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The Landscape
This region consists of short prairie grasslands which are dissected by canyons or arroyos associated with the tributaries of the Purgatorie River. The river and its tributaries, has incised the soft sandstone creating a vast expanse of canyon lands. The physical location combined with the semi-arid climate makes finding sandstone with desert varnish highly probable. Pinyon-Juniper trees and wild grasses dominate the flora of this area. To the right is a photo which gives one a good idea of what this landscape is like. Take note of the large boulders strewn across the canyon walls.
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The Purgatorie River: Photo by Kari P. |
The Picket Wire Canyon lands create a window into the past. The major rock formations include: the Morrison formation, the Purgatorie formation, and the Dakota formation. The cap-stone in the area consists of the Dakota sandstone, which is an orange-brown color. The Purgatorie formation is divided into two units. The first is called the Glencairn member and is made of marine shale, which is grey in color. The second is the Lytle member which is below the Glencairn member and is light brown or tan. One may also notice huge boulders littering the river-bottom, which have eroded off of the canyon walls (Matthew Morgan 1997). The Morrison formation which is on the valley floor, dates to about 150-million years ago to the Jurassic period. It is also divided into two parts. The lowest part is composed of lacustrine deposits, that is, rocks deposited in lakes. Shales and sandstones, found in this portion of the formation are indicative of floodplain environments and river channels. The upper part of the Morrison formation is made of fluvial deposits, which are rocks deposited by rivers. Siltstones and shales represent soil-forming environments, while the limestone layer represents a lake environment.
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Anthropomorphic Figure:
Photo by Kari P. |
Geomorphically, the area is composed of four main strata and one may notice that all of these are sedentary rock types. The area is also host to many species of cactus. The region is not only rich in geomorphology, but is also rich in archeological remains including rock art. The only reason there is so much rock art in the area is due to the semi-arid environment, which is required for desert varnish to develop on the rock surfaces. The Petroglyphs are created by pecking though the thin layer of varnish to reveal the light sandstone underneath. This is an example of the rock art that can be found throughout the Picketwire canyon. |
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The Processes at Work
So what processes are responsible for the creation of this amazing landscape? How does the altercation of the rock surface, by pecking rock art into the rock, affect these processes? Firstly, it is important to note that all landscapes are evolving (changing) toward equilibrium. Weathering and erosional processes are acting to tear this landscape down. The canyon lands are a deconstructional (or erosional) landform, and consist largely of valleys, arroyos, gullies, buttes, and spires. Climate and rock type have a great influence on the type and rate of weathering (Kenneth and Christiansen 2001:238). As discussed earlier this landscape consists largely of sandstone. Before we discuss the weathering and erosional processes we must first consider the rock type.
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Rock art and
weathering: both physical and chemical: Photo by Kari P. |
If climate and rock type influence the processes we will first consider how sandstone weathers in this climate. We can do this because the major rock groups weather in a characteristic pattern.
"Sandstone is composed mostly of quartz grains, with varying amounts of small rock fragments, feldspar, and clay minerals. The quartz in sandstone is highly resistant to chemical weathering, so chemical decomposition of the rock consists largely of an attack on the cement. The major materials in sandstone are calcite, iron oxide, and quartz" (Kenneth and Christiansen 2001:238). Another important factor to consider is the desert varnish that thrives in semi-arid climates, which is exactly what is going on in Picketwire. Ron Dorn (1984) explains,
"Rock varnish is composed of clay minerals, oxides, and hydroxides of manganese and iron, and minor and trace elements that accrete on rock surfaces mostly from desert dust" (308). A side note, Dorn has devised a way to absolute date rock varnish. This has great importance to the study and conservation of rock art. The next thing we need to do is discuss the weathering processes which are acting to tear-down this
landscape. There are two main types of weathering: chemical or biological and physical. Each of these act on different time tables. First, we should bear in mind that the rate that processes break-down a solid rock body depends on three main factors: (1) minerals which are susceptible to weathering, (2) climate, (3) and the amount of rock surface exposed to the atmosphere (Kenneth and Christiansen 2001:258). Once the weathering process is complete, erosion acts to move the decomposed material. The weathering process produces: bodies modified into spherical shapes, a blanket of loose, decayed rock debris, known as regolith, of which soil is an important part, and ions in solution (Kenneth and Christiansen 2001: 249). Chemical or biological weathering encompasses a huge amount of weathering forms. Humans, other animals, and vegetation are aiding in the erosion and weathering of this landscape. Roots act to pry rocks apart or detach from the canyon walls. Firstly, plants can tear rock apart, because their roots get into small fractures in the rock and continue to grow. Human activity is considered biological weathering. For instance, rock falls often occur when people climb on the canyon walls because the rocks are so loose. Rock art is also a form of biological weathering. When one pecks through the varnish this allows water to infiltrate into the stone. This speeds the weathering in most cases. This is because water is one of the most extreme forms of weathering and erosion. This brings us to the topic of physical
weathering.
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Example of freeze/thaw
and its
relation to the rock art: Photo by Kari P. |
In this region freeze/thaw is a major weathering process working to tear down this landscape! This is also attributed to the climate (which is semi-arid). Freeze/thaw is only possible if there is a certain amount of water and specific climate. Water infiltrates into the rock through small fissures and as the temperature falls to freezing the water freezes in the fissures and expands. Over a long period of time this will tear down the canyon walls. Evidence of freeze/thaw is found in angular rock fragments, eventually creating a talus slope on the canyon floors. Take the photo to the right; one can see the results of freeze/thaw and its relation the rock art. Now that we have an idea of what processes are acting to tear down this landscape, we must consider the fact that these processes occur at different rates. This is termed differential
weathering.
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Example of
transport-limited
landscape: Photo by Kari P. |
The processes acting on this landscape can be classified into two classes. The first is termed weathering-limited, in which the transport rate is faster than the weathering rate. The result is less soil because it is transported almost as soon as it has weathered. The canyon walls are a great place to see a weathering-limited landscape. The second class transport-limited; the rate of weathering is faster than the transport rate. This type of landscape can be found along the river up to the base of the canyon walls. For instance, there is more soil here and not much exposed bedrock. This is due to a lack of material being transported, and results in soil formation.
The two main weathering and erosional forces at work on the landscape are freeze/thaw (weathering) and the river (erosion). We have discussed weathering at length; however, there is another process at work on this landscape. That process is erosion, the transport of weathered material. An example of how weathering and erosion work can be found in the following statement,
"Accelerated erosion due to forest clearing, farming, and overgrazing is caused by more rapid and concentrated runoff, but soil formation is not speeded up correspondingly, and mass wasting may change from soil creep to slumping and earthflow" (Bloom 2004: 329). This leads us to the next
topic of hazards. |
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Hazards
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Danger! Very loose
slope: Photo by Kari P. |
A main hazard in this region is flash floods. This area consists of a massive highway system of arroyos. When it is dry these seem like simple gullies, however, there could be a storm event miles away and this would send a wall of water through these formally dry gullies. Therefore, flash floods are a huge hazard. The main river will flood at certain intervals, for instance the 100 year flood plain tells us that every 100 years the river will flood to that extent. Rock fall is another hazard here, due to the loose sandstone. The photo above is typical of the canyon walls; notice the loose rocks and the slope. Evidence of rock fall is found along the canyon walls, where many large boulders are located. Another hazard in this area is the wildlife.
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Snake track:
Photo by Kari P. |
Rattle snakes, tarantulas, and mountain lions are just a few of the critters that one might encounter. This is a very WILD environment and one should treat it as such. Take the photo to the right, this is a snake track! The temperature is also a hazard, because it is extremely hot in the summer, however, the winters are harsh. A simple twisted ankle in this environment could kill; so one should take the proper precautions when preparing to enter this environment. |
