Students will sort rocks in various ways from a river gravel bar (adapted from FOSS).
Grades: 1 - 3
Time: 2 - 3 sessions of 1/2 hour
Subjects: Earth science, mathematics, physical science
The most common earth material is rock. Rock that has broken away from the continuous layer of bedrock is found in particles of many sizes. Weathering (sunlight, chemical, frost action) breaks rock loose and erosion (moving water, glaciers, landslides) moves it to another place.
Currents of water can sort particles by size. These particles are classified (largest to smallest, fast water to slow) into the following categories:
- Boulders - larger than 25 cm
- Cobbles - 6 - 25cm
- Pebbles - 1/2 - 6cm
- Gravel - 1mm - 1/2cm
- Sand - 0.1 - 1mm
- Silt - 0.05 - .1mm
- Clay - less than .05mm
These categories are not exact, but reflect how the particles act in water. The stronger a current, the larger the particles it can move. Huge boulders move in violent mountain streams and clay settles in non-moving water over a period of days - places like lakes, marshes and oceans. Usually students can sort by eye down to the gravel level, but to sort sand, silt and clay, water is needed. Sand feels gritty, silt feels rough and clay feels greasy or slimy because the particles are so small. Clay holds its shape when squeezed and dries hard in the same shape. For that reason, it is made into bricks or clay pottery.
Students can simulate some of the sorting process by blowing, shaking sifting and moving the rocks. This activity is about careful observation, comparisons and communication.
- 2 or 3 buckets of river bar rocks and soil
- centimeter rulers
- magnifying lenses
- plastic containers (recycled butter tubs, etc.)
- clear plastic jars
- Have students take some of the pebble and larger-sized rocks from the mixture and observe their characteristics. Have groups of two or three students sort and arrange them in various ways (color, shape, rough-smooth, etc.).
- Using the rulers, students can then sort the larger rocks by size into gravel, pebbles, cobbles and boulders (which may be too large to handle). Use the recycled containers for the different sizes. Keep students thinking about the speed of water that a river would have to move each size rock.
- In the clear jars, place a few tablespoons of the fine material and then fill with water, making a mixture by stirring with spoons (at least a minute of stirring).
- Sand should settle to the bottom of the jar in about 5 seconds. Quickly pour the water into another jar, being careful not to pour out the sand in the bottom.
- What is still left in the water should be a mixture of silt and clay. Set it down so it doesn't move. After about 1 - 2 minutes, pour the water into a third jar very slowly and with a minimum of movement.
- The second jar should contain mostly silt and the third a slurry of clay and water. Have students time how long the clay takes to settle. (The water will look clear.)
- Carefully pour off any water left in the jars and set them aside to dry at room temperature.
- One or two classes later, have students feel the dry material with their fingers.
Does the "clay" feel different than the silt? How?
What is sand used for by people? (beaches, sandboxes, sandpaper, making glass, etc.)
What is silt good for? (best soil for farming and growing house plants)
What is clay used for? (bricks, pottery, modeling clay)
Variations and Extensions:
- Use different-sized screens ("hardware cloth" from a hardware store) to separate different sizes of gravel.
- Make a drawing of the water bodies which would move each size of earth materials.
- Find a source of pottery clay and have students make their own "rocks".
- Play rock checkers.