Shea Burns
November 17, 1999
UNVR195A-011
Alluvial Fans and Sand Dunes
There are several places patterns can be found in nature. Before, geologic patterns found in sea floor spreading and in lava lakes were portrayed. There are several other instances where geologic patterns can be found in nature and more specifically in and around Arizona. Some patterns can be seen in the mountains and others can be seen in the washes that drain from the mountains. Some beautiful patterns can form from the erosion and deposition of sediment from a mountain in the form of an alluvial fan. Depending on the type of erosion, a pattern may also be left behind on the mountain itself. Another prime example of a pattern that is the product of the local climate and weather in Arizona and the surrounding area are sand dunes. Beautiful dunes are formed in various shapes and sizes depending on several factors, including the prevailing wind direction, the force of the wind, and the available sand.
Alluvial fans can be defined several ways, but all the definitions have the same general meaning. Basically, alluvial fans are formed as sediment is washed out of a mountain and deposited at the base or near the base of the mountain. A stream flowing out of the mountain carries the sediment. As the stream nears the base of the mountain it loses some of its energy because the slope of the mountain loses slope. The stream loses the majority of its energy and velocity, though, because it widens at the base of the mountain from a narrow channel to a wide plain, not because the slope of the mountain is smaller. As the stream loses energy, it also loses its ability to transport sediment. The size of sediment and the amount of sediment carried by the stream is directly related to the energy of the stream. The size of the particles carried is called the competence of the stream and the amount of sediment transported is called the capacity. These both decrease as the flow moves down the mountain and loses energy. This means the sediment must be deposited and, depending on where it is deposited, an alluvial fan may form.
Alluvial fans may be low cone shaped deposits or fan shaped deposits. The shape of the fan can be related to how the stream changes when it moves from the mountain into a more flat range or plain. There are a few different changes a stream can make. It can make a gradual change from a fast moving turbulent stream to a slow meandering stream if the slope of the mountain decreases very gradually and the mountain front widens slowly. The stream can also make a sudden change and widen abruptly as it leaves the mountain front into an open valley. Generally, an alluvial fan has a range of sizes of sediment contained in it. The larger sediments from boulders to cobbles to sand are deposited at the top of the fan with finer sediments such as fine sands, silts, and muds deposited lower down on the fan. This is because the coarser sediments require more energy to be transported, so as the stream loses its energy the coarse, bigger material is deposited first and the finer sediments further down stream near the bottom of the fan. Press and Seiver provide the pictures below.
As seen in the second picture, the fan is broken up into several parts. As described at the http://geo.cas.vanderbilt.edu/Geology262%5CFlugeo.html web page, "as a fan ages it is frequently observed to entrench at its emergence from the mountain front and deposit its material more distally at the toe of the fan." Another picture, provided by http://athena.wednet.edu/curric/land/landform/alluvfan.html is shown below. Notice the conical pattern in all the pictures. The alluvial fans also have a slight upward curve in them that connects the mountain to the valley.
Sand dunes also form remarkable patterns. There are several types of dunes that exist, all dependent on factors such as the amount of sand available, the strength of the wind, and the direction of the wind. Barchan dunes, transverse dunes, blowout dunes, and linear dunes are the most common types of dunes. The following picture, provided by http://pubs.usgs.gov/gip/deserts/dunes/, shows the way dunes are formed in a generic setting.
Each dune represents a different pattern. The patterns are shown below, courtesy of Press and Seiver.
Most dunes have their own symmetry which contributes to their being part of a pattern. All but the linear dunes have a plane of symmetry, and the plane is parallel to the wind direction in each case. Pictures of some of the dunes found in nature are shown below, courtesy of the USGS at http://pubs.usgs.gov/gip/deserts/dunes/. The first and last pictures are of Barchan dunes and the wind is coming from the left. The second picture is of an isolated linear dune.
Though none of the dunes above are actually in Arizona, the Sand Hills are very close to Arizona. The Sand Hills are an example of transverse dunes. They are located 25 miles west of Yuma. The renderings below, taken from DeLorme's Topo USA computer program, show the sand dunes in 3-D from two different angles and show one contour map. The transverse dunes shown here, along with several other dunes and fans show symmetry down the middle of them. Usually, enough of this symmetry or enough of a pattern can be noticed to make a connection between the type of dune or fan one is looking at though. There are still more patterns that can be found in and around Tucson that are just as presentable or more so. Other places to look for patterns are in rock beds where ripple marks can be found in a wave like pattern. These patterns are also more permanent, because they have been lithified as part of the rock and are more resistant to weather. Anyway, keep those eyes open and do not stop looking for patterns, they can be seen everywhere.
Works Cited
