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.
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 25th.
By Juliana Olsson, Exhibits Writer/Editor, and Matthew Carrano, Curator of Dinosauria, National Museum of Natural History