As I stated earlier I mixed my fieldwork in Arizona up with a bit of a road trip through the Western US. I did the trip with an old friend from University, an geophysicist, and so it is not very strange that we spend most of our time discovering the amazing geology the National Parks in the Western US offer. We started our trip in Albuquerque, New Mexico, drove via Arizona and Utah to Nevada, from there to California and up the west coast through Oregon to Washington where Seattle was our last stop. This means we saw the Rift Valley of the Rio Grande, the Colorado Plateau with all its beautiful geological features (Grand Canyon, Monument Valley to just name two), the Basin and Range Province with its wonderful valleys and mountains (Death Valley!), the Sierra Nevada mountain range, the Coastal Range and not least but last the Cascades with its spectacular volcanoes.
While this blog is not the space (nor do I have the time) to tell you about all the astonishing things we saw on that trip, I wanted to share some of the images we took and try to explain what you can see on them. Please note that most of the explanations were made up by us on site, so no promise that what I am about to write is actually true! So here we go (and please do not be afraid to click on the images to view them in larger dimensions 🙂 ):
One of our first stops was the Grand Canyon, one of the biggest and most impressive canyons in the world. It has been formed during the last ~17 million years (other geologists think it has only formed in the last 6-7 million years) by the interaction of two geological processes: Erosion of rocks by the Colorado River and the uplift of the Colorado Plateau. Without the uplift, which was as much as 3 km in some regions of the Plateau, the Colorado River could not have incised as deep into the rocks as it has.
In the picture above you can clearly see how the different beds withstand erosion differently and form several levels. The different sedimentary rocks (the “layers” you can see) formed in the Cambrian (the bottommost layer) to the Permian (uppermost layers) and thus can give us insights into 300 million years of history. However, at the very bottom of the Grand Canyon even older metamorphic and magmatic rocks are exposed:
Between the older rocks and the horizontally layered Sediments on top there is what geologists call an “unconformity“. This represents a period of time in which no new rocks were deposited or formed but which is often dominated by erosion and uplift. As there are no rocks deposited it marks a gap in the history that rock can record. The unconformity here at the Grand Canyon represents about 250 Millions of years – quite a long time if you ask me!
The Grand Canyon is also one of the few areas on the Colorado Plateau where the basement is actually outcropping. Most of it covered by a thick sedimentary succession which has been weathered spectacular in several places. One of the most famous of these landscapes is the Monument Valley, where quite a few movies have been shot in the past. It is located near the Arizona-Utah border and is home to many big pillars of rock, so-called mesas and buttes. I guess they form one of the most photographed landscapes in the south mid western US:
The geology behind this landscape is rather simple: More or less horizontally layered sediments have been eroded over time. The valley bottom is made of easy to erode siltstones while the buttes are made up of sandstones and have a layer of conglomerate on the very top. The conglomerate forms a cover that prevents the sandstones below it to get easily weathered by rain (and wind). So what you can see are the last remains of a thick succession of sandstones and conglomerates that have been heavily eroded over time. Give it a few 10s of thousand of years (or a bit more) and not one of the buttes will remain standing!
While the monument valley is manly shaped by erosion the astonishing arches of Arches National Park in Utah have formed due to the interplay of tectonics and erosion:
After admiring the arches for some while one will soon realize that most (all?) of them are aligned in a certain direction and form within a several meter thick wall of sandstone. The image below gives you a good impression what I mean when I say wall of sandstone!
These walls are up to 30 meters high (I guess) and are pretty impressive. Walking along the “valleys” between the walls can give you claustrophobia (by far not as bad as in some slot canyons though!) but first of all it is the absence of any noise that makes a hike along the walls worthwhile. Of course it is not pure chance that all the walls are facing the same direction: They form along the same direction joints and fractures cut through the sandstones:
The arches themselves form when some less strongly cemented parts of the sandstone walls or fins are eroded by wind and weather while the surrounding strongly cemented sandstones withstand the erosion. Cement by the way is what holds the single sand grains together, it is often quartz or some form of carbonate.
The reason for the formation of the fractures and joints is less obvious: Beneath the sandstone layers that form the arches is rock salt (to be exact, there are thick shales between the salt and the sandstones). Because salt is less dense than sandstone it pushes upwards and tries to reach to surface. This leads to a so-called anticline with salt in the core of the anticline. Once the salt has pushed up far enough it will get in contact with groundwater which then starts to dissolve the salt. At one point most of the salt will be dissolved and the anticline collapses and often forms a graben valley, bound to a normal fault. The collapse of the anticline will lead to a widening of the exiting fractures and joints and to the fascinating walls and fins of sandstone we can see in the Arches NP! While this might sound very confusing you can find a very helpful cartoon on page 12 (that is 18) of this pdf.
The normal fault that forms one side of the graben valley can be easily observed near the visitors centre of the Arches NP, just on the other side of the Highway 191:
This is it for now. Part II will cover beautiful bleaching phenomena, slot canyons and volcanoes!