Montana Earth Science Picture of the Week

Layer of Clay is Focus of Debate

Photo courtesy of Milwaukee Public Museum

About the photos . . .
These two photos were both taken in Garfield County (eastern Montana). The above photo shows the Cretaceous-Tertiary Boundary . The photo near the bottom of this page shows the layers of the Hell Creek Formation, one of the best-known dinosaur "graveyards" in the world. Paleontologists from all over the world have visited this area near Jordan to search for fossils of dinosaurs and other organisms that lived here during the late Cretaceous Period over 65 million years ago.

The K/T Boundary . . .
Of special interest to scientists is a thin layer of clay that neatly separates layers of rock laid down in the Cretaceous Period (when dinosaurs lived) from layers deposited during the Tertiary Period (after the dinosaurs). Although this layer of clay, called the K/T Boundary, has been found in over 100 locations around the world, some of the most informative exposures are found in Garfield County.

Formation of the clay that makes up the K/T boundary . . .
Scientists believe that the K/T clay was formed from dust generated when an asteroid slammed into the Gulf of Mexico 65 million years ago. One reason for this belief is that the clay contains an abundance of iridium, an element that is extremely rare in sedimentary rocks but much more abundant in certain types of asteroids. Another clue contained in the K/T clay is "shocked quartz". When quartz is subjected to violent shock waves is develops an unusual pattern of tiny fractures. Shocked quartz is found around known impact craters, and in the upper part of the K/T boundary. More proof of the impact came in 1991 when geologists discovered a 110-mile wide crater buried under 3,600 ft. of sediment in the Gulf of Mexico near Chicxulub, Mexico

A debate among scientists . . .
Although there is little doubt among scientists that an asteroid crashed into the Earth 65 million years ago, there is disagreement about the role that this played in the demise of the dinosaurs. Most geologists and some paleontologists believe that this is what caused the extinction of the dinosaurs and many other species. However, other paleontologists disagree, claiming that dinosaurs had already begun to die off long before the impact. In their view the asteroid was simply the last straw.

Below: This photo shows the Brownie Butte area in Garfield County, Montana. A layer of coal, called the "Z coal", lies just above the K-T Boundary. The gray layer beneath the K/T is the Hell Creek Formation, made up of the last sediments deposited in the Cretaceous Period (end of Mesozoic Era).

Photo courtesy of David E. Fastovsky, Ph. D.
Professor of Geosciences at the University of Rhode Island

Sources . . .
1. Alt, David, and Donald Hyndman. Northwest Exposures. Missoula, Montana: Mountain Press Publishing, 1995.
2. Culotta, Elizabeth. "Dinosaurs Path to Extinction." Earth: Jan. 1993: 24-29.

Term: asteroid


Past pictures of the week
*Map of the world 65 million years ago
*More about the debate
One more photo taken near Jordan, Montana
Next picture

By Rod Benson
Earth Science Teacher at Helena High School

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