North Dakota History Geology of the South Unit, Theodore Roosevelt National Memorial Park

Stratigraphy of the Bed Rock

The bed rock of this area belong to a division of geologic time known as the Cenozoic era (See Fig. 1). To be specific the rocks were laid down in the Paleocene period of this era. The Paleocene, oldest of the periods of the Cenozoic, began approximately 60,000,000 years ago which is relatively recent from a geologic standpoint.

The beds of the Paleocene exposed in the Park are called the Tongue River formation of the Fort Union group of formations. It is named from the area around the Tongue River in Wyoming where it is typically exposed. The formation consists of shales clays, sandstones, silts, sands and lignite. In general, the shales and clays are gray to brown and the sandstones tend to be light yellowish-orange to buff and tan. The lignite is dark brown to black. The interbedded strata showing many different colors on the hillsides in the Park add much to its scenic beauty. Frequently, the lignites have burned out, baking and fusing the overlying clays, shales and sands into a red to brownish red color. This baked and fused material is locally called "scoria" but should be more correctly called "clinker."

The entire thickness of the Tongue River formation is not exposed in the Park area and only the near-basal part of the formation is seen here. The exposed part of the formation totals about 620 feet. This does not mean that this entire thickness is exposed in one spot but this is the entire thickness when all the beds are correctly placed in respect to each other. The formation dips very gently to the southeast about one to two degrees.

Fossils other than plant fossils are not particularly abundant in the Park. Many plant fossils are found particularly in the shale and clays. Sometimes these fossils are excellently preserved in the "clinker" beds even though the beds have been baked by the burning lignite.

The Tongue River formation was laid down entirely on land and contains no beds deposited in a marine environment. Visualize for yourself a large plain stretching eastward from the Rocky Mountains which at the beginning of the Paleocene period were very high and being rapidly eroded. This plain extended for unknown distances from northern Canada to the southern part of Texas encompassing about the same area as the present day Great Plains. Large rivers were coursed eastward across this plain on their way to the sea. Some of these rivers drained into the Gulf of Mexico while others drained to Hudson's Bay. Remember, however, that the climate in those times was not as it now is and probably was somewhat warmer and moister. Remember also that there is geologic evidence, too detailed to be gone into here, that the eastward extension of this plain in which is now the Dakotas was probably near sea level in elevation.

With all the foregoing pictures in mind, you are now prepared to visualize the deposition of the materials which are known as the Tongue River formation. Near the mountains in Wyoming and Montana, the rivers were running swiftly and were carrying great quantities of coarse material such as gravel. At the point where the rivers flowed from the mountains out on the plain, they lost much of their carrying power due to decreased velocity and thus the coarser gravels and sands were deposited near the mountains. The finer materials such as the clays, silts and finer sands were carried farther eastward to the Dakotas where they were laid down on this large alluvial plain. This deposition in the Dakotas was not everywhere regular and blanket-like but as the rivers turned from side to side, they deposited at one spot at this time and somewhere else a few years later. This irregular mode of deposition made the strata somewhat lenticular.

Spaced here and there on this great plain were swamps probably similar in some respects to the present day swamps in Louisiana and Florida. In these swamps, there were great numbers of trees of the Conifer family growing. Some of these trees were Sequoia and were related to the Sequoia which grow in California today. As these trees died and fell into the swamp, they began to decay due to bacterial action. However, this decaying action was not complete and before the trees could be completely decomposed, the bacteria action was arrested. The cause of the halt of the bacterial action was the fact that the bacteria had committed suicide by filling the stagnant swamp water with their body poisons to such an extent that they died. The partially decomposed trees or vegetation were later solidified by the weight of younger sediments deposited on them and thus the extensive lignite coal beds with which North Dakota is so well endowed were formed.