Erosion
Erosion literally means "to gnaw away." Erosion is the action of surface processes such as wind, water, soil and gravity that move dissolved material or soil or rock from one location to another.
The breakdown of rock or soil into sediment is referred to as mechanical erosion. Chemical erosion is when rock material is dissolved into a solvent such as water, and then the solution flows away. Eroded sediment or solutes may be relocated by just a few inches (mm) or by hundreds of miles (km). Suspended abrasive particles such as sand, rocks and boulders also make their contribution to the erosion process.
Factors that contribute to erosion
Gravity
Mass movement of soil occurs on steep slopes under the influence of gravity. The process involves the transfer of slope materials such as earth and rocks from higher grounds to lower due to the material's own weight. Gravity determines the force of the water flowing down a slope, which in turn provides the power required to start and continue the erosion process.
Water and wind erosion are responsible for about 84% of global degraded land.
Water
Rain or other water sources expose water-sensitive areas of land which can begin to break apart the soil; even the force of rain droplets falling very hard can erode firm sedimentary surfaces. As water builds up and then runs off it picks up soil, exposing roots and rock underneath which can lead to destabilization.
Wind
Wind is a major erosive force, especially in arid regions where there is sparse vegetation. Wind causes erosion by picking up and carrying away loose particles and by wearing down surfaces as they are struck by airborne particles carried by the wind. Wind erosion is much more severe in arid areas and during times of drought. Dry soil is more readily eroded.
Glacial activity
Snow and ice will melt causing water to run down hills and mountains, which creates streams and ditches.
Climate
The amount and intensity of precipitation is the main climatic factor governing soil erosion by water. The relationship is particularly strong if heavy rainfall occurs at times when, or in locations where, the soil's surface is not well protected by vegetation. Areas where there are high intensity storms and frequent winds are more prone to erosion.
Vegetative cover
Vegetation acts as an interface between the atmosphere and the soil. It increases the permeability of the soil to rainwater, thus decreasing runoff. It shelters the soil from winds, which results in decreased wind erosion, as well as advantageous changes in microclimate. The roots of the plants bind the soil together, and interweave with other roots, forming a more solid mass that is less susceptible to both water and wind erosion.
Topography
The topography of the land determines the velocity at which surface runoff will flow, which in turn determines the erosivity of the runoff. Longer, steeper slopes without adequate vegetative cover will have very high rates of erosion during heavy rains. Steeper terrain is also more prone to mudslides, landslides, and other gravity erosion.
The Rill
See photo in this cache's web page gallery. In the field of hillslope geomorphology, a rill is a shallow and narrow channel no more than several inches deep cut into the soil by the erosive action of water runoff through unprotected or vulnerable soil. Larger rills are called gullies. Rills are a type of erosion that is abundant in the area of the GZ; as you drive around you are hard pressed to find a hillside without erosive rills.
Formation and evolution of rills
Rills are created when water erodes the topsoil on hillsides, when the water shears off particles of soil that is unable to resist the force working parallel to the soil’s surface. This begins the erosion process; water breaks soil particles free and carries them down the slope. Rills are significantly affected by seasonal weather patterns, and are more likely to form or are worsened in the rainier months. Sandy and loamy soils are more susceptible to rill formation, whereas dense clay soil is more resistant. Slightly acidic rainfall can also contribute to the formation of rills by dissolving limestone and other soluble rocks.
Although rills are small, they transport significant amounts of soil each year, sometimes ten times as much carrying capacity as areas without rills. Rills cannot form on every surface, and their formation is intrinsically connected to the steepness of the hillside slope. Slope controls the depth of the rills, while the length of the slope and the soil’s permeability control the number of incisions in an area.
Please answer the following questions and send them to the CO within five days of your visit per Groundspeak policy. Do not post answers in your log. Short answers are fine.
1. Reach down and feel a handful of the sand at your feet. Based on what you've learned about erosion, where might this sand have originated? (Hint: read carefully the second paragraph under the erosion heading above.)
2. What might have been contributing factors to rill formation at the GZ? Name three factors.
3. How do rills affect the amount of rainwater flowing down the hillsides within your view at the GZ?
4. Why do you think there are so many rills in this area?
References: https://en.wikipedia.org/wiki/Rill, https://en.wikipedia.org/wiki/Erosion, shutterstock.com