In July 2013, the exhibits team went to North Dakota’s Hell Creek Formation to collect 66-million-year-old fossils. These fossils will be on display in our new exhibition on the Last American Dinosaurs while our permanent Fossil Hall is under construction. This is the 7th post in a series about our experiences in the field.
We spent so much time last summer digging around the Hell Creek Formation, we thought it deserved its own blog post. In order to know where to find fossils, you need to know where to find the right rock formations, and for that, you need to know a bit of geology.
Formations are recognizable rock layers or “strata,” and they vary from region to region. All formations have a particular size, appearance, and history. They can range anywhere from 10 to 10,000 feet in thickness, from 3 to 3,000 miles in area, and encompass thousands to millions of years of time. Geologists usually name a formation based on the location where it is best recognized as a distinct set of rock layers—the Hell Creek Formation was described after a rock outcrop along Hell Creek near Jordan, Montana.
How are formations, well, formed?
Formations can be made of igneous, sedimentary, or metamorphic rocks. The key is that each formation has a different history and appearance from the layers above and below it.
The Hell Creek Formation is sedimentary—good for us, since we were interested in finding fossils. Because sediments settle differently in different environments, geologists can distinguish between former lakes, streams, ponds, rivers, swamps, reefs, beaches, and sea-floors. Plants and animals that died in the area were sometimes buried in these sediments, and became fossils. The Hell Creek Formation was formed by sediments deposited in rivers, lakes, and floodplains from 68 to 66 million years ago, and preserves plenty of Late Cretaceous plants and animals.
Where to Look
To find the Hell Creek Formation in North Dakota, we searched along the exposed faces of buttes (small hills with steep sides and fairly flat tops that dot the western landscape). Geologic maps told us where geographically we’d find the Hell Creek Formation, but we still had to find sites where it was exposed in accessible outcrops. Time for an analogy:
Think of a stack of pancakes, with two blueberry pancakes on the bottom, four chocolate chip pancakes in the middle, and three buttermilk pancakes on top. Each flavor represents a new formation, and there are a varying number of thinner depositional layers within each formation. The oldest pancakes are on the bottom, while the ones fresh off the griddle are on top. Geology works the same way: newer things sit on top of the old. But it’s not always that simple…
Geologic forces from plate tectonics—such as volcanism and seismic activity—can fold and fracture formations. Imagine sliding your knife beneath your stack of pancakes and lifting upward, just as geologic forces might fold rocks to form a mountain. The stack will bulge up in the middle until eventually it splits, exposing cross-sections of all your different flavor formations. Similarly, if you started pushing up on the outside edges of your pancakes, you’d get a U-shaped stack: the sides will be tilted up, and you could see the exposed stack on the outside edges.
Now imagine that the chocolate chip pancakes in the middle are the Hell Creek Formation (the chocolate chips are fossils, of course), and that’s how paleontologists think about rock layers. Although it’s possible to drill down to find the layers you want to see, it’s much easier to go to the outer edges where tectonics have pushed them to the surface.
Once we identified the Hell Creek Formation and started prospecting, we soon found what we came for—spectacular Late Cretaceous plant fossils and tiny animal bones, scales, and teeth. On November 25th, you’ll be able to see some of these fossils on display in our new exhibition, “The Last American Dinosaurs: Discovering a Lost World.” Our scientists and volunteers are also preparing some of these fossils for our future main Fossil Hall, opening in 2019.
By Juliana Olsson, Exhibits Writer/ Editor, National Museum of Natural History