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8/8/2019 Taysom James Final

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The second main form of weathering is that of lithobionts; or more specifically

lichens. The lichens as they grow on the rock will slowly weather the rock that they cover

as seen in figure 6. Then as the lichen dies it will reveal an area of weak deteriorated

material that will quickly erode away. This will leave lithobiont pitting behind which is a

rough surface that appears to be covered in small holes. The North side of Mount

McDowell is heavily covered in lichens, but the entire area shows some lichen growth.

This contributes to the overall weathering of the mountain (Paradise 3).

Figure 6. This four step process shows how lichens are weathering Mount McDowell.

As it grows the lichen disintegrates the rock underneath. As time passes by the lichen dies

off in that particular area exposing the weathered material to be eroded.

There are several things that happen after the rock is weathered to move it away

from the cliff face. The first erosion process that will usually occur is rock fall. This is

where gravity has enough effect on the cliff to break off a section at natural area of

weakness in the rock and cause those rocks to fall as is seen in figure 7. The rocks falland form talus cones or small slopes of colluvial material. In figure 1 there can be seen

several different levels where cliffs are found. This is because the top layer is a resistant

layer of rock called a capstone. This layer of rock protects the rocks underneath from



weathering and slows down the mass wasting process. It is not until the softer rock

underneath is sufficiently weathered and weakened that the undercutting will collapse the

area and the capstone falls with the other material.

Figure 7. This shows the process of cliff retreat. The slope of eroded material starts

off with a shallow slope. As the material is washed away the slope increases until the

point where gravity overcomes the weaknesses in the cliff face and causes a rock fall. The

newly fallen rocks decrease the slope of the hill and re-stabilize the cliff.

Once the material has fallen to the cliff side, it takes another erosional process to

move that talus away from the cliffs. The rocks will be moved away through fluvial

processes every time it rains. As the water runs down the sides of the mountain it will

carry with it the weathered particles and sweep the mountainside clean. As seen in figure

1 there are certain areas that have little to no debris at the base. There are others, like in

the center, that have a talus cone. The smaller weathered material is easily swept away in

the rills that line the mountain. In the cases of larger rock falls however, the cobbles

would be too big to be moved by the small amounts of water that could run off the

mountain and so the talus cone remains.



Figure 8. This is a diagram of the ancient and modern floodplains of the Salt River.

Each time the relative base level dropped a new floodplain was cut which has left a series

of floodplain terraces.

Fluvial processes from the Salt River are the last process that was involved in both

erosion and deposition. Before dams were built upstream to create reservoirs, the stream

would have seasons of flooding. During a flood any of the smaller weathered material at

the base of Mount McDowell would be swept away in the torrential currents. At the same

time there would be material deposited on the banks, which over time created river

terraces. One of these terraces can be seen in the center of figure 1. This terrace can be

identified as the mesa river terrace. The fluvial processes have not done much to shape

this mountain in recent history but it did play a major part.

The last thing that should be addressed is something that gives Mount McDowell

such an interesting look. As can be seen very easily in figure 1, the mountain slopes

gently on one side giving it a unique look and exposing many layers of strata. This is

from the faulting that occurred during the expansion of the basin and range. After the

different levels of rock solidified, the tectonic action in the area tilted all of the rock. So

as seen in figures 3 and 4 there are even areas within Mount McDowell that have been

offset by this tectonic action.



Figure 9. Shown here is the end result of the tectonic activity in the area. After the

area was compressed there was a period of time when the pressure subsided allowing for

the expansion in the basin and range region. The granitic core tilted as seen and the

mountains ranges began to erode away and filled in the basins. The inverted beige

triangles represent the sediment layers.

It took millions of years of deposition, faulting, weathering and mass wasting to

create this beautiful landscape that we see today. It is made up of the conglomerates and

igneous rock that has been deposited. Salt and lichens are weathering Mount McDowell,

and continue to reshape the cliffs and slopes. As the materials weather, gravity moves the

material through rock falls and fluvial processes. All of this creates what is seen today as

a tranquil landscape.



Arrowsmith, Ramon. "Paleochannels of the Salt River: Evaluation of recharge potential."15 11 1999. ASU. 10 Dec 2008http://activetectonics.asu.edu/Environmental_Geophysics/fig1geo.GIF

Dorn, Ron. "Salt River Terraces." ASU. 10 Dec 2008http://www.asu.edu/courses/gph111/Fluvial/SaltRiverTerraces.GIF

Dorn, Ron. "Lecture on Weathering." 10 Dec 2008http://alliance.la.asu.edu/gph211/breeze/PublishWeathSoils/index.htm

Howard, Alan. "Introduction to Cuesta Landforms and Sapping Processes on theColorado Plateau." 1988. 10 Dec 2008http://erode.evsc.virginia.edu/papers/howard_colo_plat_88.pdf

Reynolds, Stephen. "Subsurface geology of the easternmost Phoenix ." Aug2002. Arizona Geological Survey . 10 Dec 2008http://reynolds.asu.edu/pubs/phx_basin_subsurface_final.pdf

Paradise, Thomas. "DISPARATE SANDSTONE WEATHERING BENEATH ."1997. University of Hawaii Hilo. 10 Dec 2008 http://www.jstor.org/stable/521309

Skontnicki, Steven. "Geologic Map of The Fountain Hills / Mount McDowellArea, Maricopa County, Arizona."Arizona Geologic Survey. 1995.

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