This area, geologically speaking, is near the center of the famous Williston Basin. This is an area of down-sinking which has progressively sunk lower and lower during most of the geologic time since the end of the Pre-Cambrian which was roughly 500 million years ago. (See Table I)
This down-sinking was a very slow process, however, and would not be perceptible to the human eye had anyone been there at the time. As the area sank, it became more and more filled with sediments. Most of these sediments were washed into this basin from highlands located to the north, northeast, and perhaps at times to the west. Greatest amounts of down-sinking took place during the Ordovician, Devonian, Mississippian, and probably Cretaceous times. The down-sinking probably continued even into the Tertiary period, most notably in the Paleocene, although the effects of the down-sinking are not so well known and not so easily noted in this latter-named period.
In all, a total of greater than 15,000 feet of sediments, most of them shallow-water deposits or continental deposits were laid down in the Williston Basin. The last sediments that were deposited, namely those of the Paleocene period, are, in this area, entirely continental in origin. There were some beds of marine origin deposited during the Paleocene, but they are exposed to the east and southeast of the area of this report. It is the beds of the Paleocene period which should be described most fully, as these are the sediments which one can see in the Park area proper.
These beds have been named the Tongue River formation from the Tongue River in Montana. This is one of the formations of the Fort Union group. Formations which immediately overlie and underlie the Fort Union group are as follows:
The beds of the Tongue River formation have been previously noted as all non-marine or continental in origin. They were deposited on land, probably when this area was considerably lower in elevation than it is now. Very likely, at the time of deposition of these beds, this area was not far from sea level. Probably if we could trace them eastward and if the deposits were still in existence, we might conceivably trace them into a marine formation. This is, of course, something that we will never know as the marine beds, if they were ever present, have been eroded away.
These beds were deposited on what can be called an alluvial plain which sloped eastward from the then newly-formed Rocky Mountains. On this alluvial plain, there were many rivers wandering back and forth depositing sediments here and there. From time to time, certain portions of the area became shut off from deposition by streams and great swamps developed in which trees and other plants grew. As these trees and plants died, they fell into the swamp where they were later turned to lignite by partial decomposition by bacteria and the pressure of overlying sediments. As the streams continued to flow to the sea, more and more sediment was deposited on this area until probably a total thickness of greater than 1000 feet of sediment now called the Tongue River formation was deposited. These strata consist largely of fine-grained sands, shales, clays, lignites, and some bentonitic clays.
The Tongue River formation the North Unit is represented entirely by the Sentinel Butte member which is the upper part of the Tongue River formation. The Sentinel Butte is characterized by being darker in overall color and having fewer lignites than the remainder of the Tongue River formation. With fewer lignites to burn, there are consequently fewer "scorias" and thus the red color is not as prominent in the North Unit as it is in the South Unit of the Park where the lower portion of the Tongue River which contains more lignite beds is exposed.
At the same time that this material was being deposited on this alluvial plain near the sea, there were volcanos erupting to the west in what is now the present-day Rocky Mountains. These volcanos threw out a tremendous amount of ash and other materials which settled down to earth far to the east of the volcanos themselves. These volcanic materials gave rise to the types of clays which we classify as bentonitic or swelling clays.
There are several different strata in the Sentinel Butte member in the North Unit which deserve description because they are so prominently displayed. Undoubtedly, the most spectacular bed in the Park is the blue bentonitic clay prominently displayed below the first observation point after leaving the Squaw Creek Campground. This bed can be traced with the eye for miles up and down the river and for some distance to the north. Because it is bentonitic, it is plastic and when wet "flows" down hill. Numerous places can be seen on the sides of the buttes where this "flowage" has taken place. This clay is very sticky when wet. When it dries, the surface of the clay shows a characteristic polygonal cracking due to the weathering which is frequently found in bentonitic type clays. This clay has been locally spoken of as the Big Blue Bed.
Above the Big Blue are two other yellowor buff-colored beds separated by a slightly darker shale. These yellow beds are siltstones and silty clays and like the blue clay underlying them, they too can be traced with the eye for long distances.
Toward the top of the buttes is one of the more prominent red beds of the area. This is a "scoria" formed when the underlying lignite burned and baked sands and clays above it. As there are fewer large lignite beds in the North Unit, there are consequently fewer "scorias". It should be noted that this is incorrect geologic usage of the term "scoria", but it is used locally for the beds described above. True scoria is an igneous rock.
Other interesting features in the Park are the sandstone concretions. These might be readily mistaken for fossil logs. In most cases they are not fossils, but are simply hard sandstone beds usually in the form of elliptical, round, or irregularly-shaped masses. The formation of these particular concretions is not entirely clear. However, it seems that very likely the formation is due to the deposition of some cementing material usually calcite or other limey material around the sand grains. Perhaps the reason why this limey material was deposited irregularly instead of uniformly, is that these particular zones of sand were originally more permeable and allowed the water to pass through them more readily. Therefore, as the water passed through these zones, readily, it may have deposited material which it was carrying in solution, cementing the sands in these spots more readily than elsewhere where the water was not passing so freely. In any event, the concretions make rather interesting sights on the sides of the buttes.
Evidences of the life of this particular area during the time of the deposition of these rocks can be seen in the plant fossils as well as the gastropod and pelecypod shells which are found scattered here and there in the rocks in the Park. There are relatively few of these fossil-bearing localities readily found.
Last Updated: 28-Mar-2006