On the Road Again - Post 8: Time & the River

Before the formation of the Rocky Mountains, the area that is now Montana was a gigantic inland sea, its bedrock at least 2.5 billion years old and its seabed thickly blanketed by seashells (a.k.a. the calcium carbonate that hardens into limestone). Three hundred million years ago, the Precambrian rocks of the Ancestral Rockies pushed their way up through that seabed. For the next hundred million years or so, these Ancestral Rockies eroded; the debris resulting from their erosion eventually solidified into new layers of sedimentary rock.

Fifty to 70 million years ago, the current Rockies began their rise, thanks to a colossal collision. When the Canadian Shield and Pacific tectonic plates smacked into each other, the lithosphere buckled and crunched, and molten rock from the mantle underneath pushed upward through the layers of sedimentary rock left behind by the Ancestral Rockies. (Click here if you missed Post 5 and would like definitions of these terms.)

The dinosaurs then wandering about would have felt all this buckling and pushing as earthquakes. Maybe it's a coincidence, maybe it's not, but the dinosaurs became extinct just as the granite from the mantle finally burst through the overlaying sandstone about 55-60 million years ago.

The evidence of this geology is everywhere in modern Montana, by area the fourth largest of the United States, by population the fourth smallest. Fewer than a million people live in this whole enormous state, so there's plenty of undisturbed room to take in both the spectacular vistas and the extraordinary sky. (I've written about the Montana sky before; "Big Sky Country" is a richly deserved nickname.)

On July 19, 1805, the Lewis and Clark expedition found itself on a remarkable stretch of the Missouri River about 20 miles north of what is now the city of Helena. Limestone cliffs (which Meriwether Lewis incorrectly thought were granite because it was raining and the wet stone looked black) rose up 1200 feet on both sides of the river. At each bend in the waterway, the looming stone walls looked as if they would block passage altogether, but then the river would curve and each time the walls opened like gates. Observing this, Lewis wrote in his journal, "I shall call this place Gates of the Mountains."

Lewis also wrote, "The river appears to have forced its way through this immense body of solid rock." This imagery, while apt, was again based on a mistake. (Lewis was no geologist.) In fact, the river predates the uplift of the cliffs around it. The Hilger Fault has been raising the northern end of the Big Belt Mountains and lowering the Helena valley for million of years. This forced (and continues to force) the Missouri to erode downward into the hard rock and carve the canyon that cuts across the mountains.

In the 204 years and 10 days since Lewis and Clark originally beheld it, the view has remained essentially unchanged. Limestone cliffs 1200 feet tall still manifest the extraordinary pressures of their formation in sensuous twists and curves and the effects of weathering in jutting ridges, domes and outcroppings. Areas of ancient Precambrian rock still alternate with the limestone. Cave openings abound. Ospreys, eagles, and falcons dot the sky. Bighorn sheep, mountain goats, otters, deer, squirrels and all manner of other wild animals roam the cliffs and flats. Pines grow thickly and reach for the sun.

And the silty waters of the Missouri continue to flow, powerfully placid.


The pictographs in the cave pictured below were painted by Native Americans long before Lewis and Clark showed up here in 1805. The youngest of the pictographs dates to 400 years ago, the oldest to 1400 years ago.


Among the fascinating rock formations in this Missouri River passage are three famous "faces." Can you find the Canyon Monster in the first photo below, the Stony Elephant (eyes and trunk) in the second, and the Eye of the Rhino (profile) in the third? (Click on the pictures if you want to make them larger.)


Cave openings and a bald eagle in flight.