Grasping Geology

"It's not gneiss to take me for granite!" This silly pun came to mind when we passed an information marker along a highway in the Bighorn Mountains of Wyoming this summer that identified the formation in the road cut as a granitic gneiss complex. While I really have no idea what it should look like, the fact that the marker exists made me look again at many of the photos I've taken that feature interesting tidbits of geology. Here are a few favorite things I've learned.

Shiprock in New Mexico is called both a 'monadnock' and an 'inselberg.' Both terms refer to a small, isolated mountain or an isolated, steep-sided hill that rises above a plain or gently sloping area. Inselberg is a German word meaning 'island mountain,' which is pretty accurate

Silhouette of Shiprock, an example of an inselberg, found in New Mexico

in this case. Monadnock apparently comes from one of the Algonquin languages and means something like smooth mountain or isolated mountain. Shiprock is visible for a long distance because it rises nearly 1600 feet above a very flat plain and can be seen from various high points in Mesa Verde National Park, about 50 miles north. Shiprock is what's left of the throat of an ancient volcano and is surrounded by volcanic dikes that radiate outward. These dikes are made up of a mineral containing a quantity of iron oxide that's distinct from the minerals found in the dikes surrounding the Spanish Peaks of southern Colorado.

The Devil's Stairsteps, a volcanic dike near the Spanish Peaks of Colorado

The radiating volcanic dikes around the Spanish Peaks are mainly granite intrusions with compressed sandstone. The overlying soils have eroded away, leaving fantastic formations with names like The Devil's Stairsteps (or Stairway to Heaven, depending on whether you're going up or down!), Profile Rock,

A few of many radiating dikes near West Spanish Peak, Colorado

Stonewall, and The Great Wall. Apishapa Arch (correctly pronounced Ah-PISH-ah-pah from a Ute word) is a road tunnel that passes through an unnamed dike near Cordova Pass not far from West Spanish Peak. Additionally, there's an area of exposed sandstone called The Dakota Wall in the same area, part of the Dakota Formation which extends from Canada to Mexico and visible along most of eastern length of Colorado's Front Range of the Rockies. The Dakota Formation, along with the Fountain and Lyons Formations, makes up much of what we see in Garden of the Gods in Colorado Springs, and the Flatirons in Boulder.

The Dakota Wall near the Spanish Peaks

So, moving from the volcanic to the sedimentary, Garden of the Gods park in Colorado Springs showcases a number of sedimentary formations -- Fountain, Dakota, Morrison, Lykins, and Lyons to name a few. In fact, some of the most iconic views in Garden of the Gods -- Kissing Camels, White Rock, Gray Rock, The Tower of Babel -- are part of the Lyons Formation. Most of the layers seen in Garden of the Gods are tipped on their sides and the softer soils have eroded away from between layers -- thus the tall, separated fins.

Kissing Camels and White Rock in Garden of the Gods, Colorado Springs

Pink pegmatite stripe found along the Arkansas River

Taking a side trip south and west of Colorado Springs, the Bighorn Sheep Canyon section of the Arkansas River between Canon City and Salida is a glorious jigsaw puzzle of geologic treasures. Just the colors alone of the rocks found there are fantastic. There is nothing "rock-colored" or even a common brown. From black, dark gray, lighter gray, and green, through to various golden tans, oranges, rusty tones, a reddish purple that reminded me of red velvet cake, and even pink. There are layers, jumbles, inclusions, cracks and seams, faults and uplifts, tilts and folds. There are names like schist, gneiss, granite, dolomite, limestone, chert, andesite, rhyolite, and mudstone. In some of the road cuts, you can see layers tilted up almost 90 degrees, and in others, chaotic jumbles in multiple colors that illustrate how geologically active this area was for a very long time.

Fallen City, a chaos of gigantic tumbled limestone blocks

The Bighorn Mountains of Wyoming, where this geologic travelogue started, provide plenty of natural wonders of the rocky kind. Shell Canyon to the north near Sheridan, and Tensleep Canyon to the south near Buffalo each have their attractions (a few of which I mentioned in my last post). Like other sections of the Rocky Mountains, some of the oldest rocks on earth can be found close to the surface -- pushed up and tilted and folded by the stresses that formed the Rockies.

One example of stresses involved is Fallen City, not far from Sheridan. Here is a dramatic example of the role of water in mountain formation. The top layer is Madison Limestone. Limestone, of course, is known to fracture and water weakens it by seeping into the fractures; freeze-and-thaw cycles, rainstorms, even the acids and minerals within rainwater and snow runoff have an effect. The next layer under the limestone at this location is slate, which becomes slippery when wet; this layer would have allowed the water seeping in to pool and collect. I find it fascinating that the blocks of limestone almost look as if they collapsed in some great crash all at once -- very much like a city falling from its foundations.

At the Greybull side of Shell Canyon is an example of the upside down nature of some geology. This deep-red rock is from the Chugwater Formation, and is very much younger than

Chimney Rock at the southern mouth of Shell Canyon near Greybull, Wyoming

the gray cliffs in the background. All of this formation was once on top of those mountains, but as the uplift and tilting occurred, it eroded away to the valley floors, leaving a red ring around the lower elevations of much of the Bighorn Mountains.

However, I've found too many other photos to include in this post, so I suspect I'll revisit the subject in the future. I do hope this short south-to-north glimpse into a tiny bit of the geology of parts of the Rocky Mountains was interesting. I think I could spend hundreds of years studying this subject and still feel like I was only scratching the surface (pun intended!). Can you tell that much of my enjoyment of photography comes from researching neat things about the photos I take?

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