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

Unusual Geologic Features of the Park

"Scoria" or Clinker

Certainly, the most noticeable and outstanding color in the Park is the red of some of the rocks. Invariably, these rocks are the ones about which there is the greatest question in the mind of the visitor. These beds are locally called "scoria" and are formed by the fusing and baking of sands and shales and clays when a bed of lignite immediately underlying them burns.

The term "scoria" is incorrectly used geologically speaking when used to describe rocks of this nature. Scoria when correctly used applies only to a dark-colored porous rock associated with basaltic lava flows from an active volcano. Thus, it is a term to be used only with igneous rocks. Inasmuch as all the rocks of the Park are of sedimentary origin, that is, laid down by water, the term should not be used here.

The correct term for this material is clinker. This material as noted previously was formed when the minerals composing them were altered chemically due to being heated to or about the fusing point. In some places, they melted and flowed like slag out of a blast furnace. It may be this flow structure plus the porous nature of some of the clinker which caused the early settlers to call this rock "scoria." It should be categorically stated that there are no igneous rocks native to the Park area and that volcanoes never existed in the badlands.

Clinker, due to its alteration by fire, does have some interesting features which show some resemblance to igneous rock. One is that in some places, it has melted enough to flow. Another feature is the columnar jointing which is found in some of the outcrops. This type of jointing can best be described as a series of cracks produced usually at or near right angle to the surface of greatest cooling. The end result is the production of numerous polygonal columns standing side by side on the outcrop. Usually, Inasmuch as the greatest cooling surface was generally the surface of the ground and inasmuch as the rocks are nearly horizontal, the polygonal columns usually cut across the original bedding planes of the rock. The jointing is being called in this report pseudocolumnar jointing. Such jointing is a feature found usually only in igneous rocks. Such columnar jointing can be well seen in the lava flows interbedded with the gravels at Tower Falls on the Yellowstone River in Yellowstone National Park and in the igneous rocks exposed in the Devils Tower in the Devils Tower National Monument in Wyoming.

Petrified Trees

An item of great geologic interest in the Park is the presence of numerous petrified stumps in certain zones in the strata (See Fig. 7 & 8). The stumps do not occur in all the rocks but seem to be concentrated in zones where apparently they were originally more abundant than they were at other levels. Most frequently, only the rootless stumps of these trees are found and seldom if ever are the roots with them. This may possibly be explained by the fact that the trees are probably representatives of a type of conifer known as SEQUOIA. The SEQUOIA and other conifers have a very shallow root system. Inasmuch as the roots were near the surface and were smaller and less durable than the stumps, they all probably decayed before petrifying. Similar reasons must also be given for the few trunks and branches although some of these are known. In one spot in the Park, a trunk measuring more than 40 feet in length was observed (See Fig. 9).

Figure 7. Stump zone in Petrified Forest area of South Roosevelt Park.

Figure 8. Petrified Sequoia stump in the Petrified Forest in the northwestern part of South Roosevelt Park.

Figure 9. Petrified log located in north-central part of South Roosevelt Park.

Interestingly enough, the petrification of the stumps is not in every place complete. On some of the stumps, it will be observed that the outside of the stump has been completely turned to stone while the inside of the stump is as yet not completely petrified and is still almost charcoal-like. The petrification may take place in either of the following methods:

(1) After the tree is buried, the water in the rock begins to percolate through it. This water does two things: first, it dissolves out with the help of bacterial action the softer cellulose material of the wood; secondly, the water carrying mineral water in solution (in this case silica) deposits the mineral in the space left by the dissolving out of the plant tissue. This goes on so that the replacement is very gradual and is almost a molecule for molecule affair—a molecule of plant tissue being simultaneously replaced by a molecule of silica. In this way, the original cellular structure of the wood is preserved so that today the stumps in many cases look exactly like old wood stumps except for the fact that they are stone.

(2) The more likely explanation is that the original pore spaces have been filled in with silica which got there by being deposited from water solutions passing through the rocks or from grains of silica being actually washed into the small voids of the tree. This is known as the process of infiltration. In this process the cell walls of the tree are still present in more or less their original form. These walls at a later date may become either replaced or infiltrated by mineral matter. The mineral matter may be the same mineral as that filling the voids or it may be a different mineral.

Sandstone concretions

There are in the Park many large and irregularly-shaped sandstone concretions which in many cases look almost like petrified logs and in some instances have probably been mistaken for them. These are not fossils because they have never been living material and they are entirely inorganic in origin.

The exact origin of these features is not known but probably they form in the sandstone beds where the sand is locally more permeable and allows the passage of underground water more freely. The underground water carries mineral matter in solution. This mineral matter may be either silica as mentioned in the case of the petrified stumps or calcium carbonate (lime). Whichever mineral it is makes little difference as it will be deposited ultimately in the pore spaces of the rock in these permeable zones. When the sand in these zones has been cemented with this mineral matter, the zones are more resistant to erosion and therefore, stand out on weathering. Thus, we have these unusual "log" and "ball" concretions present in numerous places in the Park. Frequently, they give rise to interesting erosional features in that the more resistant concretions cap less resistant sands and clays giving rise to unusual pillars and columns.

Figure 10. Little Missouri North of Medora showing its carving action on butte leaving steep sided walls.

Figure 11. Old Valley formed by Little Missouri or a Tributary.