a thin layer of geologic deposits, of varying thickness in different parts of the world, found between the geological strata identified as Cretaceous and the strata above, identified as Tertiary; also, the time point or period marking the boundary between the Cretaceous and Tertiary periods. [PJC
"According to a theory gaining acceptance (as of 1997), these deposits were formed as the debris of a large comet or meteorite impact on the earth, which threw up a large quantity of dust into the atmosphere, causing profound though temporary climatic change, and caused or hastened the extinction of numerous species, including the dinosaurs. This hypothesis was first postulated by Luis and Walter Alvarez on the basis of an excess of iridium found in the boundary layer, and was later supported by additional evidence of various types. The impact is believed to have occurred at the edge of the Yucatan peninsula in Mexico, forming what is termed the Chicxulub crater, which is partly under the Gulf of Mexico, is not evident from surface topography, and was detected primarily by gravity anomaly readings and subsurface geological characteristics.