Day 35 - Buffalo to Worland, Wyoming - 90 Miles

Today I would like to make this blog about rocks. I don't know as much as I'd like to about rocks and today, just to tease me, they kept cropping up in the conversation. It's because in Wyoming the highway department has thoughtfully placed geologic road signs along many of it's rural highways to help you identify the rocks alongside which you are driving.

For instance, this morning as we were climbing to the top of Powder River Pass, 9,666 feet above sea level, we passed a sign just before the summit that announced that the rocks at the top were 3 billion years old. Evidently, these rocks called "Gneiss" are some of the oldest rocks on earth.

Naturally, I wondered how Gneiss is formed. I found this answer at https://www.reference.com:

"Gneiss is formed from the high-temperature metamorphism of existing igneous rocks, generally granite or diorite. The rocks that form gneiss are exposed to extreme pressures and temperatures of between 600 and 700 degrees Celsius. These temperatures cause the individual minerals to migrate, forming distinct bands through the rock."

"Gneiss does not have any particular mineral composition, and the term refers only to the banded texture. However, it only forms from igneous rocks and so tends to contain minerals not found in metamorphic rocks formed from sedimentary rock, such as marble. Despite the general appearance of banding, the distinct structures are elongated granular structures, so unlike schist, gneiss does not cleave along its bands."

"The processes that creates gneiss can recreate normal granite if carried on long enough, and gneiss can also form from gabbro or shale. Schist and quartzite may be formed alongside gneiss, along with other types of metamorphic rock formed from igneous rocks."

"Gneiss is very common and actually forms the majority of the Earth's lower crust, so any deep tunneling encounters gneiss. The oldest rocks yet found are gneiss, with the oldest dated to over 4 billion years old, older than most estimates of the beginnings of life on Earth."

Okay, with the 3 billion-year-old rocks at the very tippy top of the mountain you expect the rocks further down the mountain would steadily get older. But that's not what we experienced. As we descended from Powder River Pass the rocks appeared to get younger and younger as evidenced by the highway department informational signs. By the time we reached the valley floor, the last sign we passed said that the rocks were only 200 million years old. So what gives?

The answer is (sort of) simple: Between about 70 and 45 million years ago, this region was compressed by tectonic stresses. This thick block of Earth's crust popped up along thrust faults on its SW and NE sides, folding the younger rock layers underneath. This event is called the Laramide orogeny after the Laramie Mountains in Wyoming. The Bighorn Mountains are one of the largest structures formed at that time, along with the Wind River, Beartooth, Gros Ventre - Tetons, Uinta, and other mountains in the region.

Once upon a time in the 1970s I learned about this phenomena in a Western Nevada College geology class, but this is the first time I can remember seeing it in person. For me, it's pretty hard coming to terms with 3 billion year old rocks, let alone forces in the earth that could shove that layer of rock so hard that it upthrusts, squishes up against younger layers of rock, and then folds them back upon themselves.

Putting the geology aside for a moment, let's go back to the top of Powder River Pass where we had lunch today. Once we had parked at the roadside rest and Concetta had set about preparing lunch, I took the opportunity to wander around a bit. Of course I wanted to get photographs of the Gneiss since I hoped to write about it (photo top left), but I also wanted to get photos of the flora. At an altitude of 9,666 feet, I was immediately surprised to see so many alpine flowers. There were blue ones and red ones and yellow ones and white ones and they all appeared to be pretty robust.

I'm not sure I did them justice, but I had fun trying to include as many colors in one photo as I could. Evidently, noting the general health of the flower and plant life, the top of the world that we encountered at our lunch stop must provide just the right nutrients, water, and temperature for these plants to thrive. Considering it probably gets ten feet of snow and temperatures of minus forty, I'd say the little guys are pretty hardy.

TO BE CONTINUED.......