Trailside Geology - Bridger Bowl

When buying ski passes, or heading up Slushman’s with protein bars and avalanche beacons, we don’t normally think about the mountains themselves—our minds are on deep powder and first tracks. But just for a moment, forget the skiing and let the millions of years of mountain building, volcanism, inland sea formation, and glacial periods wash over you. Earth’s history is right under our feet—or, in this case, our skis.

One-third of the way up the Bridger lift, calcite crystals can be found in limestone boulders that were once high on the ridge before being carried down-slope by erosion. Throughout the area are signs of extensive rock movement, as evidenced by "slickensides," or the smoothly filed sides of the stones as they slid against each other under intense pressure during folding and faulting while the mountains were forming.

About 40 million years ago, when Montana was a hotbed of volcanic activity, the Bridger Mountains rose during the fourth phase of mountain building in the area. With the help of a large thrust fault, the rocks buckled and the continental crust pushed the range up. However, much of the rock that would later be uplifted was laid down millions of years before.

While on the ridge enjoying a fine drift of powder, imagine the land before you as the bottom of an inland seaway, the last of which left this area during the latter part of the Cretaceous Period, about 75 million years ago. Many of the rocks found in the Bridgers were laid down during this time, as the landscape changed from a deep ocean to tidal pools, leaving behind traces of life-forms long extinct. Along the ridge, fossils, like coral and crinoids (a type of primitive sea urchin) from the Lodgepole Limestone formation, are evidence of prolific marine life in the prehistoric oceans.

Ross Pass (just south of Ross Peak) is the dividing line, geologically speaking, for the Bridger Mountains. On one side the rock is primarily sedimentary and on the other side an oblique fault exposes pre-Cambrian rock—some of the oldest rock around—on the west side of the Bridgers. As different strata dip at varying angles due to faults and folds, the beds are tilted and rocks of various ages are exposed.

Starting at the Deer Park Chalet, both the Colorado and the Eagle formations are underfoot. Those formations date back to the Cretaceous period—between 65 million and 136 million years ago—when most of the Rocky Mountains were formed. They are a salt-and-pepper-colored sandstone, created from the consolidation of grains of sand, likely deposited on an ancient beach. Terrestrial, brackish, and marine fossils have been found in this formation. Thick-shelled pelecypods (a type of mollusk) in the uppermost part of the formation, south of Cache Creek, again indicate that part of the formation is marine.

While going up the Bridger lift, notice a little knob to the right of the Deer Park Chalet. It is likely that this is a moraine, or rock deposited by glaciers. This particular moraine dates back to the Wisconsin glaciation, the fourth and youngest stage of the Pleistocene (Ice Age) stage.

Below the North Bowl, in Powder Park, is more evidence of the glaciers that were here between 10,000 and 75,000 years ago. In fact, the North Bowl was formed by a circular land formation that probably housed glacial ice and now houses an avalanche gun perch. However, there is still a question as to whether the South Bowl housed any ice at all.

One of the most interesting features of the Bridger Bowl area is that the layers of rock are not in the usual order—that is, from oldest to youngest with the youngest layer being on top. In this case, because of an overturning of an anticline—an upward arching of layered rocks—and then later, the breaking off or eroding of one side of the anticline, the older layers are exposed on the top. The buckling of the sedimentary beds was so severe that the beds were completely overturned and their tops are now on the bottom.

The limestone prevalent on the top-most layer of the ridgeline can be matched up to a similar layer that existed prior to the rise of the mountain range, about 20,000 feet below the top of the ridge, rock that was once on a sea floor. This other layer faulted under the valley while the Bridger Range was uplifted.

So, the next time you ride the lifts, or stand on the ridgeline, imagine what this land looked like millions of years ago, and consider how good the skiing must have been when the North Bowl was glaciated.