Lesson Plan on Sediments


    The objective of this exercise is to show students how the grain size of sediments varies in different depositional environments, specifically streams, small inland lakes, and large lakes such as one of the Great Lakes.  Hopefully, students will also learn how geologists can predict what grain sizes to expect in other modern depositional environments and how grain size can help geologists determine paleo depositional environments in ancient, sedimentary rocks. In order to do this lesson plan, you will need samples of a few sediments.  You need not have the same number as suggested in the “Example Exercise” below, or even sediments from the same depositional environments.  You can quite easily alter the exercise if you have less samples or samples from different environments (e.g., a beach, a dune, or an alluvial fan).  Samples like those used in the hypothetical exercise below can be collected during educational cruises aboard Grand Valley State University’s (GVSU) D.J. ANGUS (Lake Michigan, Spring Lake, and the Grand River; boat berthed in Grand Haven, Michigan) or the W.G. JACKSON (Lake Michigan, Muskegon Lake, and the Muskegon River; boat berthed in Muskegon, Michigan).  If you are interested in arranging an educational cruise for your students, go to the GVSU Annis Water Resources Institute web page (http://www.gvsu.edu/wri/education/).  Whether or not you can arrange a cruise for your students, you can always be on the lookout for good sediment samples for your teaching collection.  Hiking, fishing, swimming, and vacation trips all offer opportunities to collect such samples.

    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.
 

Example Exercise

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

LM-1

LM-2

LM-3

LM-4

GR-1

GR-2

GR-3

GR-4

SL-1

SL-2

SL-3

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?

FYI:
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.
 


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LIVING WITH THE GREAT LAKES
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GRAND VALLEY STATE UNIVERSITY
DEPARTMENT OF GEOLOGY
ALLENDALE, MICHIGAN 49401