Of the many scenic river canyons that slice through the Front Range foothills, none can match the depth and grandeur of the Royal Gorge, whose nearly vertical walls soar 1,250 feet above the Arkansas River just upstream from Cañon City. While magnificent when seen from the lookouts on either rim, the vista becomes positively breathtaking when you gaze into the chasm from North America’s highest suspension bridge, from which the river lies a dizzying 955 feet below.

The Royal Gorge is so narrow—only about 40 to 50 feet across—and precipitous because the Arkansas River sliced through a dome of especially hard rocks, including gray granite and dark metamorphic gneiss, in this spot, creating a terrain so rugged that U.S. Highway 50 must leave the river to skirt around it via a lower valley. This prompts the question: Why did the Arkansas River carve a gorge through these hard rocks instead of taking the path of least resistance down a lower valley floored with softer sedimentary rocks?

The reason is that the landscape that existed about five million years ago, when the river began carving the Royal Gorge, was quite different from what we see today. Back then the dome’s resistant rocks were buried beneath several thousand feet of soft sedimentary rocks that formed a westward extension of today’s Great Plains. The Arkansas River flowed eastward across this gentle upland, oblivious to the dome of hard rock lurking below.

With each passing year, floods dragged rocks and sand across the riverbed, causing abrasion that scoured the channel deeper into the sedimentary stack. When the bed finally lay directly atop the hard rocks, additional abrasion carved a shallow notch into the dome. This notch then served as a “guide groove,” trapping the river like a carpenter’s initial incision ensnares the blade of a cross-cut saw to prevent it from skittering across the wood’s surface.

The Arkansas River’s guide groove forced it to carve ever deeper into the hard rocks with which it had unwittingly surrounded itself. Meanwhile, smaller, less-powerful streams excavated the surrounding, soft sedimentary rocks with comparative ease, cutting the lower-elevation valleys that today lie both north and south of the Royal Gorge.

Today these valleys seem like logical paths that a less confused river should have taken to reach the Great Plains, but the river, of course, wasn’t confused; its early course just serendipitously overlay the buried Royal Gorge Dome—a fortuitous circumstance for all who have enjoyed the spectacular scenery.

Terri Cook
Terri Cook
Terri Cook is an award-winning freelance writer based in Boulder. More of her work can be found at down2earthscience.com.