunnel through Yerba Buena Island, in San Francisco Bay, is in sandstones and shales of the Mesozoic. The river turned in our direction after bending by a wall of its canyon, and the wall had eroded so unevenly that a prominent remnant now stood on its own as a steep six-hundred-foot hill. The hill, structurally, consisted of two distinct rock formations, awry to each other, awry to the gyroscope of the earth—just stuck together there like two artistic impulses in a pointedly haphazard collage. The rock of the lower half of that hill dated to three hundred and thirty million years ago, in the Mississippian, and the rock above the unconformity dated to two hundred and ninety million years ago, in the Pennsylvanian.
