Palisades Geology
The Palisades
The Palisades, forming the ridge crest on the northern horizon, is made up of three geologic formations: the Bighorn Dolomite of Ordovician age, the Jefferson Limestone of Devonian age, and the Madison Limestone of Mississippian age.
Depositional Environment
All of the formations named above
are carbonates either limestone or dolomite. That means they consist of grains,
fragments of fossils, and precipitates all made of calcium carbonate, CaCO3. All
of these formations are of marine origin, having been deposited in ocean waters
up to several hundred feet deep. Obviously they are no longer at sea level so
they must have been lifted up to their present position.
Mountain Building
The Northern Rocky Mountains were lifted up during several episodes of mountain building. This mountain building was apparently caused by the interactions of what are called tectonic plates. The entire outer portion of the earth, including the crust and the outer part of the earth's mantle, behave as though the material is rigid. The earth is divided into a dozen or so major plates and about two dozen minor plates. As these plates move about on top of a less rigid, softer underpinning, they tend to run into other plates. When two plates collide something has to give. In many cases one of the plates is pushed up as the other plate glides underneath. The most dramatic example of this is the Himalayan Mountains and Tibetian Plateau, raised to enormous heights as the Indian-Australian Plate crushed into and slid under the Asian Plate. A similar event occurred in the history of the Northern Rockies pushing up the Beartooth Mountains to heights not unlike those of the Himalayans today. Subsequent weathering and erosion have taken their toll on the Beartooth Mountains, reducing them to a somewhat lower elevation than the Himalayans. During this episode, the rocks that now make up the Palisades, were pushed up from below sea level to heights as much as 20,000 feet above sea level. As they arose, they were warped from a horizontal to a nearly vertical attitude, with bedding dipping at a very steep angle. In fact the bedding was pushed up to vertical and then beyond vertical, so that they now dip in an overturned condition, back toward the south. This uplift occurred along a major thrust fault, which occurs along the north edge of the Palisades. A subsidiary thrust fault brings Precambrian rocks over rocks of younger age in several places in this region. As the thrusts moved forward, one part extended beyond the rocks on either side, producing "tear faults" along the margins of the forward moving portion. This motion produced the offset of the Palisades along the Willow Creek Fault, as well as several other tear faults. When driving along the Red Lodge Mountain ski road, one can observe this offset, where the Palisades seem to end abruptly just above the Palisades Campground. The extension of this same Palisades ridge can be seen offset much farther to the south of the West Fork of Rock Creek. It can then be followed past Rock Creek, at the Point of Rocks, offset again on the flanks of Mount Maurice, and then continuing south toward the Montana-Wyoming border near the Clark Fork Canyon.