Triassic/Permian

When we saw the top part of this layer we stopped at "The Neck," on the North East end of Canyon Lands National Park we were actually standing on the Glen Canyon Group which is the upper layers of the Triassic. We were standing in the Navajo Sandstone which looks like great rolling sand dunes, because they are petrified sand dunes. From the neck, if you look east, you can see down into a big canyon and see alot of different layers. The first we see is the Navajo Sandstone which is a white cross-bedded layer, underneath that is the Kayenta Formation. According to the Canyon Lands Pamphlet the rangers gave us in the beginning of the park, the Kayenta is formed from meandering rivers, and there are deposits of lime stones and shales in these layers. The Wingate Formation is right underneath and it is evidence of another really dry time, created by rolling sandstones and this layer too will have the cross-bedding we see in the Navajo. These three layers keep the cliffs pretty sheer and straight due to composition although with the Kayenta it is a bit 'stair step' look to it because of it's lesser Resistance to weathering. As we drop down we come to the lower parts of the Triassic Era and the cliffs turn into steep to shallow slopes usually covered mostly with eroded rock and sediment from the higher layers, but here we see the Chinle formation and the Moenkopi. they are layers of shale, clay, sandstone. They are formed under water and swamps. In the Chinle we find a lot of petrified wood. In that layer as well we find a lot of Uranium.
Here we get to the Permian Era about 250 million years ago. It is best seen as you look around the canyon and see where the shallow slope immediately drops off into anther canyon. There is a white layer of rock that runs around the rim and it is made of sandstone. Sandstone is less resistant, so it creates these crevasses. That layer is the top of the Cutler Group called the White Rim Sandstone.

Green River Overlook

Coming south east on I-70 through the town of Green River(where the Arby's has a great bathroom according to Renee:) Exit mile 181 at elevation 4782. Walking out to a pavilion looking south the first outcrops we see are the North end of Arches National Park. Also the first visible evidence of the Salt Valley Graben( graben being a German word for Ditch). We have learned already that grabens and Anticlines form in many ways. This one forming from salts deep below the earth's surface moving upward and creating a dome. Cracks at the surface allow water to creep down and dissolve the salt creating a cavern of open unsupported space beneath the ground. The earth collapses and leaves these fins above ground that we can see from this point.

Jurassic

The Jurassic layer is one of the higher layers we see in the Arches National Park and Canyon lands. The Slickrock Member in the Entrada Sandstone of the Jurassic is where we find the Delicate Arch of the A.N.P The bottom most layer of the Entrada is a layer consisting of silty sandstones called the Dewey Bridge formation. It is easily recognized because of it's pattern of erosion and it's deep red color. It looks like bricks and does not weather flat like a lot of the sandstone but more in a cobble stone look. On top of that sits the Slickrock member who's composition is sandstone, making it weather in sheer straight cliffs. Leaving smooth straight cliffs called "arch formers" that are red in color and have cross-bedding in them. The relationship between these two layers is where the formation of arches begins. The top most layer is called the Moab Tongue. This layer can be seen at the very top of the Delicate Arch, it is a white cross-bedded sandstone.

Entrenched Meanders at Dead Horse Point

Approximately 10 million years ago, the area now known as the Colorado Plateau was uplifted. It was uplifted by tectonic activity which lifted the area approximately 10 thousand feet in 10 million years. This is a significant amount of uplift in a short period of time. At that time (10 million years ago) the Colorado River was near its base level. Being near its base level, it began to meander. When the Colorado Plateau was uplifted, it caused the river to begin cutting through the underlying rock. As the river attempted to reach its base level, the land was continuously being uplifted. This caused the Colorado River to scour deep trenches into the underlying rock. As the river continued to cut down further into the rock, they became entrenched by the rock. As you can see in the picture, the Colorado River is meandering, and also notice the rock face on each side of the river hundreds of feet high. 

Formation of Arches

For arches to form, there needs to be three main criteria present.

1. first a thin wall of sandstone (as seen in picture) need

 to be present. These thin "walls" of sandstone are called fins (see fin formation on blog). These fins allow for the arch to open up all the way through the wall.

2. The next criteria that must be present is that the overlying layer of sandstone must have an underlying layer of shale. Having the less resistant rock underlying the more resistant layer causes the underlying shale to be eroded beneath the sandstone. When the underlying shale is eroded away, there is no support under the sandstone. This causes the sandstone to slowly collapse. 

3. The contact between the sandstone and the shale is the third criteria necessary for arch formation (arches form at a contact between overlying sandstone and underlying shale). The second picture shows the contact between the two layers very well along with the early stages of arch formation.

 

                          

 

This picture shows the contact between the sandstone

 and the shale very well. You can also see, in the 

center of the photo the beginning formation of an arch. 

Different Types of Arches

According to Wikapedia "The choice of "bridge" vs "arch" is somewhat arbitrary. The Natural Arch and Bridge Society identifies a bridge as a subtype of arch that is primarily water-formed. By contrast, the Dictionary of Geological Terms defines a natural bridge as a "natural arch that spans a valley of erosion." http://en.wikipedia.org/wiki/Natural_arch#Weather-eroded_arches
While a bridge is one form of an arch, there are actually places in the world where the arch is so big that it is an actually highway on top making it a literal bridge. Carter Caves State Park is a cave eroded arch (made of limestone that has a paved road on top of it.
There are also Coastal Arches that form in two ways, Discordant Coastlines Which form at a 90 degree angle to the water, and Concordant which form parallel to the coast line.
The ones we are all most familiar with are the Natural Arches that from in Arches National Park. There is a cool Description of how they form in this blog, but also at this link. http://en.wikipedia.org/wiki/Image:Arch_form.gif

Dead Horse Point

Dead Horse Point

By: Tania Tolbert

On the last day of our trip we stopped at Dead Horse Point. This lookout is located at the end of a thin peninsula that overlooks many neat things. We learned and observed what entrenched meanders looked like and how they formed. The river in the picture above is the Colorado River and is responsible for creating and cutting the entrenched meanders. We also were able to see the evaporation ponds of a potash mine. This mine is located in the Paradox Formation and creates potassium chloride evaporites. These evaporites are used a crop fertilizers. 

Behind me is an anticline (not very visible) and in the distance are the La Sal Mountains.