Montana Earth Science Picture of the Week

Montana Rocks!

Map courtesy of Montana Bureau of Mines and Geology
Click here to see a more detailed geologic map.

This map uses color to show the ages of rocks that are at or nearest the surface throughout Montana. In most locations the rock (bedrock) is covered by soils. In some places it is exposed in road cuts, cliffs, valley walls, canyons, outcroppings, or on steep mountain slopes.

The Age of the Dinosaurs . . .
Throughout the green area rocks made up of sediments that were deposited in the Mesozoic Era lie closest to the surface. This area is dominated by horizontal layers of sedimentary rocks such as sandstones and shales that originated as sands and muds at the bottom of an ancient sea. Since the dinosaurs roamed Montana during the Mesozoic Era (250 to 65 million years ago), the green area is the best place to find fossils of dinosaurs. In parts of central Montana the horizontal layers were intruded by magma during a period of volcanic activity that formed some of the isolated mountain ranges in this part of the state, including the Bear Paws, the Little Rockies, and the Highwoods. The red areas on the map indicate the presence of igneous rock that formed as this magma/lava cooled. Other periods of volcanic activity resulted in the presence of igneous rocks at surface in southwestern Montana.

After the Big Guys Died Off . . .
The eastern portion of the state sits on sedimentary rock formed from sediments that were laid down during the Cenozoic Era (65 million years go to present), after the dinosaurs died off. In this part of the state the rock layers that would contain dinosaur fossils lie beneath the rocks of the Cenozoic Era.

The Messy West . . .
Rocks in the western portion of the state are much different from the well-ordered horizontal layers of sedimentary rock found in eastern Montana. Surface rocks in the west are made up of a variety of older rocks that have been faulted and/or folded by the movement of crustal plates, and also intruded by volcanic activity. The blue areas on the map are mostly Madison limestone that was laid down on the floor of a tropical sea about 350 million years ago. The lighter brown areas are places where you will find sedimentary and metamorphic rock that formed between 2.5 and .5 billion years ago. The oldest rocks in the state (3.8 to 2.5 billion years old) are represented by the dark brown on the map.

Terms: Mesozoic Era, Cenozoic Era


List of Past Pictures of the Week
*Maps of North America during past Eras
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By Rod Benson
Earth Science Teacher at Helena High School

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