As Old as Dirt

A drive through the Prickly Pear Canyon, on Interstate 15 north of Helena, makes a perfect afternoon drive.  The narrow, twisting canyon offers, arguably, the best stretch of driving on I-15.  Below the road, Prickly Pear Creek (and further north, the Missouri) twists like a blue cord on the narrow canyon floor.  Cliffs, in bands of red and green, rise above the road, topped by pines and the clear blue of Montana’s sky.

Next time you drive down that stretch of road, think on this: you’re winding your way through some of the oldest exposed rock in Montana. One billion years ago a shallow inland sea, or perhaps a massive lake, covered western Montana from what is now Butte to the Canadian border. As the waters receded, the mud from the sea floor dried, forming (over millions and millions of years) the multi-hued mudstones that color the Prickly Pear Canyon. Geologists have named these rocks “Spokane Shale” after the Spokane Hills to the east of Helena. Iron in the mud caused the color differences. Layers that formed with little to no oxygen exposure became green. Layers that had more contact with oxygen turned pinkish-red (caused by the iron particles oxidizing-essentially rusting). That’s all well and good, but these are some seriously old rocks, normally so many other layers of rock would cover them you would never see these mudstone flats. Thank Montana’s seismic activity. Seventy million years ago (for those of you keeping score, that’s about nine hundred and thirty million years after the shale began to form) the earth crumpled and shook in a series of events that formed the Rocky Mountains. These earthquakes twisted and folded the area, giving us the Prickly Pear Canyon and the leaning layers of Spokane Shale that make it so beautiful.

Tags: geology, intro science, Helena area, roadtrips, highway markers, Missouri River