Exploring Capitol Rock, MT

Wide-angle view of Capitol Rock, MT.  Image credit: Bill Mitchell (CC-BY)
Wide-angle view of Capitol Rock, MT. Image credit: Bill Mitchell (CC-BY)

Several weeks ago, I took a road trip with some friends across the northern part of South Dakota as part of a ham radio adventure. When we reached northwestern South Dakota, we were having so much fun that we decided to continue into just across the border into Montana.

At the state line between South Dakota and Montana, we found that there was a relatively high point (Capitol Rock) which we could probably access with our vehicle. Capitol Rock is in a national forest, so no permission would be needed to go there. It would be a good place to do ham radio (primary goal), and it would get me close to some rocks (bonus)!

As we drew closer to the summit of the hills, I couldn’t help but think that the rocks looked a lot like the ones in my research area in northeastern Montana, in the Hell Creek region (Hell Creek and Tullock/Fort Union Formations).[1]

Sadly, I didn’t get quite as close to the outcrops as I would have liked (we were on a bit of a schedule), but I did get some pictures and made a few observations.

North half of Capitol Rock.  Image credit: Bill Mitchell (CC-BY).
North half of Capitol Rock. Image credit: Bill Mitchell (CC-BY).

Here we had flat-lying sedimentary strata, presumably of roughly Cretaceous-Paleogene age (somewhere around 80-50 million years ago, Ma) (introduction to geologic time). These would have been shallow marine or terrestrial sediments from along the western interior seaway, which was on its way out at the end of the Cretaceous (66 Ma, [1]). I would expect to find some fossils preserved in the sediments, and from those, a fairly accurate date on the strata could be obtained. There may even be some volcanic ash deposits which would allow for direct dating using the U-Pb system or the K-Ar system (Ar/Ar dating) .

At the top of Capitol Rock were several massive units with a slight orange color (probably from oxidized iron). Beneath those were some more finely bedded strata, with bed thicknesses probably around 3-10 cm (eyeball estimation), and displaying some rough texture (popcorn texture?). Underneath those were some fairly easily eroded strata of generally uniform grey color. The image below has these observations annotated.

Northern portion of Capitol Rock, annotated.  Image credit: Bill Mitchell (CC-BY)
Northern portion of Capitol Rock, annotated. Image credit: Bill Mitchell (CC-BY)

The ground under my feet for that previous picture was still above average terrain. Here is an additional picture, taken from the south (looking north), which shows that the light-grey sediments are underlain by more yellow-orange units.

Distant photograph of the lower portion of stratigraphy underlying Capitol Rock.  Image credit: Bill Mitchell (CC-BY).
Distant photograph of the lower portion of stratigraphy underlying Capitol Rock. Image credit: Bill Mitchell (CC-BY).

Upon returning home, I decided to see what description I could find online of Capitol Rock’s geology. It seems there are a number of different descriptions of it.

Capitol Rock, located in the Long Pines Unit in Montana, is a massive white limestone uplift that resembles the Nation’s capitol building.
Montana Office of Tourism

Capitol Rock, located in the Long Pines land unit in Montana, is a massive white sandstone remnant which originated as a volcanic ash deposit. This unique formation resembles the Nation’s Capitol Building in Washington, DC.
US Forest Service

The Bureau of Land Management (BLM) has an interesting discussion of the geology of this area from the perspective of firefighting, specifically in the avoidance of fibrous or asbestos-like minerals which are present in some of the formations in the area:

Brule Member, White River Formation [ed: Formations are a larger stratigraphic unit, and can include multiple Members] – may only be present at Capitol Rock (SE 1/4 sec. 17; T3S; R.62 E) in the Montana portion of the Sioux District. Located at the base of the Arikaree Formation. Massive pinkish gray, calcium containing, clayey siltstone: nodular claystone: and channel sandstone. Contains abundant vertebrate fossils. Thickness 0-30 ft. The member is composed of massive pink clay, exposed in the badlands just Southeast of Reva Gap, well-bedded, hard pale green sandstones alternation with very pale brownish gray clay.
Weathering causes a tread and riser affect much like a staircase. Both the sandstone and the clay are generally calcareous and Bentonitic. The lower portion of the vertical cliffs in Slim Buttes is generally Brule.

Chadron Member, White River Formation – only located in the southern Long Pines within Montana. Found at the base of the Arikaree formation and beneath the Brule Formation at Capitol [R]ock (SE 1/4 sec. 17 T, 3 S., R. 62 E). Basal conglomerate sandstone overlain by beds 10 to 15 ft
thick of dark gray bentonite and cream colored siltstone. Thickness 0-100 ft.
Bureau of Land Management

Well, that’s a puzzling bunch of information, isn’t it! Various sources are suggesting limestone, sandstone from volcanic ash, and a mix of sandstone and siltstone. There’s one more source to check, too: the geologic map. Specifically, we’re interested in the Ekalaka 30’x60′ quadrangle from the Montana Bureau of Mines and Geology!

In the geologic map (look along the right [eastern] edge, near the “T 19N” mark; Capitol Rock is ~1 km NE of the “Tar” label] we see the Fort Union Formation (informal Ekalaka member) at the base of the hills (i.e., under my feet), which is consistent with observations and the relatively detailed presentation from the BLM. It is also consistent with my experience that the Fort Union Formation is generally yellow-orange (in contrast to the drab, grey of the Hell Creek Formation). Then things get trickier. The rocks right at Capitol Rock are mapped as “Tar”, which is the Tertiary Arikaree Formation.

So, what is the Arikaree Formation? Well, the USGS has this to say:

Arikaree formation: gray sandstone with layers of concretions; contains volcanic ash and, locally, channels filled with conglomerate; known only in southeastern Montana.

On the other hand, the North Dakota Department of Mineral Resources breaks the Chadron, Brule, and Arikaree into distinct formations unto themselves.

I suspect this is all hitting at an important point: mapping is really hard, as is saying the rocks over here are the same as the rocks 40 km away. These difficulties are compounded when different scientists use different terminology, such as when the mapping is done by state geological surveys. The same rocks may change names when a state boundary is passed. Sometimes researchers will use the terminology from one state to apply to the rocks on both sides of the boundary, and then the literature is filled with multiple terminologies for the same rocks. It can also be very difficult to correlate rocks laterally over large distances, especially when there is poor outcrop over those distances (i.e. between buttes).

Here’s my interpretation of what’s going on at Capitol Rock: it is composed of siltstone, sandstone, and altered volcanic ash [still good for U-Pb dating!]. This volcanic ash is high in erionite, an asbestos-like mineral. Naming of the unit could include either the Arikaree Formation, or the Brule Member of the White River Formation. An age of 37–30 Ma seems reasonable.


[1] Renne, P. R., Deino, A. L., Hilgen, F. J., Kuiper, K. F., Mark, D. F., Mitchell, W. S., III, Morgan, L. E., Mundil, R., Smit, J. (2013) Time Scales of Critical Events Around the Cretaceous-Paleogene Boundary. Science 339: 684-687, doi: 10.1126/science.1230492.