Module 2 Part 3 - Earth Surface Landforms
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Lets look at landforms produced by those four agents of sedimentation: gravity, water, wind, and ice.

Agent - Gravity: Landforms that mainly result from the action of gravity upon rocks or sediment include such deposits as talus aprons, desert landforms like mesas and buttes, and landslides (called mass movements in geology).

Talus aprons or slopes are deposits of broken rock that have been weathered from nearby rocky cliffs and fallen down hill. Talus slopes are the first step in the erosion and transportation of rock material from high mountain areas to low areas for deposition. Although a talus slope of broken rock at the base of a cliff might seem to be motionless, in geological time, the rock pieces are slowly making their way down hill to a stream for transportation out of the mountains.

In the desert Southwest of the United States, there are many examples of landforms produced by the retreat of cliffs of flat-lying sedimentary rocks. Plateaus are large flat areas many square miles in size that are surrounded by cliffs. Mesas are large flat-topped hills with cliff sides that are smaller than plateaus but larger than buttes. Buttes are small flat-topped hills. As plateaus erode to mesas and mesas to buttes, the final end product landform looks like a chimney and that is exactly what they are called.

Figure 25: Mountain talus slope - Cache la Poudre River valley

Mass movements: Mass movements involve the transport of rock, soil, ice, or debris downhill by gravity. This includes falls, avalanches, slides, flows, and creep.

Falls: such as rock falls where large masses of rock move down valley walls in mountainous terrain, usually caused by a trigger like an earthquake or heavy rains. These are the fastest mass movements and often result in masses of rock falling upon highways running through mountains, like I-70 west of Denver.

Avalanches: these fast-moving mass movements can consist of snow or debris, including rock, soil, trees, etc. Avalanches move down chutes or passages on the mountain valley walls that are wide at the top and narrow at the bottom.

Slides: slides move as coherent masses down a glide plane. Rock slides frequently involve layers of rock moving down sloping bedding planes. This occurs where the rock layers parallel the slope (called a dip slope). Slumps can also be considered slides because they move along a glide plane. A slump is a mass of soil or soil and rock (soil and rock together is often called regolith) that becomes water saturated and moves along a curved glide plane downhill. The slump block leaves a small cliff of exposed soil or regolith above it called a head scarp and has a bulging bottom margin called a toe. Slumps occur in steep soil slopes that become saturated with water.

Flows: Like the name implies, flows go down hill with a fluid movement. Debris, earth, and mud flows have increasing amounts of water in them. Fast-moving mudflows can contain 60% water and can move very fast indeed. California makes the news every year with catastrophic mudflows that occur after heavy rains on a thick-soil landscape. When mudflows move out onto a flat area, they often have a rounded or lobate margin like the one in Figure 26.

 

Figure 26: Mudflow - north of Dubois, Wy. Maher.

 

Creep refers to very slow movement of soil down a valley slope.

Part 3 - Earth Surface Landforms
Module 2 Home | Page 1 | Page 2 | Page 3 | Web Sites and References

Part 1 - Earth Surface Processes | Part 2 - Describing the Earth's Surface | Part 3 - Landforms