No. 26: Mar-Apr 1983
Scientists have long searched for a cause for the profound geological and biological changes that apparently occurred between the Cretaceous and Tertiary periods. When an iridium-rich layer was found in several areas at this important boundary, many claimed it as proof of an asteroid impact or some other catastrophism that would nicely explain the massive worldwide changes that occurred. With this preamble in mind, consider the following abstract from an article in Science:
"Analyses of the clay mineralogy of samples from the Cretaceous-Tertiary boundary layer at four localities show that the boundary clay is neither mineralogically exotic nor distinct from locally derived clays above and below the boundary. The significant ejecta component in the clay that is predicted by the asteroid impact scenario was not detected."
(Rampino, Michael R., and Reynolds, Robert C.; "Clay Mineralogy of the Cretaceous-Tertiary Boundary Clay," Science, 219:1983.)