Section 1: Formations: Easy as Cake!

Layers of sand, silt, rock, and other substances were laid down in North Dakota over millions of years. Some layers were brought by water eroding the surfaces of the mountains to the west. Other layers were brought by wind. Each of these layers has a name which represents a time period. Each layer has its own story of where it came from, the conditions under which it was formed, and the plant and animal life trapped within its layers. Dino Dakota by Kris Kitko (MP3)

Williston Basin Stratigraphy
Image 1: Stratigraphy. Geologists use stratigraphic columns such as this one to show when the layers of sediment were deposited. This stratigraphic column is for North Dakota and shows only the formations that can be viewed from the surface. This column will change a little in the future as geologists learn more about the foundations of North Dakota.

We can use a chart, called a stratigraphic (strat ih GRAF ick) column (See Image 1.) to view the order of North Dakota’s stratigraphy. Stratigraphy (stra TIG raf ee) comes from two words. “Stratum” comes from a Latin word that refers to something spread or laid down. “Graph” comes from a Latin word that the Romans took from a Greek word meaning writing. So, a stratigraphic column is a way of “writing” the geology of North Dakota.

The stratigraphic column shows the layers that underlie North Dakota’s surface. Some of these layers are exposed, others are deeply buried and we only know about them through core sampling. Each layer corresponds to a time period usually measured in millions of years. Each layer has its own story of climate and plant and animal life. If we look into a particular formation, we have a pretty good idea about what kind of animals or plants we will find there. If we study the Hell Creek Formation of the late Cretaceous (kree-TAY-shush) Period, we will find fossilized bones of dinosaurs that lived in that Formation. Hell Creek ended about 65 million years ago. The layers that were deposited after Hell Creek contain different kinds of rocks and different fossils.

Perch
Image 2: Perch. A perch died millions of years ago and was pressed into sediment where its bones were replaced by minerals. Once the process has been completed, there is no carbon material remaining. Fossils cannot be dated by carbon-dating. They are dated by determining which stratigraphic layer the fossil was found in.

The epochs, periods, and eras are dated in terms of millions of years. Though these dates change a little (by a few million years) as geologists accumulate more information, they are helpful in making sense of how long it took for geological changes to take place. The dates are determined through scientific study. Carbon dating can be used on materials that are less than a million years old. However, life forms that appear as fossils in rocks cannot be carbon dated because all of their carbon-based matter has been replaced by minerals. (See Image 2.)

These images show us how formations were deposited. It wasn’t always an even process. Sometimes a geologic “accident” such as a shift in the earth’s crust, might create a “bite” or depression that would be filled in by other layers of sediment. By knowing the composition of each layer, geologists can determine which layer settled to a lower point. These images show us how formations were deposited. It wasn’t always an even process. Sometimes a geologic “accident” such as a shift in the earth’s crust, might create a “bite” or depression that would be filled in by other layers of sediment. By knowing the composition of each layer, geologists can determine which layer settled to a lower point. These images show us how formations were deposited. It wasn’t always an even process. Sometimes a geologic “accident” such as a shift in the earth’s crust, might create a “bite” or depression that would be filled in by other layers of sediment. By knowing the composition of each layer, geologists can determine which layer settled to a lower point. These images show us how formations were deposited. It wasn’t always an even process. Sometimes a geologic “accident” such as a shift in the earth’s crust, might create a “bite” or depression that would be filled in by other layers of sediment. By knowing the composition of each layer, geologists can determine which layer settled to a lower point.
 

Another method for dating strata is potassium-argon dating. Potassium and Argon are found in volcanic ash. A layer of volcanic ash from any eruption anywhere in the world can be dated. Geologists collect this information and use it to track ash layers. This method can be used to date rocks as old as 4.5 billion years old.

It might be helpful to think of the layers on which North Dakota sits as being like a layer cake. Layers were laid down over long periods of time, but little accidents or “bites” removed parts of those layers. The bite may have been a river flowing through a relatively level layer that formed under an inland sea. Sometimes a shift in the earth’s crust caused the surface to drop in one area. After the layers were in place, erosion by wind, water, or ice, cut away the layers, so that today, parts of all the layers are visible in some parts of North Dakota. (See Cake Images.)

A good place to see formations is in the badlands of western North Dakota. We are attracted to Theodore Roosevelt National Park because of the beautiful exposed layers of ancient rock formations that form the backdrop to the Little Missouri River. These rock layers were deposited during the Paleocene (PAY lee o seen) Epoch between 60 and 50 million years BPE. The two main layers are the Bullion Creek and the Sentinel Butte formations of the Paleocene Epoch. Bullion Creek, the older (and lower) formation, is bright yellow and tan. Sentinel Butte is grayer and browner. Other layers add to the colors, including HT Butte clinker which is the hardened red rock left from burned coal. Bentonite clay is the light bluish-gray that adds another layer of color to the cliffs in the badlands. Sentinel Butte and Bullion Creek formations contain fossilized animals (not dinosaurs).  There are also plant fossils including the petrified trees that you can see if you hike through the park.

Why is this important?   Though North Dakota (and everywhere else, too) was put together in cake-like layers, the effects of wind, water, ice, and tectonic shifts along the earth’s crust have opened those layers to give us geological information of our very distant past.

The organization of the layers and the fossils they contain teach us how the earth formed and how plants and animals lived, died, and evolved. The geological evidence captured by formation tells us that geological formations, plants, and animals are part of a dynamic, ever-changing earth.