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The Dakota Hogback
When you are driving west out of Denver on I-70, you will likely notice a striking series of steeply tilted, multihued rock layers as you make your way past Denver’s western suburbs and into the foothills. These layers are revealed in dramatic fashion just outside Morrison, Colorado where an Interstate 70 roadcut (just before mile marker 259) passes through a geologic formation called the Dakota Hogback. Many motorists passing through this busy corridor are amazed by these dramatic looking rocks, but few would guess that this was once the coastline of a prehistoric sea.
Flanking the eastern side of the Rocky Mountains between the foothills and the Great Plains, the Dakota Hogback is a thin strip of rock that extends some 200 miles from southern Wyoming to northern New Mexico. You can see this hogback at many scenic vantage points along the Front Range, such as at Red Rock Amphitheater near Morrison and Skyline Drive west of Canon City. The ridge is composed of Middle Cretaceous age (100–95 million year ago) sandstones of the Dakota Group, the geologic unit from which the hogback gets its name. They are the oldest Cretaceous rocks in the Southern Rockies. The Dakota Group is renowned for preserving impressive dinosaur tracksites across the Colorado Front Range. Dakota rocks also contain an economically significant reservoir of oil and gas.
What’s a hogback, anyway?
A hogback is a long, narrow ridge with steeply sloping sides that are slanted at nearly equal angles. The steep slopes of a hogback often meet at a prominent V-shaped crest that can extend for many miles, such as those seen along the Front Range.
You may notice that the Dakota Hogback appears to be a different shape than the surrounding foothills. Unlike most hills and ridges that are dome-like or convex/rounded in shape (meaning they are steep on the bottom and then slowly flatten out at the top) hogbacks are unique in that one side is straight and the other side is concave, or bowl-shaped.
How Hogbacks are formed
So, how do these ridges get their unique shape?
Hogbacks are created by the erosion of tilted layers of rock, or strata, when the overlying layer is harder than the underlying layers.
Remember from from above how hogbacks have a flat slope on one side and a convex slope on the other side?
The flat slope of a hogback is formed from from a hard, erosion resistant layer (often sandstone or limestone), and the convex slope consists of the softer rocks like shale. The softer rocks on the convex side of the hogback erode more quickly, and the overlying harder formation is left behind as the rocky escarpment that makes up the distinctive hogback ridges. This process of erosion is an example of what’s known as differential erosion.
In the instance of the Dakota Hogback, the flat eastern slope where we find dinosaur tracks is formed by the erosion resistant Dakota Sandstone. This hard sandstone formation overlies the softer shales of the older Morrison and Ralston Creek formations, which form the concave western slope of the hogback.
Hogbacks formation can be broken down into a three stage process:
- Deposition. First, step involves the deposition, which is the process by which sediments are laid down after being transported by water, wind, or ice. When sediments are deposited, they accumulate in horizontal layers that build up over time in a layer-cake type fashion. Over millions of years, these sediments transform into horizontal layers of rock.
- Tilting. When we see layers of rock, or strata, that are tilted instead of horizontal, we can infer that some kind of geological force, such as mountain building events, earthquakes, plate tectonics and/or faulting must have moved them into that angled position. For example, the distinct layers of rock at the Grand Canyon are nearly horizontal, meaning that they are still in their original positions relative to the horizon. In contrast, the tilted layers at the Dakota Hogback roadcut on I-70 (see image below) means that geological forces tilted them to their side.
- Weathering and erosion - Hogbacks are created by the erosion of tilted layers of rock when the overlying rock layer is harder than the underlying layers. Remember from from above how hogbacks have a flat slope on one side and a convex slope on the other side (see fig x)?
The story of the Dakota Hogback
The geologic story of the Dakota Hogback goes back to the Middle-Late Cretaceous, approximately 100 million years ago. It’s hard to imagine Colorado as a flat, subtropical region near sea level, but such was the case during this period of geologic time. A fluvial environment nourished diverse forests that dominated the warm, wet landscape. Dinosaurs were reaching the peak of their reign on Earth, and a massive inland sea called the Western Interior Seaway was transgressing across North America. For a time, present day Colorado served as the part of the west coast of this sea. By the end of the Cretaceous, however, this massive seaway would extend its range from what is now the Arctic Ocean to the Gulf of Mexico, engulfing present day Colorado and splitting North America into two large land masses called Laramidia and Appalachia.
The orange box in the map below shows where present day Colorado once existed in this ancient North American landscape. The red star indicates the location of present day Denver.
Beginning around 70 million years ago, the landscape began to change dramatically with the onset of the Laramie Orogeny, the mountain building event that is responsible for the uplift of the Rocky Mountains. As the mountains lifted up, the Western Interior Seaway retreated and 1.7 billion year old Precambrian basement rock lying deep in the Earth’s crust was forced to the surface, creating the great mountain peaks that we see today. As geologic forces drove the Rocky Mountains upward, the sedimentary rock layers lying above them, such as the Dakota rocks, were forced to the side. Like a drawbridge opening up, these sedimentary layers were tilted relative to their original (horizontal) position. In some locations such as the Flatirons outside of Boulder, Colorado, these layers are tilted as much as 60 degrees from the horizontal!