The terrain never ends at Big Sky Resort. Image Credit: Chris Kamman
Lone Peak is a world-famous recreational paradise. Whether cruising down groomers or tearing up powder-filled backcountry, few thrill-seekers pause long enough to consider the dynamic geologic processes that carved Lone Peak’s distinct ridges and bowls.
Beneath the resort’s snow-clad hype, there is a rich history written in stone. The exposed slopes of Lone Peak are layered, like an onion. Each distinct layer is a different chapter in the mountain’s rich geologic story.
For much of Earth’s history, Big Sky, along with the rest of the West, was submerged beneath the waves of the ancestral Pacific Ocean.
The white line highlights the geography of the watery West 425 million years ago. During this epoch, coral reefs filled the oceans while the first plants began to grow on land. Image Credit: Paleomap Project.
Sediments are pieces of the land transported and deposited by wind or water. Sedimentary rocks form from the build up, compaction, and cementation of these sediments. Image credit: eschooltoday.com
The erosive forces of wind and water carried sand, silt, and mud from ancient landforms to streams and rivers. Sediment-laden rivers flowed to the sea. As the water slowed and deepened, a silent snowfall of particles rained onto the ocean floor. Sediment drifts and banks built up, layer by layer. After countless years, these drifts compacted to form sedimentary rocks, such as sandstone, siltstone, and shale.
Today, evidence of this ancient watery world can be found on the flanks of Lone Peak. Alternating layers of a black, fine-grained rock reclining in thin beds and a coarser-grained, yellowish-tan rock lying in thicker beds span the eastern-facing slopes of the Peak.
Only recently, in the past 100 million years, did Montana begin to emerge from sea.
The Cretaceous world was warm. The land was covered in shallow inland seas. Giant marine reptiles hunted in their depths and ammonites flitted across their shallow, sunlit waters. Dinosaurs ruled the land. Credit: Paleomap project.
The dense Pacific ocean crust collides and sinks beneath the buoyant coast of the Western United States. The deeper it sank; the hotter it got. Eventually, the crust melted, fueling mountain building. Image Credit: usgs.gov
Lone Peak, and the Rocky Mountains, grew during a tumultuous period of Earth’s history. After the breakup of Pangaea, the North American continent moved West, away from the coasts of Europe and Africa, to collide head on with the floor of the Pacific Ocean. The dense floor of the Pacific Ocean slid under the buoyant West Coast to form a trench or subduction zone.
The collision of the Pacific Ocean floor and the North American continent built the Rocky Mountains. The overriding North American continental crust rose and buckled into a high plateau, like a carpet sliding across a wooden floor and colliding into a wall, while the ocean floor began to melt. Melted rock, known as magma, rose and built up beneath the Earth’s surface.
Big Sky’s layered slopes.
Lone Peak is a laccolith, a volcano that never erupted. Instead, hot melted crust rose vertically from the magma chamber and solidified between the older layers of sedimentary rock.
Geologists believe that if you were to cut Lone Peak in half, it would look like a Christmas tree with a light-grey igneous rock, known as dacite, forming the trunk and branches.
The heat of the magma intruding between the sedimentary layers and baked the older rocks. Like bricks hardening in the sun, heat changed sandstone and shale, making them more resistant to erosion.
Once, the Madison Range was like the Himalaya today, a high plateau towering above the seas. Over millions of years, wind and water wore away the softer sedimentary rock. The resistant dacite core and hardened sedimentary rocks endured. Lone Peak was left towering over the other peaks of the Madison Range.
Broken pieces of dacite weathered by wind, rain, and snow.
Glaciers put the finishing touches on the silhouette of Lone Peak. Mountain glaciers sculpted cirques, bowl-shaped valleys, while melting valley glaciers left epitaphs of their retreat in the Mountain Village: mixed piles of rock, gravel, and sand. To this day, landslides pile rocky debris at the foot of cliffs, such as Headwaters.
Water, fire, ice, and wind carved the peak that dominates the skyline of Big Sky. Our singular peak would be nothing, if not for its unique geologic history.