The Dakota Hogback
Travelers driving from the Colorado high plains west toward Rocky Mountain National Park pass through one of the more interesting and remarkable examples of the geologic history of the Rockies. Yet few notice and even fewer stop to study the interesting layers of rock found in the Dakota Ridge (or Hogback, because of its sharp top edge).
The ridge stands like a barrier between the plains and the foothills, with a few water-cut gaps here and there, where road builders took advantage of the natural passage. Watch for the ridge as you travel U.S. Highways 34 (west of Loveland) and U.S. 36 (west of Lyons) to Rocky Mountain National Park.
In a way, the Dakota Ridge is essential to understanding the Rocky Mountains and their long history, going back millions of years. Before the present mountains rose, much of the region was a shallow sea. As sediments fell to the bottom of the water, they were compressed over the centuries into soft sedimentary rock. Thus, oyster and clam shells, sand and mud, any substance that could be broken into tiny bits or dissolved, built slowly into layers of sandstone, shale, limestone, and "mudstone."
These layers vary in thickness and cover thousands of square miles, mostly deep under the earth's surface, never to be seen. We can see a tiny exposure of these ancient rocky layers because millions of years ago the mountains began to rise, taking the layers of surface sedimentary rocks with them, up to nearly 30,000 feet above sea level. The soft sedimentary rocks were quickly weathered and washed away from the high mountains. But on the edge of the foothills, where the layers were scraped and pushed only slightly upward, the western edge of their remnant still stands at about a 45 degree angle - the Dakota Ridge.
There is an interesting reason that the ridge survived many years of exposure to rain, snow, ice, and wind: The top layer is a hard sandstone. the Dakota Sandstone, from which the ridge gets its name. It protects the softer shales and limestones beneath it from weathering and erosion.
The hogback can be seen west of the towns of Loveland and Lyons, approaching Rocky Mountain National Park. A cut through the hogback west of Denver on Interstate 70 reveals the various layers.