Introduction
The class traveled west about a half-hour from Denver.
The route cut through Lakewood City on US-6 W. On turning southwest on the I-70 W at West Pleasant View and heading towards Grand Junction and Jefferson County, we knew we were almost there because the urban sprawl that is Metro Denver finally surrendered to the lush greenness of Hayden Green Mountain Park and the whimsically-named Tin Cup Hogback Park. It was towards a spur of the latter where the class headed, an outcrop that towered over I-70.
Observations
Like many other segments of the Rocky Mountain range that spans New Mexico to British Columbia, Tin Cup Hogback takes its name from the classic ridge formed by tilted strata. The western ridge that encloses Tin Cup Hogback Park (a ghost mine), as well as the eastern wall of the manmade pass that gives I-70 access to State Highway 26, is the classic ridge topped by a sharp summit and characterized by steeply sloping sides.
Generally, hogbacks are homoclinic ridges, formed when a monocline of steeply-tilted strata of rock juts out from surrounding rock formations. The two strata forming a hogback are sedimentary rocks with variable weathering rates. Since softer rock erodes more rapidly than the harder rock that overlays it, erosion gradually pushes the former back to the point where both rock strata are adjacent. As weathering continues to work on the softer rock, cliffs (of the harder sedimentary layer) emerge more and grow steeper. Strictly speaking, hogbacks are defined by a steep dip slope exceeding 30 to 40 degrees and approximately symmetric slopes on each ridge face (Diviner 1). Overhead observation confirms that the sides of the ridge facing Tin Cup Park are distinctly steep while the side bordering the west of Thunder Valley resembles a “cuesta” for being a homoclinic ridge with a more gradual slope. Viewed from a distance, however, it is clear that this horseshoe-shaped formation is really a steep-ridged hogback (Figure 1 below).
Shot from about Ten Miles’ Distance
On gross inspection of multiple sedimentary strata lying near the surface, it is evident that this ridge was formed into a tilted or otherwise deformed structure. Typically, softer and more easily eroded shale lies between layers of harder sandstone which may be upthrust to form the valley-and-ridge formation characteristic of tilted strata. Stepping away from the exposed rock face caused by excavating a roadway through the ridge, one should have discerned the asymmetrical-slope profile of this homoclinic-ridge hogback: a less sharply-angled slope on one side and the characteristic steep scarp on the opposing slope. Again, the steeper side owes its formation to rapid weathering, thence undermining and mass wasting of a less resistant shale layer (Easterbrook 39).
There is support for the view that Tin Cup Hogback was formed about the same time as the main Rocky Mountain range itself, during the Laramide orogeny approximately 30 million years ago (Wernicke, England & Sonder 205). The formation in question may be integral to the Dakota Hogback that extends from New Mexico to Colorado and thence to Wyoming. Height varies, owing to weathering, and the chain has numerous gaps formed by bodies of water. Underlying this ridge is the large Dakota Formation of sandstone.
Conclusion
The vibrant colors of upthrust rock strata revealed by road building belie the fact that Tin Cup Hogback may be much older than the Laramide period. Up to the second half of the last century, there were reports of fossilized footprints for Diplodocus, which lived during the Jurassic. This highly accessible rock formation, therefore, bears further study for its geologic and paleontological clues.
References
Divener, V. “Structural Control of Fluvial Landscapes”. Crustal Structures and Landforms (course notes). Long Island University C.W. Post Campus. n.d. 2010. Web.
Easterbrook, Daniel J. Surface processes and landforms. (2nd Ed). Upper Saddle River, NJ: Prentice Hall, 1999.
Wernicke, Brian P., Philip C. England & Leslie J. Sonder. “Extension in the Basin and Range Province and East Pacific Margin Tectonomagmatic Evolution of Cenozoic Extension in the North American Cordillera.” Geological Society (London) Special Publications 28, (1987): 203-221. DOI: 10.1144/GSL.SP.1987.028.01.15.