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Saturday Science: Edible Rocks

Saturday Science: Sedimentary Sandwiches

If you’ve ever gone rock collecting, or even just hiking in the woods, you’ve probably noticed that rocks come in a seemingly infinite number of types: different patterns, textures, colors, materials, consistencies, and on and on. All of this variation can be broken down into three main types of rocks: sedimentary, metamorphic, and igneous. They’re differentiated by how they are formed. Today’s activity will give you an idea of how sedimentary rocks are formed. And, even better, you’ll get to eat it when you’re finished!


  • Graham crackers
  • Peanut butter
  • Chocolate chips
  • Mini marshmallows
  • A butter knife
  • A plate


  1. Break one of your graham crackers in half as if you were making a S’more.
  2. Spread peanut butter on one side of one-half of your cracker and put it down on the plate.
  3. Sprinkle some chocolate chips and mini marshmallows onto the peanut butter. You now have four different layers in your sedimentary sandwich.
  4. Put the other half of your graham cracker on top and make a sandwich. I know you’re hungry, but we’re not quite done yet. Have patience!
  5. Spread some more peanut butter on top of your sedimentary sandwich. Sprinkle it with chocolate chips and mini marshmallows like you did before.
  6. Break another graham cracker in half.
  7. Add another graham cracker layer, then repeat the peanut butter/chocolate chip/mini marshmallows process one last time.
  8. Put your last cracker half on top for a sedimentary sandwich that has 13 layers. Bon appétit!


Sedimentary rocks are, as their name suggests, formed out of materials called sediments. Sediments are small particles of matter, like silt, sand, or small chunks of seashells that can be carried by water, the wind or even glaciers, from one place to another. Sediments are made by erosion. For example, as a river flows over stones, it breaks them down bit by bit into tiny particles of sediment and carries them along with the water. That sediment may settle down somewhere else later on and, over a long period of time, become part of a sedimentary rock.

The sediments in your sedimentary sandwich came in four types: graham crackers, peanut butter, mini marshmallows, and chocolate chips. You put them together in layers because sedimentary rocks form in layers. First, a layer of sand might get laid down, and then some mud or clay on top of it, and then some tiny pebbles, and then some more sand, and over time all of these sediments harden together like cement to make a rock that is made of many layers. Since sedimentary rocks are made of many tiny particles pressed together over time, instead of by massive heat pressure inside the earth’s crust (like metamorphic rocks) or out of magma or lava (like igneous rocks), they are often soft or easy to break. Some of them can even be damaged with a scratch from your fingernail!

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