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.
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.
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.
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.