Theory:

Soil on earth is not formed in a day. It takes thousands of years for its formation. Millions of years ago, the earth was one big solid rock. Due to the changes in climate and many other factors, these rocks started to erode slowly. All the soil on earth, we see around, is formed due to such type of processes. At first, a rock gets eroded into small rocks, further eroded into much smaller stones and finally soil was formed. So if we can see the vertical cross-section of the earth surface, we will see the distinct layers of soil.
 
Activity: Layers of soil
  • Take a glass tumbler, filled with water in three-quarters of the tumbler.
  • Dig the garden soil with the help of trowel (a small garden tool with a short handle and a curved part for lifting plants, digging small holes, etc.) to some depth.
  • Take a little amount of soil and put it in the glass tumbler. Stir it well with a stick and mix the soil and water well together.
  • Let it stand undisturbed for a while, and we observe that the particles of different sizes in the soil separate to form distinct layers in the glass tumbler.
  • The most dense layer of gravel settles down in the bottom.
  • Above this layer lies, the layer of sand particles followed by silt and clay.
  • The rotting dead leaves float on the water surface because they are lighter than water and are called Humus.
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Soil profile
A vertical section of the earth's surface through which distinct soil layers are classified is called a soil profile.
Soil is formed when rocks break down. This process is also called Weathering. The weathering of rocks occurs because of rains, flowing water, winds, temperature, and climatic conditions.
The decaying organic matter present in the soil is called as Humus.
 
Soil is generated by breaking down of rocks by the sun's heat, wind, rainwater, and freezing of water crevices  (a narrow crack in a rock, wall, etc.) by the weathering process. During this process, the rocks are damaged to form tiny particles due to the prolonged exposure to weather elements.
Weathering is a prolonged process as it takes thousands of years for a large rock to turn into minute particles such as soil.
Nature of the soil texture and availability of minerals entirely depends upon the rocks from which it is formed. This rock is usually called as the Parent Rock.

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Weathering process
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Soil formation occurs due to the following processes occur years together:
  • Heating of rocks by sun, and cooling by rain.
  • In winter, water got frozen in rocks.
  • Ice in rocks led to cracks.
  • Growth of roots of trees led to the cracking of rocks.
  • Day and night temperature variations
  • Action of wind and water.
Important!
In colder countries, water is used to split huge rocks. During night time, water is poured in between the cracks of rocks, and as water freezes, it broke the rocks along the cracks. Cracking of rocks happened because water volume is higher in the form of ice than in liquid, which broke the rocks.
Reference:
http://www.handsonlearning4all.com/wp/?p=2359