Mountains, Belt Butte, and the Great Falls Coal Field

By editor

Belt, Cascade County, Montana

As one stands on the rolling plains east of Great Falls, the distant silhouette of Belt Butte commands attention, a sentinel rising abruptly above the low hills that cradle the town of Belt. This solitary butte reveals a chapter of Earth’s history stretching back over a billion years, its summit crowned by the deep reds and muted greens of the Spokane Shale -- mudstones deposited in a shallow, ancient sea long before the dawn of animals with bones or shells. These Precambrian sediments, laid down in a world devoid of complex life, form the same resistant strata that sculpt the rugged walls of Prickly Pear Canyon to the south. The story told by these rocks is one of a silent ocean, its quiet waters settling silt and clay into layers destined to become the Belt Supergroup, one of the thickest and most extensive sedimentary sequences in western North America.

Belt Butte’s name is no mere coincidence. It christened not only the town but also Belt Creek and the Big and Little Belt Mountains to the south, all linked by the characteristic “belt” of sandstone that encircles this prominent hill. The Belt Supergroup earned its name from these very formations, extending its reach through many mountain ranges, each a monument to a distant Precambrian world more than a billion years old. The passage of time since then is almost incomprehensible, yet the rocks speak with clarity, as if whispering the slow rhythm of continents drifting and seas rising and falling.

To the northeast, the Highwood Mountains punctuate the skyline with jagged volcanic peaks, their summits carved from dark, dense shonkinite -- an uncommon igneous rock named for exposures near the small community of Shonkin. Shonkinite’s presence here is remarkable; it appears in only a few places worldwide, and its dense, fine-grained texture hints at the violent forces that formed these mountains roughly 50 million years ago. These volcanic cores thrust upward through the softer sedimentary layers surrounding them, creating a dramatic contrast between rock types and ages.

Belt Butte itself reveals a striking geological paradox. Its Precambrian mudstones rest atop much younger rocks, a phenomenon resulting from the immense compressive forces of the Laramide orogeny, the mountain-building event that shaped the Rocky Mountains between about 70 and 50 million years ago. During this time, the Lewis Overthrust Fault pushed massive slabs of ancient rock eastward over younger Cretaceous strata. This tectonic collision caused billion-year-old rocks to override formations a mere seventy million years old, a reversal of the usual order of geological layering.

Beneath Belt Butte, the foothills cradle the Great Falls Coal Field, a patchwork of coal deposits discovered in the 1870s. These seams formed in swampy environments during the Early Cretaceous period, roughly 100 to 70 million years ago, when the Western Interior Seaway intermittently flooded the interior of North America. Dense forests of conifers, cycads, and ferns grew in these wetlands, their fallen branches and leaves accumulating in waterlogged soils. Over millions of years, the plant material compressed and transformed into coal. The seams here, while discontinuous and relatively thin compared to other coal regions, fueled the smelters and railroads of central Montana for decades, especially as the region industrialized in the late 19th and early 20th centuries.

The Great Falls Coal Field lies mainly within the Armington area, just south of Belt, where miners extracted Cretaceous coal layers lying beneath younger sedimentary rocks. These mines became crucial to regional industry, providing much-needed energy for the burgeoning rail network and the metallurgical plants. The coal’s economic importance peaked in the early 1900s, supporting the growth of Great Falls as a center of smelting and transportation. However, by the mid-20th century, the mines closed as coal demand shifted and alternative fuels emerged. Still, the coal seams remain buried beneath the hills, a reminder of the energy locked in Montana’s ancient landscapes.

The geological complexity of the region unfolds further as the Little Belt Mountains rise to the south. These mountains, distinct from the Big Belts, formed through a series of domes created by blister-like pockets of magma pushing upward about 50 million years ago. This magmatic activity warped the overlying sedimentary layers into rounded uplifts, each dome cored by igneous rock of similar age to the volcanics of the Highwood Mountains. These domes not only shaped the modern landscape but also influenced drainage patterns, soil development, and vegetation.

Walking among these formations, one cannot help but recall the words of geologist Charles D. Walcott, who studied the Belt Supergroup in the late 19th century. He remarked, “The rocks of the Belt series are among the most instructive and interesting of any known in the world, revealing clearly ancient geological processes and environments.” Walcott’s careful observations laid the foundation for our understanding of this Precambrian sequence, highlighting its global significance beyond Montana’s borders.

The intersection of ancient Precambrian rocks with younger Cretaceous sediments, volcanic intrusions, and coal deposits reveals a landscape shaped over eons by a succession of natural forces. Each formation--whether the reddish mudstones of Belt Butte, the dark shonkinite peaks of the Highwoods, or the buried coal seams of the Great Falls field--offers a distinct chapter in a vast geological narrative. The physical evidence beneath our feet holds stories of ancient seas, mountain-building upheavals, tropical swamps, and volcanic upheavals that together forged the terrain seen today.

This region’s geology has not only scientific interest but also human history etched into its folds. The coal mines fueled the engines of progress in Montana’s early industrial age, while the mountains and valleys shaped settlement patterns and transportation routes. The enduring presence of these formations invites us to consider the vast scales of geological time and the persistence of natural forces that sculpt the earth’s surface.

Though the Belt mines closed decades ago, the layered earth beneath the rolling foothills preserves its secrets, awaiting the curious eye and steady hand of the geologist or naturalist. To gaze upon Belt Butte and the surrounding mountains is to witness a dialogue between time and rock, where every outcrop and stratum retains a record of deep history, patiently awaiting discovery.

See also

• Mountains, Belt Butte, and the Great Falls Coal Field at Belt, Cascade County
• The Lewis Overthrust Fault and Marias Pass at East Glacier Park, Glacier County
• A Perfect Defile: The Prickly Pear Canyon at Wolf Creek, Lewis and Clark County