Capitol Rock Close-Up

Close-up outcrop photograph of Capitol Rock, viewed from the north-northeast. Image credit: Bill Mitchell (CC-BY).

Two years ago, I came tantalizingly close to Capitol Rock, an outcrop in southeastern Montana (45.572189, -104.087964) just a few miles over the border from Camp Crook, SD. Unfortunately, I did not have an opportunity at that time to explore the outcrop from any closer than about a quarter mile, but I did find the Ekalaka Quadrangle 30’x60′ (pdf) geologic map.

Recently, I was out in the area again, and this time made sure to have time to take some pictures and see some of what was to be seen. Let’s start with the quarter-mile view, which is roughly equivalent to what I saw last year.

Wide view of Capitol Rock from the east. Image credit: Bill Mitchell (CC-BY).

Capitol Rock has three major parts to it: an easily eroded base, a laminated sandstone middle, and a massive sandstone top. A handy turn-out from the forest service road leads right to the base of the outcrop.

The easily eroded base is made of fine, chalky, white sediment sediment, and it remains in horizontal orientation. In several places, this unit is at least superficially porous. Surprisingly, there are occasional chert clasts in the otherwise fine sediments—I’m not quite sure how those would have been deposited or formed here.

Basal unit of Capitol Rock. Foot for scale. Image credit: Bill Mitchell (CC-BY).

Chert clast embedded in the basal sediments at Capitol Rock. Foot for scale. Image credit: Bill Mitchell (CC-BY).

Above the basal unit is a somewhat more resistant, coarser-grained set of beds. These strata are finely bedded, and have a tendency toward spheroidal weathering. Occasionally interbedded with the spheroidal beds are 1–3 cm thick, well-cemented strata of a white or pink color [discoloration?].

Spheroidal weathering of finely-laminated strata. Hand for scale. Image credit: Bill Mitchell (CC-BY).

Laminations in the unit displaying spheroidal weathering. Hand for scale. Image credit: Bill Mitchell (CC-BY).

Non-spheroidal bed 1–3 cm thick and slightly orange-pink in coloration, within the spheroidal beds at Capitol Rock. Hand for scale. Image credit: Bill Mitchell (CC-BY).

The spheroidally-weathered unit also seems to have one or more channels within it.

Contact between spheroidally-weathered strata (above) and easily-weathered basal unit (below). Possible channel cut at right. Outcrop height in image is ~10 m. Image credit: Bill Mitchell (CC-BY).

Contact between spheroidally-weathered strata (above) and easily-weathered basal unit (below). Possible channel cut at right has been annotated. Outcrop height in image is ~10 m. Image credit: Bill Mitchell (CC-BY).

The upper unit at Capitol Rock has more massive sandstone (see wide view above). I didn’t notice many channels in this unit, although I didn’t get very close. A butte just to the north of Capitol Rock provided a good photograph (below).

Massive unit of Capitol Rock, seen in the butte immediately to the north of Capitol Rock. Cliff is ~30–40 m tall. Image credit: Bill Mitchell (CC-BY).

Although I have those observations, I don’t have much for interpretation of them. The depositional environment seems to be relatively low-energy (give or take the chert clasts), evidenced by the flat strata, fine grain sizes, and relatively few cross-beds. Changes in the rock types would suggest changes in the sediment sources or the depositional environment (or both). There may be post-deposition alteration effects as well, such as cementation of the spheroidally-weathering strata.

View SSE from the butte just north of Capitol Rock. Truck for scale in pull-out near Capitol Rock. Image credit: Bill Mitchell (CC-BY).

Capitol Rock is an interesting outcrop, and if you’re in the area, I’d recommend a stop. The rocks are interesting, there are US Forest Service campgrounds nearby, and the view is quite nice. These units can probably be correlated to the Slim Buttes in South Dakota (~45 miles east).

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