The Pierre Shale is a geologic formation or series in the Upper Cretaceous which occurs east of the Rocky Mountains

Rocky Mountains

The Rocky Mountains are a major mountain range in western North America. The Rocky Mountains stretch more than from the northernmost part of British Columbia, in western Canada, to New Mexico, in the southwestern United States...

 in the Great Plains

Great Plains

The Great Plains are a broad expanse of flat land, much of it covered in prairie, steppe and grassland, which lies west of the Mississippi River and east of the Rocky Mountains in the United States and Canada. This area covers parts of the U.S...

, from North Dakota

North Dakota

North Dakota is a state located in the Midwestern region of the United States of America, along the Canadian border. The state is bordered by Canada to the north, Minnesota to the east, South Dakota to the south and Montana to the west. North Dakota is the 19th-largest state by area in the U.S....

 to New Mexico

New Mexico

New Mexico is a state located in the southwest and western regions of the United States. New Mexico is also usually considered one of the Mountain States. With a population density of 16 per square mile, New Mexico is the sixth-most sparsely inhabited U.S...

.

The Pierre Shale was described by Meek

Fielding Bradford Meek

Fielding Bradford Meek was an American geologist and paleontologist.The son of a lawyer, he was born in Madison, Indiana. In early life he was in business as a merchant, but his leisure hours were devoted to collecting fossils and studying the rocks of the neighborhood of Madison...

 and Hayden

Ferdinand Vandeveer Hayden

Dr. Ferdinand Vandeveer Hayden was an American geologist noted for his pioneering surveying expeditions of the Rocky Mountains in the late 19th century. He was also a physician who served with the Union Army during the Civil War.-Early life:Ferdinand Hayden was born in Westfield, Massachusetts...

 in 1862 in the Proceedings of the Academy of Sciences (Philadelphia). They described it as a dark-gray shale, fossiliferous, with veins and seams of gypsum

Gypsum

Gypsum is a very soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO4·2H2O. It is found in alabaster, a decorative stone used in Ancient Egypt. It is the second softest mineral on the Mohs Hardness Scale...

, and concretions of iron oxide. The Pierre Shale is about 700 feet (210m) thick at the type locality.