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| | K-T Boundary Bed, Hell Creek Formation |
 | | Quoting from Retallack (1996), the 'impact bed' is "one centimeter thick, gray, smectitic, and layered, with shocked quartz and an iridium anomaly." This layer is not acid-leached as the underlying layer was, and it probably settled-out of the atmosphere over a longer period of time, thereby avoiding the prolonged effects of acid-rainfall. |
 | | This rock formation boundary is, for all practical purposes, arbitrary, and was defined in the early part of this century as "the lowest persistent bed of lignite" that is above the highest in-place dinosaur (Calvert, 1912; Brown, 1952). |
 | | Paleocene erosion of the iridium-containing boundary clay layer is probably the most likely culprit, although the effects of burrowing animals (called "bioturbation") during the earliest Paleocene also could destroy any sign of the clay layer. |
| www.scn.org /~bh162/k-t_boundary.html (900 words) |
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