A Volcanic Island in the Rocky Mountains

By editor

Great Falls, Cascade County, Montana

The Missouri River carves a deep and twisting canyon between Wolf Creek and Great Falls, a landscape that seems out of place amid the broader Rocky Mountain front. Here, the canyon walls rise dark and solemn, formed of volcanic rock -- basalt and rhyolite -- whose origin dates back roughly 76 to 73 million years ago. This volcanic episode occurred long after the Laramide orogeny, the great mountain-building event that thrust up the main ranges of the northern Rockies some 70 to 80 million years ago. The volcanic formations in this canyon are thus a geological interlude, a sudden volcanic island amid the sedimentary seas of the Rocky Mountain foothills.

Driving along U.S. Highway 91, which traces the river’s serpentine course, one cannot help but notice the contrast between the volcanic ridges and the neighboring sedimentary layers. To the south, the Gates of the Mountains reveal bright white Madison Limestone cliffs, their pale hue stark against the dark basalt. Southeastward, the reddish Spokane Shale colors the slopes of Prickly Pear Canyon in a warm ochre. The volcanic rocks occupying the Missouri River canyon are remnants of a once-active magma chamber that intruded through the sedimentary strata and ultimately erupted onto the surface, forming a series of lava flows, ash deposits, and volcanic plugs.

Among these features, Mount Chisholm commands particular attention. This volcanic plug formed when magma forced its way into fractures within the volcanic rock and solidified into basalt, a rock notably harder than its surroundings. Over millions of years, the softer volcanic fragments eroded away, leaving Mount Chisholm as a prominent outcrop rising above the landscape. Nearby, narrow ridges formed by dikes -- vertical sheets of magma that filled north-south fractures in the volcanic rocks -- now stand as slender, jagged lines against the sky, their resistant basalt pillars shaped by relentless erosion.

The Missouri River itself has been an agent of transformation for millions of years, slowly eroding the volcanic field to form the canyon we witness today. The river’s persistence carved a corridor through this volcanic island, exposing layers of geologic history in its walls. Approximately 20,000 years ago, during the last great glaciation, the Continental Ice Sheet advanced southward from Canada, damming the Missouri River near the north edge of the Highwood Mountains, some 70 miles northeast of Great Falls. This glacial dam created Glacial Lake Great Falls, which flooded the river valley upstream nearly 80 miles, reaching as far south as Townsend, about 90 miles from this overlook.

The lake persisted until roughly 15,000 years ago, when the ice sheet retreated enough to release the stored waters, allowing the Missouri River to resume its ancient course. The draining of Glacial Lake Great Falls left behind sediment deposits and shaped the valley’s present form. The interplay of volcanic rock, glacial action, and river erosion has sculpted a landscape unlike any other on the Rocky Mountain front.

The human history entwined with this geology is equally compelling. The Hardy Bridge, spanning the Missouri River below this overlook, was constructed in 1931 by Great Falls contractors Angus McGuire and Evarts Blakeslee. This steel truss bridge, named for the nearby community of Hardy, remains in use today, carrying traffic along the old U.S. Highway 91. Though Interstate 15 now bypasses this route, the Hardy Bridge and the highway preserve the experience of early 20th-century travel through a geologically rich corridor.

When Lewis and Clark passed through this region in 1805, they noted the striking geological formations of the Missouri River valley. William Clark wrote in his journal: "The country appears to be of the most rugged and mountainous nature; the river runs through a narrow defile with walls of basalt rising on each side, black and precipitous." Their expedition recognized, even then, the unusual volcanic character of this stretch, a feature that distinguishes it from the sedimentary landscapes farther east and west.

The volcanic rocks here are not mere relics; their mineral composition and texture tell the story of a fiery past. Basalt, an extrusive igneous rock rich in iron and magnesium, cooled rapidly from molten lava, forming a dense, dark matrix studded with tiny mineral crystals. Rhyolite, more silica-rich and lighter in color, represents a more viscous lava that produced explosive eruptions, scattering ash and fragmented rock over the landscape. The coexistence of these rock types marks a dynamic volcanic environment, where differing magma compositions shaped the terrain.

In the canyon’s flora, one finds species adapted to these basaltic soils. Ponderosa pine (Pinus ponderosa) clings to the rocky slopes, its deeply furrowed bark and long needles resilient to the dry summers and cold winters. Douglas-fir (Pseudotsuga menziesii) grows in shaded pockets, while shrubs like serviceberry (Amelanchier alnifolia) and chokecherry (Prunus virginiana) provide food for local wildlife. The canyon’s fauna includes mule deer (Odocoileus hemionus), which browse among the shrubs, and the occasional sighting of a mountain lion (Puma concolor) stalking through the rugged terrain.

Geologist Charles R. Keyes once described the volcanic complex near Great Falls as "a remarkable volcanic island in the midst of the sedimentary rocks of the Rocky Mountain front." The phrase captures the essence of this geological anomaly -- a volcanic outcrop that interrupts the otherwise sedimentary sequence of the region. For those who stand on the canyon rim and gaze upon the jagged ridges and towering plugs, the scene is a vivid reminder of the Earth’s restless activity over deep geological time.

The Hardy Bridge, meanwhile, is more than a crossing; it is a link between the human and geological histories of the Missouri River canyon. Built during the Great Depression, the bridge embodies the era’s engineering skill and resilience. Its steel trusses, painted a muted gray, span the river with a quiet dignity, complementing the dark volcanic cliffs that rise on either side. Travelers on old U.S. Highway 91 can still feel the weight of history beneath their wheels as they pass through this volcanic corridor.

In conclusion, the volcanic formations along the Missouri River near Great Falls offer a window into a fleeting but intense volcanic episode in Montana’s geological past. These rocks, shaped by forces of fire, ice, and water, create a landscape that challenges the notion of the Rocky Mountains as simply sedimentary peaks and valleys. Instead, here stands a volcanic island, bold and enduring, inviting careful observation and appreciation by naturalists, geologists, and travelers alike.

See also

• A Volcanic Island in the Rocky Mountains at Great Falls, Cascade County
• A Perfect Defile: The Prickly Pear Canyon at Wolf Creek, Lewis and Clark County
• The Experiment at Great Falls, Cascade County