Rock Layers and Fossils

(Truck driving past a rock wall.)

[Abby Brown] This is a very unique spot in Iowa where we can learn lots about our geological history, by studying the changes in the layers of this rock.

(Closeup of a wall of rock.)

Where could I be in Iowa? Come explore with me.

Sometimes the rocks that tell the history of Iowa are in places where people drive by every day without ever recognizing how special they really are. Just like the rocks we're exploring today in northeast Iowa in a town called Graf.

(Drawing of the state of Iowa with a location marker on the northeast side of the state designating the town of Graf, Iowa.)

Where we can see layers and layers of ancient rocks right on the side of the road.

By studying the layers from the bottom all the way up to the top, we can tell so much about Iowa's history. Millions and millions of years ago, this rock was deposited in Iowa. My good friend, John, lent me one of his favorite geology tools to take a closer look.

(Abby uses a small pickaxe to break up the rock. As she is striking the ground, the rock is flaking apart in large junks causing dust particles.)

Some of the layers are made out of shale. They're softer because they're made out of clay and sand. Some of the layers are made out of limestone. They're a whole lot harder.

(Abby uses the pickaxe to try and break a piece of limestone. The ax makes a ringing sound when it hits the limestone. The limestone does not break or flake off.)

These layers of limestone and shale are evidence that Iowa was once covered in a warm, tropical sea. The alternating layers tell us that the sea sort of came and went, or there were times where it was much deeper and much shallower. The limestone contains even more evidence of Iowa's history. Can you guess what it is?

The limestone in Graf, Iowa, is so special because it contains fossils. These fossils are actually sea creatures.

This fossil is a nautiloid cephalopod. These creatures lived in the warm, tropical sea that once covered Iowa.

So how does a creature once living and swimming around in the sea become fossilized and part of a rock?

(Two sea creatures come into the frame from the right and left. Both have cone shaped bodies, two eyes and tentacles like an octopus where their mouths would be. Just like all forms of life, sea creatures from long ago died; and when they died, they sank to the bottom of the sea and into sediment or mud. The sea creatures fall to the ocean floor at the bottom of the screen sinking below the brown layer that represents mud. Eventually more and more sediment piled on top of them, until the creatures were completely buried. Layers of mud are added to the frame from the bottom up. Each layer gets lighter in color then the last. Representing the passage of time. Over time, that sediment turned into sedimentary rock, and so did the remains of the creature. They literally turned into rock. The layers of mud on the screen change from brown to gray in color and flexes of white have been added to represent the formation of rock.)

The nautoid cephalopod is just one type of fossil. Do you think there could be other types of fossils in other places in Iowa?

[Announcer] Funding for Find iowa has been provided by Pella and the Gilchrist Foundation.