Sunday, August 02, 2020

Geology of the National Parks Through Pictures - Rocky Mountain National Park

My next post about the Geology of the National Parks Through Pictures is a park we hit up after the wife did a race outside of Denver.


You can find more Geology of the National Parks Through Pictures as well as my Geological State Symbols Across America series at my website Dinojim.com.

Colorado State Geological symbols can also be found HERE.

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Even though there are a lot of scenic views within Rocky Mountain National Park, the basis for Rocky Mountain National Park is geology. The reason the mountains are here is geology and the features seen on the mountains are geological. So even though this isn't a strictly geological park, geology in imbibed everywhere. 

The obligatory entrance sign

The mountains within Rocky Mount National Park are a result of orogenic events (mountain building) that took place over three periods within the planets history. The first orogeny (mountain building event) took place 1.7 billion years ago during the Precambrian. This event laid down the foundation of faults that will later be activated during the most recent mountain building. The next orogeny, known as the Ancestral Rockies Orogeny, was 285 million years ago during the Pennsylvanian. Afterwards, the Ancestral Rockies were entirely eroded away, but again the faults were left behind. The final mountain building event was the one which produced the mountains we see today. This occurred 70 to 40 million years ago and is known as the Laramide Orogeny.

The Earth is made up of giant plates which are all moving around. When two plates are moving towards each other they push each other up, forming mountains. This is what is currently happening in the Himalayas where the Indian Plate is moving northward into the Eurasian Plate. As the two plates collide, the edges get "wrinkled". It is this wrinkle that are the mountains.  Here is a view of the Rocky Mountains from the visitors center. 



 The Laramide Orogeny occurred due to the presence of a plate off the western coast of the United States known as the Farallon Plate. The Farallon Plate was an oceanic plate that was pushing up against the west coast of the United States and Mexico. Since the plate was made up of oceanic crust, it was denser than the North American continental crust and therefore ended up going beneath the North American Plate in a process known as subduction. As the plate went under North America, it still ended up compressing North America, forming those mountain wrinkles. The mountain building occurred along those long ago created faults that were reactivated during this renewed time of mountain building. Eventually, most of the Farallon Plate was completely subducted, leaving behind only a small piece known as the Juan de Fuca plate off the coast of Washington and Oregon.

 Within the park there are also significant amount of glacial features, this divot out of the side of the mountain being one of them. When a glacier is forming on the side of a mountain, the snow continually accumulates near the peak. Eventually the snow builds up enough that it compresses down into ice, and then eventually the entire ice block starts to flow downhill. As the ice flows downhill it picks up rocks and starts to erode into the mountain that it is sitting on, creating a bowl shaped depression. That bowl shaped depression is what eventually becomes known as a cirque, once the glacier melts entirely away. 

 Here's the view of another cirque from a distance.

 Here are a whole bunch of glacier features, as well as some glaciers still present (where some of the snow and ice doesn't melt completely in the summer time). Within this photo we also have some more cirques. Since cirques erode down into the mountains, creating a bowl shaped depression, these often eroded down into bedrock. The bedrock creates a nice seal against water escaping and eventually can create a lake. This specific type of lake is known as a "tarn". When two cirques are on the the same mountain but going in different directions (i.e. back to back), they can form a ridge between themselves. This steep, knife-like ridge of rock is a glacial feature known as an "arĂȘte", which you can also see within the central portion of the photo.

 Here is a view looking down into a cirque. Although there is snow here, it is not a glacier until the snow starts to build up over time and compact into ice. Right now these are just seasonal snow fields.

After the glaciers move downward from the mountains, they start to fill up the valleys between the mountains. This ends up eroding the valleys in a specific pattern. When rivers erode out valleys, the valley cross section forms a "V" shape, where the stream erodes straight downward at the bottom of the V. However, when a glacier enters a valley, it is much larger and erodes over a much larger area. The result is a valley with more of a "U" shape, created as the glacier erodes on all of the edges of the valley. Here is a look towards the mountains centered on a glacial U-shaped valley.   

Another erosional glacial feature is when the glaciers make it further down slope and erode out and widen existing valleys. The valleys were initially eroded by streams and rivers, but glacial erosion widened and flattened these stream valleys far beyond what was initially dug out. After these valleys are eroded out by glaciers, eventually the glacier melts and water returns to the former streams. The large flat areas are then filled in as a series of interconnected lakes called paternoster lakes.


One of my favorite features of western National Parks is the Continental Divide. This is essentially a high point of the country where water at the divide goes one of two ways. Within this picture (which I am facing south when I took), water on the left, or eastern side all flows to the Atlantic Ocean, while all of the water on the right, or western side, all flow to the Pacific Ocean. The signs are not often perfect, but they are pretty close. The Continental Divide is actually a line of mountain ridge tops that runs up the entire length of the continent, dividing the drainage between the two oceans. There are other divides within the country as well, such as the one that divides water going north into Canada's Hudson Bay, or water going into the Great Salt Lake, which is an end-basin where water goes to die.

Saturday, August 01, 2020

Geology of the National Parks Through Pictures - Natural Bridges National Monument

My next post about the Geology of the National Parks Through Pictures continues the Spring Break trip of 2020 where we visited the Needles District of Canyonlands. On the same trip we hit up one of our last remaining parks in Utah.



You can find more Geology of the National Parks Through Pictures as well as my Geological State Symbols Across America series at my website Dinojim.com.

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Natural Bridges National Park
A couple of hours drive south of Moab, UT brings us to one of the most remote parks in Utah and the second to last that we have yet to visit. Natural Bridges is a small park, with one loop road that brings you to the overlooks of three natural bridges and also has trails to each bridge. We ended up hiking the shortest trail down below the third arch and got some great photos in the process.

The entrance sign shot.

Each of the bridges is given a name in Hopi to honor the ancestral Puebloans who once lived here. The loop is a one-way drive so you can hit up each of the bridges in order. The first bridge is known as Sipapu bridge, which means "the place of emergence", an entryway by which the Hopi believe their ancestors came into this world.

 Here is a zoomed out, panoramic view of Sipapu bridge from the overlook. The bridges occur within the Permian age Cedar Mesa Sandstone (~286 million years old), which is formed from near-shore sand dune deposits (the whiter, more resistant layers) and has frequent river and lake deposits (the redder, softer layers). This is the same formation that produces the needle formations within the Needle's District of Canyonlands National Park.

 This region had been occupied off and on throughout the past several thousand years, with the ancestral Puebloans farming and building houses within the alcoves of the cliffs using the Cedar Mesa Sandstone as building stones. The houses were built between 1000 and 1270 CE, when eventually they left, likely due to environmental changes.


The second bridge on the loop is Kachina bridge, which is named for the rock art on the bridge that resembles the symbols commonly used on kachina dolls. Although "bridges", such as those seen here, and "arches", such as those seen at Arches National Park, are similar in structure they have different definitions. By definition, a "bridge" has water running underneath it, while an "arch" does not. However, they also form differently. While an arch forms from the slow dissolution of the cement within the sandstone and eventual erosion of the sand, a bridge is formed by river processes.
The three different bridges formed slightly differently, but all three of them have the same basic formation. This area is part of the Colorado Plateau, a region that has been slowly lifted upwards over time. As the ground surface is lifted up, the rivers cut down into the ground. This creates what is known as an entrenched meander, meaning a bend in the river that is firmly in place within the surrounding rock. Many rivers within the Colorado Plateau have formed these features including the Grand Canyon and Dead Horse State Park. Even though the meanders are entrenched, that doesn't mean they stop eroding the surrounding landscape. The rivers slowly cut through the outside of the meander by the forces of erosion and the increased water speed on the outside of the meander, eventually cutting off the old meander. This is the exact same process that goes on in all meandering river systems, like the Mississippi River, however since this river is entrenched, when it cuts off a meander it is doing so below a rock ledge, leaving behind a bridge of rock. Image from the NPS.


Here is a zoomed out view of the Kachina bridge, which is located on the right of the image. This view we are facing the outside of an entrenched meander, with the cut-off meander far in the background. The current river now flows in the entrenched meander at the front of the photo.

 The final bridge, and the one we hiked to, is known as Owachomo bridge and means "rock mound", in regards to the mound of rock on top of the bridge seen on the left side of this photo.

 View from below the Owachomo bridge. As long as water continues to flow under these bridges they will continually be eroded until they collapse. However, since this area is in the high desert, rain fall is relatively low and therefore the rivers are more like a trickle of streams at most times, preserving the bridges for many years to come.

 Here is a view of some of the cross bedding within the Cedar Mesa Sandstone, which indicates that this particular part of the formation was once a sand dune. The cross beds are formed when sand is blown over the top of a dune crest and rolls down the opposite side. As it rolls down the leeward side, or slickface, the sand piles up in parallel layers that are at an angle to the ground surface, which is what is preserved as cross beds within the rock unit.

 Here are more preserved cross beds in the Cedar Mesa Sandstone. As dunes shift directions, based on the wind direction, the cross beds change direction to match. This results in stacks of cross beds often going in different directions.

References