2e. The Stratigraphic Column of North Dakota

The “stratigraphic” column shown here lists all of the rocks formed in North Dakota. The deposits of lignite coal are in the Sentinel Butte Formation of the Fort Union group of rocks. It is 55-60 million years old. To compare, the Bakken Formation produces oil and is about 359 million years old, a huge time difference! North Dakota coal is relatively young compared to the state’s geologic history.

The Powder River Basin and Williston Basin are also significant coal producing strata, and they are all part of the Fort Union group of rocks. The coal comes from lake, river and swamp deposits.