In many parts of the world a thin clay or marly unit marks the boundary between Cretaceous and Tertiary rocks. In marine sequences this boundary is defined by the first appearance of typically Paleocene marine plankton in the clay. In continental rocks, the boundary sediment yields the stratigraphically highest occurrence of a Cretaceous assemblage of fossil pollen. Detailed analyses of the marine boundary sediment at Caravaca, Spain, permit a three-fold subdivision: the lowest is apparently a fallout deposit of impact ejecta, preserved as a 0.5-cm lamina of red clay. The main subdivision is a black or dark gray clay or...