A quebrada is a valley or canyon formed by the action of flowing water. The Quebrada de Humahuaca is located in the northwesternmost corner of Argentina, on the eastern side of the Andes, where active uplift of the land has caused rivers to cut down and create quebradas of many sizes and shapes, resulting in a stunning landscapes of shear cliffs and massive incisions.
This view to the west is down an east–west-oriented tributary quebrada near the town of Tilcara that feeds into the much larger Quebrada de Humahuaca that extends north–south. We went trekking with llamas (www.caravanadellamas.com), who carried our gear. My llama was named Puca, which is the indigenous word for reddish color.
The Quebrada de Humahuaca is an attraction for tourists because of its brightly colored rocks, which have been exposed as the Andes have been uplifted and rivers have cut down to expose rocks that range in age from Precambrian to very recent. The older rocks that have been somewhat metamorphosed generally have dark colors of greens and greys. In contrast, there are white and light grey limestones that formed from a variety of organisms in shallow marine environments. But what most catches the eye are the reds, oranges, purples and yellows of sedimentary rocks that were deposited in continental environments such as rivers, where exposure to the atmosphere caused iron in the sediment to oxidize (to rust!).
On our three-day trip with the llamas , we were able to trek into the mountains east of Tilcara, to a much higher valley that is oriented parallel to the Quebrada de Humahuaca, which at Tilcara is at an elevation of 2500 meters (about 8000 feet). The scale of this vast landscape is impossible to capture in a photo.
Here are Jay and Adela, our guide, in the high valley near the house of Adela’s father, where we stayed over night. His house is located at about 3100 meters (10,000 feet). There are no roads and at an age of 78 her father lives without electricity or running water and must walk or ride his burro to town for supplies.
These north–south valleys are all formed from east–west-oriented compression and are a series of folds (imagine pushing on a rug until it is folded into hills and valleys). We trekked around the high valley that is a large-scale syncline; that is, layers are inclined toward the center of the valley, although there are many complications to this overall structure.
We were able to see many small-scale features such as this syncline. Cactus for scale is about 2 meters (6.5 feet) high. The larger-scale features are more easily seen from the air or using Google Earth.
A popular place to visit is the Serrania de Hornacal, located east of the town of Humahuaca. People go to the Mirador de 14 Colores (viewpoint of 14 colors) to see colors best displayed before the sun is getting ready to set. You can imagine that many photographers come to this part of the world and take great pains to be in the right place for the perfect exposure of colors in the early morning or late afternoon light.
This photo of the Serrania de Hornacal was taken in the late afternoon, but unfortunately rare spring clouds appeared to obscure the sun just before we arrived at the mirador. Nevertheless, it was a thrilling sight, especially because for geologists this is one of the finest possible examples of flat irons in the landscape. Flat irons are formed when layers (usually sedimentary) are inclined steeply and then eroded into shapes that look like the bottom of the tool used to iron clothes. The colors are a result of the different sedimentary layers that are exposed by erosion when water flows down the slope.
But only when I looked at Google Earth did I realize this cliff of tilted sedimentary layers is part of a large-scale plunging synclinal fold. All of the layers are tilted toward the center of the fold (we were viewing the north limb); in contrast, in anticlines all of the layers are tiled away from the center.
The reason for all of these folds will be explored in a future post.