Rocks Tell a Story

Peggy Ashbrook

Sedimentary rocks, formed by an accumulation of sediments (tiny pieces of rocks or minerals) in a water environment, tell a story that many students may be familiar with. They may have visited areas where water or wind carried sediments and deposited them in rivers, lakes, oceans, or dunes. The rocks are often visually or texturally interesting and may have the added attraction of containing fossils.

We can understand the stories rocks tell more easily if we have experience with the materials that make up a rock. Here are a few suggestions on how to experience these rock materials:

• With permission if needed, dig clay or sand from the ground to bring back to the classroom. Examine it, and wash a cupful in water on a tray to see what else is in the sample, perhaps “dirt,” organic matter from plants, small pebbles, and shells.
• Take a field trip to a beach on an ocean, lake, or river to see sediments accumulate.
• Use various colored clay, softened and thinned, to paint on fabric squares. Soften the clay by letting it sit in water for a day or two (pour off the unabsorbed water).
• Add a small amount of sand or clay to paint for painting pictures.
• Pour water into sand to see how it can move sand (when finished, do not dispose of the sand in the sink drain.)
• Mix sand and clay with water in jars to shake and watch the sediment settle. Make one jar with ¼ cup sand, one with ¼ cup clay, and one with 2 tablespoons each, and seal tightly with hot glue inside the lids and tape outside. Ask questions before shaking: What do you think will happen if we mix the sand or clay with water? How long will it take the sand to mix into the water, how long will it take the clay? What will happen when you stop shaking? What did you find out?

When the clay and sand that the children have been working with dry out, the children will notice that they no longer stick together. The sand is once again individual grains and the clay, although it’s hard to see particles, it feels “dusty” and easily broken.

Students can record their ideas about why clay and sand feel different, and how they hold together when wet and when dry as part of these explorations of earth materials, part of National Science Education Content Standard C, Earth and Space Science.

In the following activity it is very important to always use the words “pretend rock” so children do not get the idea that rocks are human made. Rocks are made through natural processes. Many descriptions of rock formation for young children say that the rock formed when sediment was buried under tons of more sediment and dirt until it turned into solid rock, omitting the role of cementing materials. Sediments are cemented together when water carrying dissolved minerals seeps into the spaces between the particles and the minerals precipitate out from the water in the spaces, cementing the grains together. In the activity, plaster of Paris will be added to the sediments in the cup to act as the cement.

Variation seen in sedimentary rocks comes from many differences including color of parent material, source of parent material, particle shape, particle size, and the environment in which the sediment was deposited. You can offer various sediments in this activity to produce a variety of pretend rocks.

Pretend Rocks

Objective:
To notice the range in grain size in sedimentary rocks and think about how such rocks are formed.

• Samples of various sedimentary rocks, including sandstone and shale, made of different-sized particles
  *Rock can be purchased through local stone dealers or scientific supply companies or collected locally.
• Sand
• Clay
• Spoons to serve the sand and clay
• Paper towels
• One 5–8 oz. paper cup and craft stick for each child
• Plaster of Paris
• Disposable containers to mix plaster
• Water
• Pebbles, dirt, and small shells (optional)

1. As the collected rocks are brought in, a discussion about where rocks come from develops. Ask if anyone has ever seen a rock being made to learn the students’ ideas. Encourage the children to look closely at the rocks and compare them to each other in color, size, shape, texture, and weight. Have them use a magnifier to see the grain size. What size are the pieces that make up these rocks? Can you see them or feel them? Are any of these rocks the same? Note that a rock can be many sizes and have different names such as sand, pebble, stone, and boulder, and still be rock.
2. Compare the actual rocks to those pictured in a rock identification book. The “match” that young children make is usually based on color and shape rather than other distinguishing properties or origin. At this age they are beginning to understand the use of an identification book, not the complexities of rock composition, so no corrections are needed.
3. Tell the students that now they are going to feel the raw materials that make sandstones and shales. Have the students feel soft, damp clay and damp sand, keeping the materials separate. Accentuate the difference in textures by using clay that does not contain grit so its texture is very smooth. Where have you seen clay or sand? How are these two materials alike or different? What size are the pieces that make up the clay and sand?
4. Ask for ideas on how to make a “pretend rock.” Then tell the children that you have a recipe to try.
   Give each child a small paper cup to fill about half full with damp sand or very wet clay. You might also provide pebbles, shells, or dirt to be added. Make one pretend rock of just clay and one of just sand so the children can later compare these types of pretend rocks. Have the children stir their chosen material(s) using a craft stick (it doesn’t accidentally flip sand the way a spoon does).
5. Using a finger, test to see if the mixture “is a rock yet.” Tell the children that a “cementing” material must be added, then add a heaping teaspoon of mixed plaster of Paris (follow the package instructions and do not wash the remainder down the sink). Have the children stir the mixture thoroughly and describe the mixture.
6. Review the process for sedimentary rocks formed by an accumulation of sediment: Rock-forming is happening all the time, not in schools or factories but in nature. It takes a long time for the sand grains and clay minerals to pile up in the same place, become buried as more sediment is deposited on top, and for water to carry dissolved minerals into the sand and clay minerals to become the glue that holds the pieces together. By tomorrow—a much shorter time—our pretend rock materials will be cemented and become hard.
7. After 24 hours, have the students peel off the paper cup to reveal the pretend sedimentary rock. Doing this as a group will allow the children to compare rocks and talk about how their rocks are made of different size particles.

As a followup to the activity, make a snack “rock” with a variety of cereals particle sizes, including the puffed rice cereal in the original Rice Krispies Treats recipe (www.kelloggs.com/brand/rk/index.shtml). The melted marshmallows are the cement!

Peggy Ashbrook (scienceissimple@yahoo.com) is the author of Science Is Simple: Over 250 Activities for Preschoolers and teaches preschool science in Alexandria, Virginia.

Teacher’s Picks

Marie Faust Evitt, a teacher at the Mountain View Parent Nursery School in Mountain View, California, is always on the lookout for stories that incorporate accurate science. She finds children absorb information more easily through an engaging tale. These are some of her favorites featuring geology and rocks.

A Gift From the Sea. Kate Banks. 2001. Frances Foster Books, Farrar, Straus, and Giroux.
The sumptuous illustrations and lyrical text describe the journey a rock takes from a volcano through the ice age and early civilization to the bottom of the ocean and finally to the beach where a boy finds it. Though the text is simple, you can use it with older children to help them speculate about the history of rocks they find.

Rocks in His Head. Carol Otis Hurst. 2001. Greenwillow Books, Harper Collins.
Understated humor punctuates this true story about the author’s father whose passion for rock collecting as a boy eventually leads him to become curator of mineralogy at a science museum. Learning about this natural-born scientist researching, labeling, and displaying his beloved rocks will inspire students to follow their dreams.

Grand Canyon: A Trail Through Time. Linda Viera. 1997. Walker.
This rich description of one the seven natural wonders reads as a story. Facts about the canyon’s record of geologic time are interwoven with information about the animals, plants, and people who live in it today.

Internet

Geology of the National Parks: Virtual Tours
http://geology.wr.usgs.gov/docs/parks/project/index.html
From rock formations at Bryce Canyon National Park to the stones used to build our nation’s capitol in Washington D.C., this site presents the outstanding geology of 29 different parks.

Images of Clay: A joint initiative of the Clay Minerals Society and The Clay Minerals Group
www.minersoc.org/pages/gallery/claypix/index.html
A look at highly magnified photos of different clay minerals will help children understand that clay, like sand, is made of particles.

Copyright © 2006 NSTA