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Soil Genesis and Development, Lesson 3 - Soil Forming Factors

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3.6 - How Parent Material Affects Soil Profile Development

Parent material, from which soil develops, comes from many different sources. This is due to the fact that parent material is not static. Soils form in parent material that is not just bedrock weathered in place. Parent material is classified based on its mode of transportation: ice, water, gravity, wind, lakes and oceans, or in place.

Parent Material Deposited by Ice

Parent material transported by ice is known as glacial till. This parent material is found where glaciers have deposited material, such as in terminal moraines or lateral moraines.  Ice itself is a poor sorter of soil particles. The force carrying particles is able to carry particles of all sizes the same distance. Thus this parent material contains everything from the smallest clay-sized fraction to rocks, pebbles, and boulders. 

Parent Material Deposited by Water

Water is a good sorter of soil particles. Larger particles (i.e., sand) settle out of water first because they are heavier and require more energy to carry them. Smaller particles (i.e., clay) settle out of water last because they are lighter and can be carried greater distances. Larger, sand-sized fractions are found closer to the water source and clay-sized fractions further away. Parent material transported by flowing water (streams and rivers) is called alluvium. In addition, parent material formed from water flowing from glaciers is known as outwash or glacial outwash.

 

Figure 6. The soil across this landscape was formed from glacial till and glacial outwash, but more recently has been added to by alluvium.
Image courtesy of Jim Ippolito; Northern boundary of Rocky Mountain National Park, Colorado.


Parent Material Deposited by Gravity

Material transported due to gravity is known as colluvium. Gravity is a poor sorter of particles, and thus soils which develop at the base of mountains, for example, contain particles from clays through rocks, pebbles, and boulders.

 

Figure 7. Notice the colluvium at the mountain’s base (lower left in the photo).  Image courtesy of Jim Ippolito; Northern boundary of Rocky Mountain National Park, Colorado.


Parent Material Deposited by Wind

Parent material transported by wind has many names. The most common names are loess or aeolian. Parent material transported from volcanic eruptions is called tephra; it can be carried great distances by wind after being spewed from a volcano. Wind is a good sorter of soil particles. Heavy particles, such as sands, tend to bounce across the landscape surface. Clays tend to clump together and act as larger particles. The silt-sized fraction tends to become suspended in wind and transported great distances. Great Sand Dunes National Park in Colorado, U.S.A. is a classic example of aeolian sands.

 

Figure 8. The valley to the West of this site is the source of soil, with sands settling at the base of these 14,000-foot peaks.
Image courtesy of National Park Service, http://www.nps.gov/grsa/index.htm


Parent Material Deposited by Lakes and Oceans

The terms lacustrine and marine classify parent material transported by lakes or the ocean. Again, this is water acting on parent material, and thus there is a high degree of particle sorting. Lacustrine and marine deposits sometimes have visible horizontal layers present, which indicate settling material deposition.

Figure 9. Lacustrine deposit. Image courtesy of NRCS

Parent Material Formed in Place

Finally, parent material weathered in place is called either residual or residuum. The degree of sorting is low because no other factors have influenced its movement.


Summary


Types of Parent Material
 Mode of Transportation Type of Parent Material  Degree of Sorting by Particle Size 
 Ice  Glacial Till  Low
 Flowing Water  Alluvium; Outwash  Medium - High
 Gravity  Colluvium  Low
 Wind  Tephra; Loess; Aeolian  High
 Lake  Lacustrine  High
 Ocean  Marine  High
 In Place (non-transported)  Residual (residuum)  Low

 

Click the image below for a slide show describing parent material weathering.  Note: This presentation may take several seconds to load.

 Parent Material Animation icon
 Click the picture to view the animation.
Animation courtesy of Jim Ippolito, CSU. 2005.



 
Question 12:  Rocks of various sizes which rolled off and collected at the base of a mountain served as the parent material for soil.  Which best describes this type of parent material:

Alluvium
Loess
Colluvium
Marine

 

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