Simulated rock layers created by Stephen J. Reynolds showing the sequence of events in the formation of a canyon.
Begin the day by reviewing how sedimentary rock forms – deposited sediments are compacted and cemented together.
Describe how today we will model how layers of sediments are deposited and will use logic to derive rules or laws that can be used to examine layers of rock in the real world. You may want to review the principle of uniformitarianism – that the processes that shape the world today also operated in the past – if you covered that previously. By making observations from our models, we can make some assumptions about how sediment is deposited and how rock layers formed in the real world many thousands and millions of years ago.
Distribute 1 clear plastic cup to each student. Tell them to fill their cups a little less than 1/2 full with water. Students should also get out a pencil and a piece of paper or their lab notebooks to take notes on.
Place sets of sediments on the desks or tables where groups of 4-6 students can access it.
Tell students that they will be gradually creating several layers of sediment in their cups. We have added water to the cups because most sediment is transported and deposited by water. There are 2 important keys to this activity. First, it takes time for each layer to form, so listen to directions and don’t rush. Second, sediment should be slowly sprinkled into the cup in a nice even layer, so don’t dump spoonfuls in all at once or the layers on the bottom will be ruined by the sediment coming in from the top.
Ask students to pick one of the sediments. Carefully, gently, and slowly sprinkle 2 spoonfuls of sediment over the surface of the water. Spend a moment observing the sediment as it slowly settles on the bottom of the cup. Have students take a peek at their neighbor’s cups and observe any similarities and differences in the way the different types of sediment settle.
Optional: If you are using the squeeze box, add a layer of your own, dry, to the squeeze box.
Have students share some of their observations. Generally, one of the first observations will be that gravel settles fastest while the powdered clay stays suspended, perhaps even floating in a layer on the surface. Allow this observation to lead into a review of what was learned about how sediments are deposited according to the size of the grains – small particles are picked up easily by running water and deposited only when water becomes still while large particles are difficult to transport and are deposited first. You may also want to remind students of the guesses they made when observing sedimentary rocks – that mudstone must have come from somewhere with a lot of mud like a delta, bay, while conglomerate must have come from somewhere with a lot of gravel, like a fast-flowing river.
Summarize these ideas on the board and have students copy it into their notes or lab notebooks: “Depositional environment – The size of the grains in a rock tells you about the environment those grains were deposited in. Small grains, like clay and silt, are picked up easily by running water and deposited only when water becomes still, like in a lake, bay, or delta. Large grains are difficult to transport and are left behind in rivers when the silt and clay gets washed away.”
Look at the cups again. Even the silt should have mostly settled into a flat layer by now. Discuss the process that turns sediments into rock. Have students imagine real world sediment layers, perhaps on the bottom of a lake, and what would have to happen to turn those sediments into rock. What kind of rock would it be? What would it look like?
Students should pick a new sediment and add a second layer of 2 spoonfuls on top of the first. Remind students to add the sediment slowly by sprinkling. Spend a moment observing the sediment settle and comparing each others’ cups.
Optional: If you are using the squeeze box, add a second layer.
Ask the students to describe the surface of each layer. Answers should resemble: “It’s flat.” “It’s even.” “Even though the gravel layer is more bumpy than the other surfaces, all the layers are flat.” Probe a little deeper and ask why the layers are flat. Answers might include: “Because of gravity.” “Because the sediment drifts down through the water and collects on the bottom.”
Summarize these ideas and formalize it as the law of original horizontality. Write it down on the board. You can choose to tell students about Nicolas Steno or not depending on your goals. Students should copy the summary statement into their notes or lab notebooks: “Law of Original Horizontality – When layers are deposited, they settle in flat, horizontal layers.”
Quickly observe the cups again. Students should then add a third layer to their cup. Spend a moment making observations and comparisons.
Optional: IF you are using a squeeze box, add a third layer.
Ask students about what happens at the sides of the cup. Are there walls or sides on a lake or in the ocean or in a delta? How far would the layer continue if there were no walls? Discuss the spread of sediments in the real world. If it seems appropriate, suggest a mini-experiment that students can try in their cups, such as adding just a pinch of sediment or sprinkling sediment just in one place as the sediment flowing from a river might be dumped into a lake just in one place.
Summarize these ideas as the law of lateral continuity and write it down on the board. Students should copy it into their notes or lab notebooks: “Law of Lateral Continuity - when a sediment layer is laid down, it will extend in all directions until it runs our of material or hits a wall.”
Quickly observe the cups again. Students should then add another layer to their cup. Spend a moment making observations and comparisons.
Optional: IF you are using a squeeze box, add another layer.
Ask the students about the order of the layers. Where is the first, the oldest, layer? Where is the most recent layer? Discuss the ordering of layers.
Summarize these ideas as the law of superposition and write it down on the board. Students should copy it into their notes or lab notebooks: “Law of Superposition - in an undisturbed series of rock layers, the youngest layers are on the top and the oldest layers are on the bottom.”
Discuss what “undisturbed” means and what you could do to the cups to mess up that order. Answers might include: “Stir it up.” “Turn it upside down.” “Tilt it sideways.” “Crush the cup.”
Ask the students what might happen in the real world to disturb rock layers that form. Answers might include: “Erosion.” “Bulldozers.” “Earthquakes.” Discuss how each of these disruptions might affect the rock layers.
Optional: If you have prepared a squeeze box, you can now use it to demonstrate how tectonic forces might disturb rock layers. Show the students the layers in your box. Then, squeeze the box to observe how the layers fold and fault, changing the sequencing of the layers.
Suggest a final experiment to the students. Place a pencil below the edge of their cups, being careful not to let the contents spill. Discuss what forces in the real world might tilt a series of rock layers in this way. Now add a final layer to the tilted cup. Observe how the layer is laid down flat relative to the table and the surface of the water, but how the new layer lies at an angle to the layers below. Review each of the 3 laws with the students and see how each rule was affected by the tilted cup.
Finally, instruct the students to draw a side-view picture of the cup with their sediment layers. Ask them to label each layer with a description of the layer and the depositional environment that would have laid down that kind of sediment. They could also write a paragraph below the picture describing the sequence of events that created their cup.
A few minutes before the end of class, clean up. The sediments can be collected in large waste containers and the cups themselves can be thrown away.