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 marl, containing reworked extraterrestrial debris, laid down in an oxygen-deficient environment. The uppermost boundary clay is lighter gray in color, transitional in lithology to the overlying Paleocene sediments, which were deposited after the recovery from the terminal Cretaceous convulsive event. The boundary clay unit on land, represented by a section in Raton Basin, New Mexico, consists of a lower white clay, which is apparently a fallout deposit, and an upper carbonaceous shale. Boundary sections elsewhere are similar to those sections. The sedimentology of the boundary sediment records the convulsive environmental changes at/after a terminal Cretaceous event.