Aside from sediments, the other items your students might use for this exercise can be as simple as pencil and paper. But, if available, your students might use magnifying lenses or stereo microscopes, magnets (to test for the mineral, magnetite), and grain size cards. (The latter can be purchased through scientific supply catalogues.) The “Example Exercise” below assumes that half of each sediment sample is a dry sample in a small plastic dish or other container and the other half is in a baby food, jelly, or peanut butter jar filled with water. The bottles of sediment allow the students to observe settling rates of various grain sizes and may also give the students a better feeling for the sorting (variation in grain size) of the samples. They also can observe layers form as the grains settle. Such layers are typical of sedimentary rocks. Plus, generally students love to shake bottles of sediments and watch the grains settle! It’s fun!
The “Example Exercise” below uses four samples from Lake Michigan (LM); four from the Grand River (GR), which empties into Lake Michigan; and three from Spring Lake (SL), a small inland lake that empties in to the Grand River near the mouth of the stream. The Lake Michigan samples are taken at various water depths and distances from the shore so that students can observe that the grain size generally decreases with decreasing energy (increasing water depth and distance offshore). The Grand River samples all look different – some are fine grained and well sorted (all the grains are about the same size). Other samples are much coarser (sand and/or gravel) and may or may not be well sorted. And the samples contain variable amounts of organic material such as leaves, bark, and wood. The variety of samples from a single river helps the students appreciate the variable energy levels present at different locations in a stream (e.g., inside and outside of meanders; deep and shallow water). The Spring Lake samples are consistently very fine grained (clay with perhaps some silt) and are rich in fine, organic matter. (Fresh samples from Spring Lake may have a rotten egg smell due to the decay of organics by anaerobic bacteria.) The consistently fine sediment gives the students a sense for the low energy that is found throughout the lake. The organic-rich nature of the sediments helps the students gain an appreciation for the amount of organic material that accumulates in the lake largely because of algal blooms. For this particular exercise the students are provided with a map (e.g., Figure 1) so that they can determine the locations from which the samples were collected. Again, you can collect samples from similar environments or vary different ones – the idea is to show students that variations in sediments can be seen between different depositional environments.
Before assigning the exercise you will need to discuss the concept of depositional environment with your students. You will also need to discuss grain size (gravel, sand, silt, clay) and how important energy is in determining the grain size observed in a depositional environment. You may wish to review “The Basics of Grain Size within a Lake System”.
A follow-up exercise can be done in which you place
sediment in numbered dishes, vials, or bottles and ask the students to
determine which was collected from a stream, beach, inland lake, or whatever
environments the students studied. You might also ask the students
to hypothesize as to the characteristics expected for sediments from an
environment that they did not study.
1. Study the sediment from each of the locations. In each case examine the sediment first in the plastic dish and then in the bottle and indicate the grain size (clay; silt; sand, including the sand size, fine, medium, coarse, etc.; granules; pebbles) and types of animal or plant remains present, if any. Be sure you do not mistake “clumps” of sediment for a single grain – if necessary break up the clumps with your fingers to determine the actual grain size. Then examine the sediment in the bottle and record your grain size estimate again. Finally, indicate the type of rock (e.g., sandstone, siltstone, or claystone) the sediment would become if it were lithified (made into a rock). Stereo microscopes, hand lenses, grain size cards, and magnets are available for your use.
Sample Dry Sample Sample in Bottle Rock Type
2. Did examining the sediment in the bottles commonly cause you to change your estimates of grain size? (For example, did the rates the grains settled help you determine the grain size?) Explain.
3. Examine the Grand River samples again along with their locations on the map. Explain why the grain sizes might vary at the different locations. If possible, give a specific explanation for the grain sizes observed in the four samples.
4. Now reexamine the samples from Lake Michigan. Without looking at the map (don't cheat!) and only using the dry samples (again, don't cheat!), arrange the samples as best you can in order from nearshore/shallow water to offshore/deep water. Give your suggested listing below.
5. Now use the samples in the bottles to make any adjustments to your proposed order that you wish to make. Give your new suggested listing below.
6. What evidence did you use to put the samples in order? Did the samples in the bottles change your suggested listing? Explain. Now check the map and discuss how well you did.
7. Ultimately, what controls the grain size distribution in Lake Michigan?
8. Why are the grains in the Spring Lake samples the size they are?
9. Observe the sand sized samples with a hand lens or under the microscope. What is the dominant mineralogy of the sand? ______________ Estimate the percentage of that mineral in each sample consisting mostly of sand-sized grains.
10. Why do you think that mineral is so dominant?
11. What other minerals do you see in the sand-sized samples?
Quartz: Glassy, transparent, orange or yellow if stained with iron
Feldspar: Light colored, opaque
Garnet: Red or green, opaque
Magnetite: Black, opaque, shiny, metallic, magnetic, rounded
12. Based on the observations you made concerning grain sizes above, what grain size(s) do you predict you would see in Lake Macatawa in Holland? __________________. The Macatawa River, which empties into Lake Macatawa? ______________________ Lake Michigan in the shallow water off Holland? _____________________ Explain your reasoning.
13. Now extrapolate to ancient rocks in slightly different settings.
For some 400 million-year-old, sedimentary rocks, you suggest that the
depositional environment for Rock A was a nearshore, shallow sea, whereas
Rock B resulted from deposition in a stagnant, freshwater swamp.
List two pieces of evidence you may have used to make your interpretations.
Be sure to be specific and indicate which evidence indicates which depositional
environment. Explain your reasoning.