I've recently moved back to Colorado after living in Louisiana for five years. When I lived here before (three different times--I can't stay away), the scenery was the main thing I was after. I would always try to climb the highest mountains, to find the widest, biggest, most glorious vistas. Nowadays, though, that's not so much what I'm after. I still appreciate the scenery as much as ever, but I often find myself stopping at the places you don't see in coffee table books. These days I want to listen to the mountains as well as look at them. I want to learn what they have to say about their history, and the history of the Earth as a whole. It turns out they have a lot to say, and it's not always the most beautiful ones that have the best stories to tell.
Consider the mountain below:
It's called Yarmony Mountain, and it sits at a bend in the Colorado River at a tiny little nothing of a town called State Bridge. Yarmony Mountain is certainly pretty, with its red and tan rocks and Pinyon tree plumage, but it's no Pike's Peak.
Still, when I came around a bend in the highway and saw it, I pulled over to take pictures. Lately I've been trying to teach myself geology, and Yarmony Mountain looked like a nice example of something called an angular unconformity, which I promise is not as boring as it sounds. When I got home and looked up that totally not-boring term in a book on Colorado geology, there it was: a picture of my mountain. I had happened across an actual textbook example of something I had been reading about; something that--at least to me--makes this little unknown mountain just as awe-inspiring as Pike's Peak.
Here's why. If you look the layers of rock, you see that the red and white layers are tilted nearly vertical, while the darker layer of rock that caps the mountain is almost horizontal. That arrangement speaks volumes about the history of the mountain, and the landscape around it.
Looking at the vertical layers, as you move from left to right (or deeper into the mountain and to the left) the rocks get older. I don't have a detailed enough geologic map to identify them in detail, but I know the leftmost red layer is called the Maroon Formation. It's the same rock that forms the famous red peaks near Aspen called the Maroon Bells. If you look closely at it, it's mostly made of coarse sandstone with some larger rocks and gravel. That tells geologists it formed from streams powerful enough to carry real rocks, not just sand and silt. And that means those streams were coming down a steep slope--they were mountain streams, in a long-since-eroded mountain range called the Ancestral Rockies. They carried the sand and gravel out of the mountains, and dropped it at their flanks in broad, fan-shaped aprons of debris called alluvial fans. Eventually those fans solidified and reddened to form vast beds of red sandstone. The alluvial fans on the other side of the ancestral Rockies formed similar red rocks, which can now be seen at places like Garden of the Gods and Red Rocks Amphitheater.
Today all these rock layers are sharply tilted or almost vertical, but when they formed they were close to horizontal. This reflects a basic geologic law called the Law of Original Horizontality, which goes back to the 1600's. Gravity being what it is, most layers of sediment are flat when they're first deposited.* In the case of the Maroon Formation, they stayed flat for over 200 million years. Then, around 75 million years ago, the modern Rocky Mountains started to rise in a great mountain-building event called the Laramide Orogeny. Tectonic collisions raised huge blocks of granite and ancient metamorphic rock from miles underground. The relatively thin, flat layers of sedimentary rock above them were lifted high in the air, and sometimes tilted almost upright, as they still are at Garden of the Gods and Yarmony Mountain.
As soon as the mountains started lifting all these rocks, erosion set in and started wearing them down. At Yarmony Mountain, it planed down the vertical layers of sedimentary rock to form the near-flat surface, which the dark layer of rock rests on. That dark rock is totally different stuff from the rock below it. It formed from great floods of lava that erupted and flowed across the region around 24 million years ago. Further geologic forces have tilted it slightly in the ages since, but it's still mostly flat. The line of contact where it meets the ancient rocks below it represents about 270 million years of missing geologic history. That history is recorded in rocks in other places, but not at Yarmony Mountain.
What we do see at Yarmony Mountain, though, is an incredible chronicle of past ages; of the rise, fall, and rise of mountains, and of cataclysmic volcanic events. It's all recorded as layers of rock that can be read like a book by scientists who know what they're looking at. Thanks to all their work deciphering that book, and writing actual books to explain it to people like me, I can come across a place like Yarmony Mountain and know I'm looking at something remarkable. It's really just as remarkable as Pike's Peak, but in a different way. At Pike's Peak you're looking at vast heights and distances, but at Yarmony Mountain you're looking at vast expanses of time. It's a different kind of epic vista, and it's not as easy to appreciate, but once you learn to see it, it's just as awe-inspiring.
__________________________________________________
* Geologists today know the law is more like a rule of thumb, because not ever rock layer is perfectly vertical when it's first deposited, but most broad rock beds are initially pretty flat.
