July 10: Fort Mountain

"Square Butte"

View Northeast

Square Butte

© 2002 Airphoto, Jim Wark. All rights reserved.

Shortly after leaving their upper canoe camp in mid-July of 1805, both captains took note of "a round mountain on our right abt. 10 miles appears inaxcessable." They called it "fort mountain." There are two other more or less flat-topped buttes in the immediate vicinity, but they were not visible from the Missouri River.

"Battle Hills"

To see labels, point to the image.

Interactive aerial photo showing present names for prominent landmarks

© 2002 Airphoto, Jim Wark. All rights reserved.

Fort Mountain

Its straight vertical battlements and the panoramic view from its brink make it look just as a fortified mountain should. Countless generations of Indian people had already seen it as an "inaxcessable," safe retreat too, and thought of this and the other two prominent buttes as "Battle Hills." The treeless heights of both Square Butte and Crown Butte are indeed inaccessible by wheeled machinery such as plows and combines, so they remain among the few pristine islands of the mixed prairie that once covered the arid high plains.

How this landmark came to be called Square Butte is not known for sure. Before the advent of aerial photography it may have seemed more square when viewed from the valley, especially its east face, than either Shaw Butte or Crown Butte. But the geometrical distinction is relatively unimportant, compared with the story of the geological events that created these and many other similar formations in this vicinity. The three laccoliths in this famous group were not all created at the same time. Shaw Butte has more rounded edges than the other two because it still has more of its original sedimentary cover, which indicates it is the youngest.1

Haystack Butte—Lewis's "—Shishequaw Mountain"—was formed by the same processes at about the same time, give or take a few million years. Clearly, however, conditions in the sandstone layer at that place created a laccolith of a different, though equally distinctive, shape.


1. David Alt and Donald W. Hyndman, Roadside Geology of Montana (Missoula, Montana: Mountain Press, 1986), 351-55. Shonkinite is a dark igneous rock consisting of augite, a dark silicate mineral compound containing iron and magnesium, plus white or pink crystals of potassium feldspar. It is similar to the basalt that can be seen as outcrops of the Idaho batholith around Lolo Hot Springs, but it contains more potassium than basalt does.

Funded in part by a grant from the Montana Cultural trust