Squeezed: How is Sedimentary Rock Formed?

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Author: Janice VanCleave


How is sedimentary rock formed?


  • 2 standard-size bed pillows with covers removed
  • Marking pen
  • 1-inch (2.5-cm) piece of masking tape
  • Ruler
  • Helper


  1. Fluff the pillows to make them as large as possible:
  2. Lay 1 pillow on a table with its longer side facing you.
  3. Draw an arrow lengthwise on the piece of tape.
  4. Hold the tape so that the arrow is parallel to the tabletop. Stick the tape in the middle of the side of the pillow facing you.
  5. Use the ruler to measure the distance from the tabletop to the arrow on the piece of tape.
  6. Ask your helper to gently lower the second pillow on top of the first one. Again, read the measurement on the ruler.



The height of the bottom pillow decreased when the second pillow was placed on top.


Sedimentary rock is formed by deposits of sediment (small particles of material carried and deposited by wind, water, or ice). Sediment can be bits of shells, rocks, or the hardened remains of plants and animals. After thousands of years, thick layers of sediment can build up.

The pillows in this experiment each represent a layer of sediment. In order for sediments to form rock, they must be held together. The squeezing process of compaction is one way in which sediments are held together. Over time, sediments become compacted because the weight of upper layers of sediment squeeze the lower layers of sediment closer together, in the same way that the second pillow squeezed the first pillow. Sedimentary rock forms when the layers of sediment harden.

Height of Lower Sediment Layer

Let's Explore

  1. Would larger pillows affect the results? Repeat the experiment, using queen- or king-size pillows.
  2. Would the number of pillows affect the results? Repeat the original experiment, using four or more pillows.
  3. How does time affect the results? Repeat the original experiment, using at least four pillows. Measure and record the height of the bottom pillow on a chart similar to the one shown. Continue to measure the height of the pillow at set intervals, such as after 2, 6, 12, 24, and 48 hours. NOTE: Be sure to place the Pillows where they will not be disturbed.

Show Time!


    1. Another way in which sediments are held together is cementation (the process by which minerals dissolved in water cement sediments together). Demonstrate that water usually contains dissolved minerals by placing a 2-inch (5-cm) -square section of a clear, colorless plastic report folder or a small piece of clear plastic food wrap near a window. Add enough tap water to make a small pool of water in the center of the plastic. Do not disturb the plastic until the water evaporates (changes from a liquid to a gas). Then hold the plastic up to the light and examine the material left on the plastic.
    2. Does the amount of dissolved minerals in water differ from one place to the next? Determine the answer by repeating the previous experiment, using samples of tap water from different areas. These water samples can be collected during a vacation and/or from friends who live in different parts of the country. Label the plastic pieces with the area that the water sample came from, and use them as part of your project display.
  2. Water in the lower squeezed layers of sediment contains dissolved minerals picked up as the water passed through the upper sediment layers. Demonstrate this process by filling a large paper cup half full with sand, then fill the cup with soil. Use a pencil point to punch 3 to 4 small holes in the bottom of the cup. Hold the cup above a clean jar as a helper slowly pours about img cup (125 ml) of distilled water into the cup. Repeat the previous experiment, using this collected water.


Check it Out!

Sedimentary rock can be divided into clastic and nonclastic rocks. Clastic rocks are classified according to the size and shape of the rock pieces they contain. N on clastic rocks are made of two types, chemical rocks and organic rocks. Use an earth science text and/or a science encyclopedia to find out more about sedimentary rock. How does the formation of clastic and nonclastic rocks differ? How are nonclastic rocks classified? What are the different clastic and nonclastic classifications?

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