Picture of the Week #5
Section of the K-T boundary. American Museum of Natural History, August 2006
The arrow points to the iridium layer that lies between the Cretaceous (below) and Tertiary (above) layers this cross section. This is, of course, the famous layer that led the Alvarezes to hypothesize that a large extraterrestrial impact occurred at this time and ultimately caused the extinction of the dinosaurs. There is a section of the K-T boundary exposed here in Texas, up east of Waco, but I haven't had a chance to see it - yet!