(Click on images to enlarge)
For this geologist there is a great thrill of hands-on experience like running your fingers in cake frosting, fresh apple pie, or in this case, the brown clay layer of the famous K-T boundary just outside Gubbio, Italy. We had our hands-on experience May 15, 2011 during the Ceri Festival.
This layer was the first found of now about a hundred similar boundaries world-wide that contain the tell-tail iridium anomaly suggesting a massive extinction due to an asteroid impact. Geologist Walter Alavarez was studying this extinction boundary and brought back clay samples to his father Luis Alaveraz, a nuclear physicist. The father and son joined forces with Helen Mitchel and Frank Asaro, and put together the compelling story of the asteroid caused extinction. For a good overview of this work see (Rocks from space sec. ed. by Richard Norton pp 375-380).
Internet searches regarding access to this site were sketchy to discouraging in that directions were unclear or the site was fenced off and unaccessible for close examination. Take note however, that the strata containing the clay layer has no fences, is within walking distance from Gubbio, and is well known by people in the visitors information center in central Gubbio. You can buy a brochure at the information center for half an Euro that has a map and describes the significance of the site and other near by geologic formations of interests.
The site is No. 34 on the brochure map called Botaccione Gorge. Diana and I drove by car from Gubbio and there is a turnout on the right for parking. One can walk to the tilted layer and examine it closely. Unfortunately much of the clay has been dug out over the years and the site has an over abundance of small unsightly core holes apparently for paleo magnetic studies.
There is also a metal sign in Italian explaining the significance of the site. (Someone care to translate?)
Another interesting brown clay layer can be found in the limestone cavern Castellana Grotte near the town of Castellana, inland from the south eastern coastal town of Bari. The cavern is accessible by guided tour only. These caves formed in Jurassic-Cretaceous limestone, and among other fine formations. Our tour guide Laura Lisa pointed out a brown clay layer between limestone beds. We contemplated whether this layer was possibly a similar to the extinction marker at Gubbio, waiting for further studies.
For a list of papers on this subject see: