Citadel Rock is a shonkinite intrusion in north central Montana along the Missouri River. This landmark is accessible by boat only. About one mile downstream from the landmark is the Hole-in-the-Wall boat-camp. The BLM maintains this campground which includes vaulted toilets, three-sided log shelters and fire rings. The camp’s namesake, a geographic sandstone formation, is aptly named Hole-in-the-Wall.

These are fine-grained igneous rocks, dominated by dark-colored minerals, occur as dikes, sills and stocks injected into fractures in the Cretaceous age sandstones and shales. They range in age from Tertiary to late Cretaceous. They are more resistant to weathering than the enclosing sedimentary rocks, causing them to form promontory features in the surrounding terrain.

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