II.

Geology

Collisions between the plates of Earth's crust continued unabated from Eocene time (see Plate Tectonics). In the eastern hemisphere, the Afro-Arabian and Indian remnants of the former supercontinent Gondwanaland, colliding with Eurasia to the north, pinched shut the eastern end of the Tethys Sea, leaving in its place a much shrunken remnant—the Mediterranean. Compressional forces generated by the collision helped to push up an extensive system of mountain ranges, from the Alps in the west to the Himalayas in the east. Meanwhile the Australian plate collided with the Indonesian, and the North American plate had begun to override the Pacific. As a result, the seafloor-spreading process originating at the East Pacific Ridge was diverted to a direction perpendicular to the ridge axis. A great transform fault—the earthquake-producing San Andreas Fault of California—developed to accommodate this shift in motion between the plates. Other effects of the collision included the creation of the Basin and Range structure of the southwestern United States, the continued uplift of the Sierra Nevada, and the outpouring of massive basalt flows that built up the Columbia Plateau. The climate remained subtropical and moist throughout North America and Europe, but a gradual, long-term cooling trend had begun, culminating in the Pleistocene ice ages.