An Origin Story as Rich as the Rock is Old

In mid-September, I led a geology field trip in and around Lewis & Clark Caverns State Park. Myself and 15 patrons spent the day together learning about ancient landscapes, pondering geologic forces of change, and overall, enjoying a different side of the park. The following information is a small insight into the rich geologic history of the area.

Driving east from Whitehall on I-90, mountains lift up from the flat valley like a wall. Engines groan as vehicles ascend the steep climb, a testament to the challenges that bankrupted many who sought to create a road through the mountains. An audible mechanical sigh marks the crest, followed by a gradual descent back to the valley floor.

This traverse is due to the existence of about a 10-mile uplifted range called the London Hills. Lewis & Clark Caverns (L&CC) is located within the midst of this tilted landscape. The upheaval of the London Hills exposed the land around Cave Mountain to erosion and paved the way for the formation of all the wonders in the cave 2-3 million years ago. Simultaneously, it also uncovered rock dating back over a billion years, revealing an origin story as rich as the rock is old.

Precambrian: 1.1 billion years ago

Hot dry air separated land from sea. On one side of the Willow Creek fault, shallow salty waters stretched up north into modern day Canada, on the other side, a mountain range that could rival the height of the Himalayas. Today, L&CC stands at this divide. Above water, the Precambrian world was a quiet place, practically devoid of life, nothing but cracked and barren ground. Fast moving rivers carried cobbles and boulders out of the mountains, towards the algae covered rocky beginnings of the Belt Sea. Farther out, to the north, the composition of the sediments quickly changed to fine-grain particles, all the heavy debris deposited at the mouth of the mountains.

The only remnants of this once mighty division between land and sea can be inferred today by the rocks exposed. South of the land-sea division, basement rocks dating back over 2.5 billion years, resurrected from the depths of the earth, are viewable in the Madison, Gallatin, and Beartooth mountain ranges. Directly north of the division, the large boulders and cobbles solidified together to form the LaHood conglomerate (right) found in and around L&CC state park. Farther north, the fine-grain particles created the Belt Supergroup, viewed by thousands each year in the cliffs at Glacier National Park.  

Over time, the Precambrian mountains eroded down to nothing. Rising water levels slowly submerged the land of Montana, laying the foundation for life to thrive below the surface.

Mississippian: 330 million years ago

In every direction, sea met the horizon; the reflection of the sun winked off salty waves. The Mississippian sea dominated most of the future North American continent from northern Mexico to northern Canada. Tropical rays penetrated clear blue-green water, bringing life to countless organisms. Primitive sharks stalked unsuspecting prey, while crinoids, corals, and brachiopods flourished in the clean water, filtering out microscopic plants and animals for food. Life and death cycled, building up limey shells on the sea floor, giving rise to more shelled organisms.

Tour guides at L&CC are very familiar with this time in Earth’s history. These shelled organisms live on in the existence of fossils fragments found in and around the cave. The plethora of crinoids, corals, and brachiopods built up massive 10-20 foot layers of Mission Canyon Limestone.

A new age was dawning however, the Earth was to change climate and geography dramatically, the North American reign of Poseidon would crumble, ushering in Pangea.  

Pangea: 300 million years ago

Collisions between continents eventually created a supercontinent called Pangea. The once dominant seas were slowly choked off by sediments shed in uplift events. North America breathed oxygen on dry land, and plants and animals diversified. 

During the reign of Pangea, amphibians and reptiles evolved surrounded by ferns and conifer trees, blanketing much of Montana with green forests. Pangea’s 100-millionth birthday was celebrated by the rise of the dinosaurs; dinosaurs that once roamed the park. In the later years of Pangea, the world exploded in color and buzzing as flowers and pollinators evolved side by side. The Nature Trail in the park is testament to this relationship every spring (Spring Beauty- right). 
  

Regardless, Pangea’s days were numbered and the separation of the continents would cause just as much cataclysmic change as their unification. 

Rocky Mountain Upheaval: 100-55 million years ago

Driving cross country through the Great Plains to Montana, mile after uneventful mile of flat lands pass by in a blur. It is easy to miss the mild climb in the landscape; nonetheless, valleys in Montana are often several thousand feet above sea level, towering over some of the highest points in the central states.

The separation of Pangea gave rise to the Atlantic Ocean as the entire North American (NA) continent drifted westward. A collision between the NA continental plate and the Pacific oceanic plate caused continental-scale upheaval. North America was uplifted as it ground into and over the Pacific plate. This compression also raised the mighty Rocky Mountains along what used to be the western edge of the continent.

Many zones of weakness were exploited, including the old east-west Precambrian Willow Creek fault. Cave fault and the Jefferson Canyon fault followed in similar suit, slicing mostly east-west horizontally through layers of rock across L&CC state park. Cave fault and the Jefferson Canyon fault interacted to slide ancient LaHood rock over much younger rock just north of Cave Mountain, and tilt and bend Mission Canyon Limestone beneath the surface.  

Before the uplift could expose Mission Canyon limestone, the upheaval slowed. Erosion slowly gnawed away the Rocky Mountains, the memories of these once imposing mountains to be buried beneath the surface. 

London Hills Upheaval: 5-6 million years ago

The Rockies today are a formidable feature that stretches over 3,000 miles from the northern parts of Canada to the southern reaches of the United States into Mexico. The formation of the Rockies began when the previously compressed land started to relax and expand as the Pacific oceanic plate and the NA plate slowly stopped colliding. The expansion caused stress which exploited the faults and weaknesses that originally formed the Rocky Mountains; this time, raising a mountain range hundreds of miles inland. Geologically, the Rockies are a young mountain range, still growing to this day.

Following these north-south weaknesses, the Starretts Ditch (SD) fault slowly ripped through the valley separating Jefferson Valley from the Three Forks/Gallatin Valley (see fault map above). From the west, the London Hills rise abruptly from the landscape. The SD fault uplifted the western edge of the London Hills almost 2,000 feet, creating a gradual slope to the east. This uplift also exposed the land that would become Cave Mountain to erosion.

For those traveling to L&CC from the west through the Jefferson Canyon, the story of the London Hills is laid bare to the inquisitive observer. With the mouth of the canyon ahead, the SD fault acts as a gate, leaving the valley behind. Ancient LaHood rock raised by Cave fault makes up jagged walls just past the LaHood Steakhouse Restaurant. The road narrows as it winds through the canyon; limestones, sandstones, and mudstones all tilted in peculiar fashion near the Jefferson Canyon fault.

The cave is located at one of the highest points in the park. Hundreds of millions of years of geologic history pass by on the steep drive up.

Creation of a Cave: 2-3 million years ago

Accessing the entrance of the cave requires a moderate hike up Cave Mountain. The hike begins heading west, then after some steep switch-backs, the trail abruptly turns south along the bend of the mountain. Visitors are rewarded for their endeavors with a birds-eye view of the park from the cave entrance.

Every year, visitors step into the weird underground world of the caverns and remark that the stalactites and stalagmites seem almost fake in the faint glow. These breathtaking wonders formed through patient dripping over the last 2-3 million years.

However, in every crack water trickles through, there whispers a tale of ancient origins, dating back long before the creation of the cave itself:

·         5-6 million years ago: the Starretts Ditch fault raised the London Hills and erosion slowly carved Cave Mountain

          100-55 million years ago: the uplift of the Rocky Mountains tilted the rock layers of Cave Mountain

o   Cave fault and Jefferson Canyon fault interacted to bend Cave mountain, creating lots of cracks and fractures around the bend

·         330 million years ago: the shells of sea creatures built up on the sea floor eventually creating massive 10-20 foot pure layers of Mission Canyon Limestone, the rock of the cave  

          1.1 billion years ago: the Precambrian Willow Creek fault laid the foundation for east-west weaknesses in the rock, eventually contributing to the creation of Cave fault and Jefferson Canyon fault over a billion years later.

      In short, acidic ground water reacting with the pure limestone layers of Cave Mountain utilized the tilt of the rock to dissolve away the limestone and create caverns within the large layers of rock. The abundance of cracks along the bend of the mountain created more access for water, contributing to some of the largest, fastest growing rooms in the cave.

In consequence, we can affectionately say that over a billion years of geologic history played a part in producing one of the most decorated caves in the U.S.