Types Of Rock In The Badlands

The Badlands are anything but bad.

The Badlands are a unique cross-section of different land masses. Much of the sedimentary materials that make up the landscape have been washed from the Rocky Mountains. Other sediment material has been deposited from the many glaciers that have grown and receded across North America. Fossils also play their part as different plant and animal species have flourished and died out in the area.

Pierre Shale

One of the deepest forms of rock in the Badlands is called Pierre Shale. Pronounced “peer,” this shale was formed when the Badlands were covered with ocean. As plants and sea creatures died, they sank to the floor of this ocean and decomposed. Over centuries, this layer of decomposing matter became compressed, forming a layer of deep black shale that exists today.


As the ocean drained away, rivers replaced them. Trees grew along the rivers, and after a time, they. too, died and decomposed forming areas of peat. This peat was in turn buried under river clay and even volcanic ash. Time and pressure transformed the peat into veins of lignite coal.


As the western volcanoes spewed ash into the air, much of it drifted east and ended up in the badlands. Rain washed the ash into lakes and ponds where it became more concentrated and was ultimately covered with other sediment. These compressed layers of ash developed into beds of bentonite and can be spotted as the distinctive blue layers of eroded rock.


Granite is formed when many different minerals are compressed over time. Granite comes in different colors, depending on the minerals contained. Quartz and mica make up much of the granite in North America. When iron is present, the granite has a reddish tinge due to the rust that forms as the iron oxidizes. When feldspar is present, the granite has a greenish tint.

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Petrified Wood

Petrified wood formed when trees became trapped in water or clay that is high in calcium and silica. The water prevents bacteria from decomposing the wood. The high mineral content of the water or clay slowly replaces the cells of the plant with minerals. This creates an exact copy of the original tree. But instead of being made of wood, the tree is now made of stone. Many of the petrified trees in the badlands are light brown in color and so soft that they quickly erode when exposed to the elements.