National Museum of Natural History Unearthed

Digging into the Hell Creek Formation

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.

Formation Basics

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.

Lots and lots of layers with different textures and colors. Photo by Kirk Johnson/ Smithsonian Institution

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.

Let erosion do the work! We found lots of great plant fossils in the Hell Creek Formation layers on these buttes. Photo by Kirk Johnson/ Smithsonian Institution

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 25^th, 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

What is the K/Pg Boundary–Part 2

In mid-July of 2013, the exhibits team went to North Dakota to collect 66-million-year-old fossils from the Hell Creek Formation. These fossils will go in our new exhibition on the Last American Dinosaurs while our new Fossil Hall is under construction. This is the 6th post in a series about our experiences out in the field. 

Scrambling around the rocks in North Dakota, we became familiar with the boundary between the Cretaceous and Paleogene periods of geologic time, which marks the end of the age of dinosaurs, 66 million years ago. You might remember that the Cretaceous/Paleogene (K/Pg) boundary is an important place-marker chosen by scientists, and it has distinctive physical features that can be found in different locations around the world.

Standing on the K/Pg Boundary, we found evidence of the Cretaceous extinction event right at our feet (literally). Photo by Juliana Olsson/ Smithsonian Institution

Boundaries Mark Important Moments in Time

Early geologists understood that rock layers formed in a sequence, with the oldest layers below the youngest ones. This allowed them to tell relative time in any particular place by looking at the order of the rocks. But there was no real way to tell absolute time, making it difficult to know whether one stack of rocks recorded the same sequence of events as another stack exposed hundreds or thousands of miles away. Identifying distinct global events would help geologists compare distant formations.

Fossils helped. Geologists knew that some fossils were found across wide areas, but only in specific layers of rock. This meant that the original organisms were widespread but only lived for a certain period of time. For example, dinosaur fossils are found in Mesozoic rocks but not older or younger layers, and they lived all across the globe. So their fossils could be used to constrain the age of their surrounding rocks. By using hundreds of different species to carefully correlate many rock layers, early geologists developed a remarkably detailed (and surprisingly accurate) time scale.

As a result, some moments in time became more important than others. The moment when dinosaurs became extinct was especially important because it took place everywhere at about the same time—so this event was used to mark the end of not just the Cretaceous Period but the entire Mesozoic Era.

Once radiometric dating was discovered in 1907, geologists could go back to these rocks and figure out exactly how old they were. Now we know that the Cretaceous Period ended when the dinosaurs went extinct, and that this took place 66 million years ago. With improvements in technique, we can now pinpoint this date to within a few tens of thousands of years—not bad for something that happened so long ago.

Geologists still use events to mark boundaries, precisely because they represent moments in time that were important across the world at the time they took place. But scientists now work to know when these events occurred as precisely as possible.

Time and Rock Formations

A formation is a recognizable layer of rock with a specific history and geographic range, typically named after the location where it was first identified. Formations have their own “boundaries,” called contacts, where they contact a younger layer (above) and an older one (below). But these are physical borders, and in fact one formation can change over to another at slightly different times in different places. Because they form under local geological conditions, formations often do not match exactly with geologic time periods (which are based on global events).

And that’s the tricky thing about boundaries and rocks: they are totally different things. The rocks preserve the boundary, but they are not interchangeable. The boundaries between geologic time periods take place at specific, absolute moments in time, and would exist even if we had no rocks to record it (thankfully, that’s not the case with the K/Pg boundary).

The physical signals of a boundary like the K/Pg are found in formations, but which formation depends on where you’re looking. In the Dakotas and Montana, the K/Pg boundary lies at the top of the 300-foot-thick Hell Creek Formation. This formation contains the rock layer that was the land’s surface when the asteroid hit. It also contains fossils that reveal the world of Tyrannosaurus and Triceratops. Come explore their world (and relive our expedition) in “The Last American Dinosaurs” exhibition, opening November 25^th.

 

By Juliana Olsson, Exhibits Writer/Editor, and Matthew Carrano, Curator of Dinosauria, National Museum of Natural History

Colored Diamonds from Rio Tinto: The Rough Cut

It’s been a sparkly fall at the Smithsonian’s National Museum of Natural History. We recently welcomed 500 carats of rough diamonds into the National Gem and Mineral Collection thanks to a generous donation from Rio Tinto. The gemstones arrived in a kaleidoscope of colors, including rare pink diamonds. Take a look for yourself!

Credit: Courtesy of Rio Tinto Diamonds

These dazzling beauties were extracted from Rio Tinto’s Argyle Mine in remote northwest Australia, an area known for naturally colored diamonds. Unlike most polished diamonds that you might see in a jewelry store, these stones were removed from the earth and kept in their natural, “rough” state. For our team of mineral scientists, that’s the perfect form in which to study them.

Why do scientists study diamonds? For one thing, they offer a unique window into the geological history of the planet. Diamonds typically formed more than 100 miles deep in the Earth and provide an exceptional window into the geologic processes and conditions that took place there 2–3 billion years ago. Dr. Jeffrey Post, curator of the National Gem and Mineral collection, specializes in telling the stories of diamonds and other gems by searching for clues about their origins. In the case of the Rio Tinto crystals, the rough diamonds carry tiny mineral grains that might be unique to diamonds only found in the Argyle mine. If future unknown gems with the same mineral fingerprint are discovered, scientists will know that they also originated in the Kimberly region of Australia.    

Credit: Courtesy of Rio Tinto Diamonds

Diamond colors add another chapter to the history of the Earth’s deep past. Post and his colleagues have discovered that blue diamonds, like the museum’s 45.52 carat Hope Diamond, form when boron becomes trapped in a crystal’s molecular structure while yellow diamonds result when nitrogen atoms swap places with carbon. Green, a very unusual color in diamonds, occurs when crystals form in rocks exposed to radiation. Fun fact: green diamonds are thought to be the inspiration for kryptonite in the Superman series!

The cause of the pink color found in many of the Rio Tinto gems remains a bit of a mystery, but Post is leading the charge to find answers. He recently conducted research confirming that this striking color is likely the result of a shock event that took place in the gem’s history, either in the mantle or during transport to the surface of the Earth. The result: changes in the structures of pink diamonds that interact with light to produce colors ranging from brown to pink.

Explore our Gem Gallery for more information about the Smithsonian’s diamond collection. 

 

By Kathryn Sabella, Press Officer, National Museum of Natural History