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The mountains of the Boulder Batholith that cover most of Jefferson County largely avoided the effects of glaciation during the last ice age, but the bones of a few deceased glacial features remain.

Just south of I-15, less than a mile east of the Bernice exit, one can look across the Boulder River and see a field of scree coming off the mountain. It’s not just any rock slide, it is a rock glacier, a remnant of ice on a once-glaciated mountain side.

Rock glaciers are masses of mineral solids and ice that flow downhill. Sometimes they’re caused by an ice field that is covered by a landslide, which insulates the ice and preserves it, and sometimes they are just rock slides that through hundreds or thousands of years of freeze and thaw, begin to flow. Though they are common in the high mountains of western Montana, in reference to this particular feature, the late and well-known Geologist Dave Alt said it was rare to see one at such a low elevation (5,500 ft).

Though it is made of rock, it looks like a liquid frozen in its seep. It has ripples and seems smooth, but it is made up of large angular scree, some individual rocks as big as a person. 13,000 years ago the whole Boulder Mountain Range was colder, but outside of glaciers on the flanks of peaks that feed the headwaters of the Boulder River, the Boulder Mountains largely lacked glaciation.

Somehow the talus falling off of the north slope of Swede Park’s mountain was interstitially laden with ice, maybe insulating it for hundreds or thousands of years after the ice age ended, and a small valley of granite oozed north to the Boulder River. The contours are beautiful, and are a representation of features called flowage wrinkles. Given enough time, gravity works on ice and rock in similar ways to how it works on water.

The rock glacier is now a relic of the Pleistocene, succumbing to time and climate, it is now inanimate in terms of ice. More than just an earthly carcass of history though, it is an aperture for daydreams to a time when the Boulder River was fed by glacial meltwater, and wooly mammoths wandered the Boulder Valley.

Keep in mind that the terminus of this ancient flow is on private land, so if you want to access it you’ll need to hike up on the Forest Service ground and approach its flank, most easily on its western side.

I hope you do.

Bret Lian grew up in Clancy and lives in Jefferson City with his wife Lisa, their three children and a dog. The places and histories of Jefferson County and beyond have always provided endless daydreaming material to this geography-minded Montanan, who provides this column monthly to the Monitor.

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