Stages Of Soil Formation

Erosion is the movement of rock fragments and soil.

Erosion is the movement of rock fragments and soil.

Picture Organic Sedentary Rocks
Figure 17.3 Rocks. Granite: pink (left) silver (top) sandstone (right) slate (bottom)

Igneous rocks are those formed from the molten material of the Earth's crust. All other rock types, as well as soil, are ultimately derived from them. When examined closely, most igneous rocks can be seen to be a mixture of crystals. Granite is one of the commonest and contains crystals of quartz, white and shiny, felspars that are grey or pink, and micas, which are shiny black (see Figure 17.3). Many of these crystalline materials have a limited use in landscaping as formal structures rather than in the construction of rock gardens; more commonly they are used in monuments and building facades.

As granite is weathered ('rotted') the felspars are converted to kaolinite (one of the many forms of clay) and soluble potassium, a plant nutrient. Similarly, the mica present is chemically changed to form clay and yield soluble minerals. Whilst the many types of clay retain much of the potassium, sodium, calcium, etc., the soluble material is carried by water to the sea making the sea 'salty'. The inert quartz grains are released and form sand grains.

Sedimentary rock is derived from accumulated fragments of rock. Most have been formed in the sea or lakes to which agents of erosion carry weathered rock. Organisms in the seas with shells die and accumulate on the bottom of the sea. Layers of sediment build up and, under pressure and slow chemical change,

Rock Organisms
Figure 17.4 Limestone

eventually become rock strata such as shale, chalk or limestone. In subsequent earth movements much of it has been raised up above sea level and weathered again. Similarly, the sand grains that accumulate to great depths in desert areas eventually become sandstones (Figure 17.3).

Moving water and winds are able to carry rock particles and are thus important agents of erosion. As their velocity increases the 'load' they are able to carry increases substantially. The fast-moving water in streams is able to carry large particles, but in the slower-moving rivers some of the load is dropped. The particles settle out in order of size (see settling velocities). This leads to the sorting of rock fragments, i.e. material is moved and deposited according to particle size. By the time the rivers have reached the sea or lakes only the finest sands, silts and clays are in the water. As the river slows on meeting the sea or lake all but clay is dropped. The clay eventually settles slowly in the quieter waters of the sea or lake. Moving ice is also an agent of erosion, but the load dropped on melting consists of unsorted particles known as boulder clay or till.

The type of sedimentary rock formed depends on the nature of its ingredients. Sandstones, siltstones and mudstones are examples of sedimentary rocks derived from sorted particles in which characteristic layers are readily seen. Limestones are formed from the accumulation of shells (see Figure 17.4) or the precipitation of materials from solution mixed with varying amounts of deposited mud. Chalk is a particularly pure form derived from the calcium carbonate remains of minute organisms that lived in seas in former times. Many of these are attractive materials for use in hard landscaping, where care should be taken to align the strata (layers) for a natural effect.

Metamorphic rock is formed from igneous or sedimentary rocks. The extreme pressures and temperatures associated with movements and fracturing in the Earth's crust or the effect of huge depths of rock on underlying strata over very long periods of time has altered them. Slate is formed from shale, quartzite from sandstone, and marble from limestone. Metamorphic rock tends to be more resistant to weathering than the original rock.

Natural soil profiles

Sedentary soils

Sedentary soils develop in the material gradually weathered from the underlying rock. True sedentary soils are uncommon because most loose rock is eroded, but the same process can be seen where great depths of transported material have formed the parent material, as in the boulder clays left behind after the Ice Ages. A hole dug in such a soil shows the gradual transition from unweathered rock to organicmatter rich topsoil (Figure 17.5). Under cultivation a distinctive topsoil develops in the plough zone.

Sedentary Soil

Stages in the formation of sedentary soils Transported soil

Figure 17.5 The developmentfromayoung soil consisting ofafewfragments of rock particles to a deep sedentary soil is shown alongside a transported soil. A subsoil, topsoil and leaf litter layer can be identified in each soil. Simple plants such as lichens and mosses establish on rocks or fragments to be succeeded by higher plants as soil depth and organic matter levels increase.

ii III

Stages in the formation of sedentary soils Transported soil

Figure 17.5 The developmentfromayoung soil consisting ofafewfragments of rock particles to a deep sedentary soil is shown alongside a transported soil. A subsoil, topsoil and leaf litter layer can be identified in each soil. Simple plants such as lichens and mosses establish on rocks or fragments to be succeeded by higher plants as soil depth and organic matter levels increase.

Transported soils

Once rock fragments and soil particles are created they become subject to erosion. Transported soils are those that form in eroded material that has been carried from sites of weathering, sometimes many hundreds of miles away from where deposition has occurred. They can be recognized by the definite boundary between the eroded material and the underlying rock and its associated rock fragments. Where more than one soil material has been transported to the site, as in many river valleys, several distinct layers can be seen. The right-hand part of Figure 17.5 shows an example. How they are moved depends on where the loose material lies:

  • Gravity affects anything on a slope. On steep sides, e.g. cliffs, particles fall and accumulate at the bottom to form heaps of rock called 'scree'.
  • On gentler slopes particles are helped downhill by rainsplash. Raindrops striking soil dislodge loose particles that tend to move downhill. As a result, surface soil is slowly removed from higher ground and accumulates at the bottom of slopes. This means that soils on slopes tend to be shallow, whereas at the bottom deep, transported soils develop, known as colluvial soils.
  • Glaciers carry vast quantities of rock downhill and deposit their load at the 'snout' (terminal moraines). Of more significance is the enormous load that was left behind when the glaciers retreated after the last Ice Age (10 000 years ago). This is known as 'till' or 'boulder clay' (it comprises boulders down to clay size particles).
  • Material washed away in running water eventually settles out according to particle size. The river valley bottoms become covered with material (alluvium) in which alluvial soils develop.
  • Wind removes dry sands and silts that are not 'bound in' to the soil. The soils that develop from wind-blown deposits are known as 'loess' or ' brick-earth ' .

Many of these transported soils provide ideal rooting conditions for horticultural crops because they tend to be deep, loose and open. Most are easily cultivated. However, those that have a high silt or fine sand content, notably the brick-earths, may be prone to compaction.

Homeowners Guide To Landscaping

Homeowners Guide To Landscaping

How would you like to save a ton of money and increase the value of your home by as much as thirty percent! If your homes landscape is designed properly it will be a source of enjoyment for your entire family, it will enhance your community and add to the resale value of your property. Landscape design involves much more than placing trees, shrubs and other plants on the property. It is an art which deals with conscious arrangement or organization of outdoor space for human satisfaction and enjoyment.

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Responses

  • CODI MUNRO
    What are the stages of soil formation from rocks?
    7 years ago
  • niklas v
    What are the kinds of rocks and the examples?
    7 years ago
  • pinja
    What is transported soil?
    7 years ago
  • Matta
    WHAT ARE THE STAGE IN THE FORMATION OF THE SOIL?
    7 years ago
  • AMBER
    What type of erosion boundary exists between a rock and underlying rock formations?
    6 years ago
  • ESTER PIAZZA
    What is the process by which fragments of soil and rock are left behind as moving water slows down?
    5 years ago
  • sophia
    What are development stages in soil formation?
    5 years ago
  • Samuel
    How are soils formed from rocks?
    3 years ago
  • Ilenia
    What are the three stages involve in formation of transported soil?
    2 years ago
  • Fred
    What are the stages involved in soil formation?
    4 months ago
  • brhane teodros
    What are the stages of soil formations?
    3 months ago

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