3. Layers Upon Layers

Summary
The study of rock layers, or stratigraphy, is a natural way to introduce students to the fundamental principles of geology and to lead into the idea of geologic time. In this lesson, students are introduced to Nicolas Steno’s 3 major laws of stratigraphy: the law of original horizontality, the law of superposition and the law of lateral continuity. Students also add their observations of sediment sorting from previous lessons (Soil Analysis, Erosion Patterns, and the Sediment Study Project) to generate a fourth “law” concerning depositional environment – the tiny grains in mudstone were most likely deposited in very still water like a lake or delta while large gravel in conglomerate was most likely deposited in fast moving rivers and streams. While this activity has students depositing sediments in clear plastic cups or Mason jars, it is recommended that the teacher simultaneously conduct the activity using a squeeze box like the one described by Eric Muller. In this way, when the student activity concludes, the teacher can take the activity further to show how layers can become folded and faulted by plate movements. This lesson is a natural extension of the Going Further activity from the Crayon Rock Cycle lesson where sediments are mixed with sodium silicate to create home-made sedimentary rocks.

Objectives
Can describe the environments in which different sedimentary rocks are formed
Can identify and explain Steno’s 3 laws of stratigraphy
Can apply the laws of stratigraphy to describe the relative age of sediment layers

Vocabulary
Stratigraphy
Law of Original Horizontality
Law of Superposition
Law of Lateral Continuity
Depositional Environment
Optional: Principle of Uniformitarianism

3. Layers Upon Layers - Logistics

Time
45-50 minutes

Grouping
Individual, although clusters of 4-6 students need to be able to share materials

Materials

Clear, plastic, 16 oz or 20 oz party cups, at least 1 per student plus 20-30 additional cups for students to share.
At least 6 different, dry “sediments” of different grain sizes and colors, around one shoebox-full of each: playground sand, beach sand, groundcover rocks, diatomaceous earth, aquarium gravel, powdered clay, garden soil, school-yard soil, etc. Have students bring in sediments they find interesting. The activity transforms readily into an art project if you can find “pretty” sediments: colored aquarium gravels, colored sand, etc.
Spoons
5 gallon bucket or other large container for collecting waste sand
Water
Optional: Squeeze box (see Sources)
Optional: Cafeteria style trays
Optional: Photographs showing rock layers such as the Grand Canyon or other cliff faces

Squeeze Box with horizontal sediment layers

Squeeze box after squeezing showing folding and faulting of the layers

Setting

3. Layers Upon Layers - Background

Teacher Background
In this activity, students gently layer sediments on top of one another as they learn about the basic principles of stratigraphy as laid out by Nicolas Steno. The key to the activity is to allow time for each layer to settle completely before adding additional layers on top. Thus, I alternate between adding a layer and writing down each of Steno’s laws. Another key is to slowly sprinkle sediments evenly across the surface of the water. If you dump them too quickly, the lower layers will be disturbed, particularly when adding a gravel layer on top of a clay or silt layer.

Nicolas Steno (1638-1686) was one of the founders of the principles of geology. He got his start in medicine and anatomy. His foray into geology came about when some fishermen caught a large shark and Steno had the opportunity to dissect it. Upon examining the shark’s teeth, Steno noticed their resemblance to stony objects found embedded in certain rocks called glossopetrae. While the prevailing thought of the time was that the glossopetrae fell from the sky or grew in the rocks, Steno argued that the glossopetrae were the teeth of ancient sharks. These ancient teeth were buried in sediments that, over time, became stone. This led Steno to study fossils and other geological ideas and propose the three principles that form the foundation of stratigraphy: the law of original horizontality, the law of superposition, and the law of lateral continuity.

The law of superposition states that in an undisturbed series of rock layers, the youngest layers are on the top and the oldest layers are on the bottom. For example, my husband and I stack the mail on a table in the kitchen. Often, the pile builds up for a week or more before we sort through it. The mail from Monday is at the bottom of the pile while the mail from Friday ends up at the top, with the mail from the rest of the week in a chronological sequence in between. Similarly, as sedimentary layers build up on top of one another, as long as the stack remains undisturbed, the layers will form a chronological sequence with the oldest on the bottom. Viewed from the side in cross-section, you can read the layers as you would a timeline. Similarly, as students build more and more layers of sediment on the first, they will observe that the first layer they made is on the bottom, with subsequent layers building on top of the first.

Naturally, this sequencing of rock layers can be changed by geological forces, just as the sequencing of mail is changed when I sort through the pile. In a river valley, the running water will erode away the uppermost young layers. On cliff faces near the ocean, the action of the waves may undercut a cliff enough to cause a chunk to plunge into the water below, bringing younger layers down to where the older layers should be. As plate tectonics acts on the Earth’s surface, older layers may be tilted sideways or pushed upward and exposed. Students can experiment with tilting their cup on a pencil partway through the activity, tilting the lower layers and causing future layers to be laid down flat, but at an angle with the bottommost layers. If you perform the activity with the squeeze box, you can demonstrate and discuss the effects of tectonic forces that folds and faults the layers and disturbs the series.

The last of Steno’s laws, the law of lateral continuity, states that when a sediment layer is laid down, it will extend in all directions until it runs our of material or hits a wall. Thus, the sediment layers are virtually continuous sheets that extend until there is no more sediment (like if you put sprinkle a very small spoonful of sediment into the cup and it can’t cover the entire surface) or until it hits a barrier (like the edge of each layer where it hits the side of the cup).

For a discussion of depositional environment, see the Background section in the Erosion Patterns lesson. For a discussion of the principle of uniformitarianism, see the Background section in the Crayon Rock Cycle lesson.

Student Prerequisites
Students need to understand how sedimentary rocks form (see Crayon Rock Cycle lesson) and know how water velocity affects the deposition of sediments (see Erosion Patterns).

3. Layers Upon Layers - Getting Ready

Getting Ready

Create a set of sediments for teams of 4-6 students to share. Each set should contain a 16-20 oz cup full of each sediment sample with a spoon in each cup. For easier clean up and transportation, place each set of cups on a cafeteria tray.
Set out additional cups
Build a squeeze box. With a 2 by 4, some sheets of Plexiglas, a section of PVC pipe, and a few hours, you can build a great squeeze box to model how geologic forces deform sedimentary layers. See Sources.

3. Layers Upon Layers - Lesson Plan

Simulated rock layers created by Stephen J. Reynolds showing the sequence of events in the formation of a canyon.

Lesson Plan

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.

3. Layers Upon Layers - Assessments

Assessment

Collect the labeled drawings and paragraphs.
Provide your own drawing of a cup with layers and ask the students to describe the series of events that led to the formation of the cup, just as they did with their own cups.

Going Further

Spend a day looking at photographs of rock layers in landscapes such as the Grand Canyon, Bryce Canyon, Petra, or other places with amazing rock layers. Teach about the formation of these places or have students independently research them.
Go on a field trip to look at rock layers and study the rock layers up close, making hypotheses about the depositional environments that formed each layer and the sequence of events that must have taken place. See the Caldecott Tunnel field trip description for an example.

3. Layers Upon Layers - Sources and Standards

Sources
The inspiration for this activity is the squeeze box, cleverly designed by Eric Muller of the San Francisco Exploratorium. Look under Earth Science for the Squeeze Box activity.

For more information about Nicolas Steno and stratigraphy see:

The book, The Seashell on the Mountaintop, by Alan Cutler, is a wonderful, easy read that chronicles the life of Nicholas Steno. He vividly describes the life and discoveries of this anatomist turned geologist turned priest.
A great summary of Steno’s life and work can be found on the University of California Museum of Paleontology’s website.
Earth Science Australia offers supremely well organized lecture notes for their geology course based on the work of Professor Stephen Nelson of Tulane University. The information about sedimentary rocks goes into wonderful detail about how to read sedimentary layers for clues about the environment where the sediments were deposited.
Pamela Gore of the Georgia Perimeter College has also provided a fabulous overview of the basic principles of stratigraphy.
Stephen J. Reynolds has created a gorgeous collection of simulated landscape images showing the sequence of events in the formation of canyons, buttes and other landforms.

Standards

Grade 6

Shaping Earth's Surface
Topography is reshaped by the weathering of rock and soil and by the transportation and deposition of sediment. As a basis for understanding this concept:
a Students know water running downhill is the dominant process in shaping the landscape, including California's landscape.
b Students know rivers and streams are dynamic systems that erode, transport sediment, change course, and flood their banks in natural and recurring patterns.

Investigation and Experimentation
Scientific progress is made by asking meaningful questions and conducting careful investigations. As a basis for understanding this concept and addressing the content in the other three strands, students should develop their own questions and perform investigations. Students will:
e    Recognize whether evidence is consistent with a proposed explanation.
f    Read a topographic map and a geologic map for evidence provided on the maps and construct and interpret a simple scale map.
g    Interpret events by sequence and time from natural phenomena (e.g., the relative ages of rocks and intrusions).

Grade 7
Earth and Life History (Earth Sciences)
Evidence from rocks allows us to understand the evolution of life on Earth. As a basis for understanding this concept:
a Students know Earth processes today are similar to those that occurred in the past and slow geologic processes have large cumulative effects over long periods of time.
c Students know that the rock cycle includes the formation of new sediment and rocks and that rocks are often found in layers, with the oldest generally on the bottom.