Except for a stint in the service, Hank Armstrong has lived most of his 86 years in the house his grandparents built in 1910, seven miles east of Geraldine, Montana. He is a local historian, seemingly familiar with every square inch of land for miles around his native hearth and the stories of everyone who has lived there over the decades.
The center of his world, the world he sees from the windows of his house, is Square Butte, a massive flat-top mesa that rises steeply out of the surrounding plain, with rampart-like tendrils of volcanic rock snaking down its flanks. Armstrong knows exactly how many times he’s been to the top of Square Butte: 63.
“It’s always been a part of my life,” he says.
Dr. Gerald T. Davidson grew up in Harlem, Montana, in the shadow of the Bear Paw Mountains. Like Square Butte and a couple of other isolated mountain ranges in north-central Montana, the Bear Paws are made of igneous, or volcanic, rock. Early in life Davidson developed a fondness for what he calls “geological oddities.”
“Geology is something you can’t avoid if you grew up in Montana, which I did,” he said.
After a 30-year career with Lockheed Martin in California, where he did mostly computational physics, Davidson retired to Red Lodge. In his retirement he has made an intensive study of one of the oddest geological oddities in Montana — the Shonkin Sag.
The Sag, as it is often called, is where the interests of Armstrong and Davidson intersect. The Sag itself marks the intersection of two of the most dramatic periods of Montana’s geological history — an era of intense volcanic activity some 50 million years ago and the relatively recent era of the Ice Ages, which began about a million years ago and ended about 12,000 years ago.
The Shonkin Sag is a now-dry valley carved by river flow and glacial melt water during a succession of Ice Ages. It stretches for more than 60 miles from the town of Highwood, east of Great Falls, to the channel of the present Missouri River east of Geraldine. The valley is a mile or more wide and hundreds of feet deep, pockmarked by a chain of shallow alkali lakes.
Over hundreds of thousands of years, as the great glaciers of North America crept south and then retreated north before the cycle began again, these glaciers dammed precursors of the Missouri River and other streams, creating immense lakes. The freshwater seas would rise until they topped ridges or ice dams, spilling over and creating flow channels. The Shonkin Sag is one such channel. David Alt and Donald W. Hyndman, in their popular “Roadside Geology of Montana,” called it “one of the two or three most spectacular glacial melt water channels in the country.”
The Sag happens to run through land rich in laccoliths, particularly striking remnants of volcanic activity. A laccolith is a layer of igneous material injected between layers of sedimentary rock. These are formed when magma pushes up from underground, lifting the strata above it. In some cases the overlying rock erodes away, exposing giant laccoliths like Square Butte and the nearby Round Butte.
Other laccoliths were exposed by the cutting waters of the Shonkin Sag. The most stunning expanse of this formation is known as the Shonkin Sag laccolith, a few miles west of Square Butte. The dark, granite-like rock of all these laccoliths has been named shonkinite. The Shonkin Sag laccolith is a textbook example of igneous intrusion, where the shonkinite is interspersed with layers of pale Eagle sandstone, the same material as the Rimrocks that tower over Billings. That “magnificent exposure,” to quote Alt and Hyndman again, “has made the Shonkin Sag laccolith the most thoroughly studied and best known igneous intrusion of its type in the country, perhaps in the world.”
It was also the site of the Square Butte Granite Quarry, founded in 1914. Armstrong published a history of the quarry in 2000 and led a successful effort to have the site placed on the National Register of Historic Places.
It should not be surprising that so few people seem to know of the Shonkin Sag or of the old Square Butte quarry. Both are impressive, once you realize what you’re looking at, but perhaps only someone with a keen interest in geology would be drawn immediately to either feature.
That is not the case with a portion of the Shonkin Sag known as the Dry Falls. For pure, breathtaking wonder, there is nothing quite like it in Montana.
The Dry Falls are about 10 miles northwest of Geraldine, surrounded by miles of gently rolling wheat fields and pastureland. You approach them on a narrow dirt road that can be impassable after a rain and park your vehicle in a wide spot in the road that is marked only by a nondescript sign advising you that this is private property, and to enter at your own risk. The Highwood Mountains — “shonkin” was supposedly the Blackfoot Indian name for the Highwoods — loom up to the south, and as you near the falls you walk through curious layered piles of rock covered in light-green lichen, looking for all the world like gigantic petrified cow pies.
After a walk of a quarter mile or so, you reach the rim of the Dry Falls. It is a dizzying, otherworldly spectacle. You look down 250 feet or more to Lost Lake, a long fiord-like body of deep-green water. To the west are the Dry Falls, a horseshoe-shaped wall of shonkinite half a mile across, like the black walls of a great fortress. Verdant carpets of vegetation reach up the lower portions of the cliffs, creating the impression of a prehistoric rain forest or some South American Shangri-La.
Armstrong, standing on the rim of the Dry Falls last June, spoke with wonder of the Ice Age marvel.
“It boggles your mind to think how much water came over this one spot,” he said.
To imagine that volume of water, consider this: given the depth of Lost Lake, Davidson said, the falls probably dropped 300 to 350 feet when the Shonkin Sag was brimful and water poured over the top. At 300 feet deep and half a mile across, the Dry Falls would have been just about double size of the largest cataract at today’s Niagara Falls.
“It’s one of the most spectacular geological sites in Montana and it really ought to be a state park,” Davidson said.
The Lost Lake Valley also bisects another notable laccolith. The horizontal bands of intruded shonkinite on the north side of the lake are very similar to those at the Square Butte quarry.
Davidson and Armstrong have long been puzzled by the relative obscurity of the Shonkin Sag and its associated features. Early in the last century, as Alt and Hyndman indicated, the laccoliths in the area were extensively studied, particularly by geology students and professors from Harvard and Yale. But the Shonkin Sag itself seems to have been relatively neglected, the subject of more conjecture than research.
When Armstrong began studying the quarry near Square Butte, he said “it was almost completely forgotten, even by people around here.”
In a conversation once with Dick Berg, director of the Rock and Mineral Museum at Montana Tech at Butte, he learned that Berg had never heard of the quarry.
Davidson had similar experiences. When he worked for Lockheed in California, he used to attend the annual conferences of the American Geophysical Union. He made a point of trying to find geologists who knew something about the Shonkin Sag, but he never met anybody who had even heard of it.
“It’s just so little known,” he said. “I’ve tried to interest geologists in it, but they’re just totally unaware. … This is sort of off on the fringes, and nobody is doing anything that I have found.”
With time on his hands in retirement, Davidson has tried to fill in the gaps of what is known about the Shonkin Sag. In popular accounts that mention the Sag, he said, there has been a tendency to simplify the geological history of the area. The descriptions generally speak of one or perhaps several catastrophic overflows of glacial Lake Great Falls, as the inland sea in this area was called, and they usually describe this as having happened at the end of the last Ice Age.
“I just wondered if the explanations of it held any water,” Davidson said, and then apologized for his small play on words.
His research, gathered through years of studying the landscape, reading published reports and creating detailed topographic maps of the area, has led him to believe that the Sag was the work of ages. He is convinced that different parts of the Sag were created during different periods of glaciation, and that the melt water overflowed not just one lake several times, but several lakes on numerous occasions.
As Ice Age succeeded Ice Age, the ancient Missouri — which flowed north to Hudson Bay before the continental glaciers pushed its channel ever farther south — was dammed at different points, forming lakes of various sizes and shapes.
Davidson believes some parts of the Shonkin Sag could have been formed very early, many hundreds of thousands of years ago, while the relatively pristine channel seen near the town of Highwood was probably formed by the last overflow of the ice-dammed lake during the most recent Ice Age.
Topographical evidence seems to suggest that the channel that carved the Dry Falls and created Lost Lake was also formed in a later episode, maybe 80,000 to 200,000 years ago.
“Those are just guesses,” he said. “I’m just placing them in the context of the Ice Ages.”
For the modern visitor, it hardly matters how or exactly when the Shonkin Sag and its awe-inspiring Dry Falls were formed. And though Davidson likes to talk of creating a state park and protecting the landscape of the Sag, Armstrong points out that the whole area has so far been protected by its very obscurity. At the Dry Falls there are no signs, no litter, no evidence of people at all but for a few faint trails in the sandy soil.
One day last summer, Lucretia Humphrey of Great Falls, accompanied by her sister Clarissa Metzlercross of Seattle, were enjoying the near-solitude of the Dry Falls. Metzlercross had never visited the site before. Humphrey and her husband had seen it a year earlier for the first time, having gotten lost or turned back by muddy roads on two or three other attempts to visit the area.
“How can people not know about it?” Humphrey asked, gesturing to take in the whole vast scene. “There’s so much in Montana. I mean, it’s right under our nose.”
This article first appeared in the Summer 2013 issue of the Montana Quarterly. If you’re not already a subscriber, you really should be. Check it out.