VARVE DATING

Dating Sedimentary Strata

EVOLUTION

 SYNOPSIS

 PRINCIPAL CONCEPT

 ASSOCIATED CONCEPTS

1. Varve counts in natural samples will vary in a predictable way, fitting a normal curve.

2. There are reasons why varve count variations occur.

3. Varve counting is one of many methods of geological dating.

4. Several geological processes, combined with seasonal changes, can be inferred from patterns seen.

5. Distinction between relative and absolute dating

6. Uniformitarian processes in geology

7. How one can do scientific extrapolation.

ASSESSABLE OBJECTIVES

Students will.....

1. complete their worksheets accurately (individually, in pairs, or in groups).

2. calculate the time taken to form a known thickness of lake bed deposits by extrapolating from a small sample of those sediments.

3. recognize that the geological history of Earth covers at least many millions of years (rather than only a few thousands of years.

4. describe and explain the evidence for this time frame.

 MATERIALS

See RESOURCES below for details in obtaining real varves, or using the pictures provided (items #5, 6, & 7).

1. piece of polished shale billet, about 1 cm cube (or picture of same)

2. 10x-40x lens (magnifier, dissecting scope, or compound scope)

3. small metric ruler (or picture of same)

4. 10 mm piece of millimeter graph paper

5. forceps to maneuver the graph paper

6. calculator (optional)

 TIME

STUDENT HANDOUTS

(see end of lesson for the formatted handouts)

1. Two-sided handout of the lesson, giving background information, purpose, materials, procedure and discussion questions.

2. Two-sided Worksheet (for answering questions and recording data).

3. Key to Worksheet (for teacher use)

 TEACHING STRATEGY

1. Have materials in trays available for convenient student access, with one set for every two students, including hand lenses or access to microscopes.

2. Have copies of the lesson and worksheet pages for all students.

PROCEDURES

1. Distribute copies of the lesson and worksheet pages to all students.

2. Students are to read Introduction and answer the Check Questions.

3. Students continue with the lesson, working in pairs, getting the materials and following the procedure provided. All observations are to be recorded on the worksheet.

4. When all teams (pairs) have recorded the number of varves per millimeter, ask for this information from each team, and display these numbers on the board or overhead. Students are to complete their histogram using these class data.

5. Students answer discussion questions (they can discuss them within each team). If necessary, this part can be completed as homework, to be collected the next class day.

 ASSESSMENT

1. Observe and record active participation by all students

2. Prepare quiz to assess the objectives listed above.

EXTENSIONS

& VARIATIONS

1.Interested students could (for extra credit?) look for more information about the Green River Formation in Wyoming (using internet). What kinds of fossils have been found there? Can they find a picture of a 200 meter cliff there? Can they find someone who has been there? (interview). Prepare a report on this topic.

2. Explore other methods of dating (both absolute and relative methods). List and explain those methods, and the time ranges for which each can be used.

3. Seriously consider spending a session with the interactive tutorial on radiometric dating: Deep Time. This new lesson on the ENSI site emphasizes the basis for the high level of reliability in geological age dating, and also points out the pseudoscience from those predisposed to supporting a young Earth premise. This lesson also provides an introduction to the excellent online interactive tutorial on Virtual Age Dating.

RESOURCES

1. If possible, read about varves in Strahler, Arthur N. "Science and Earth History", 1987, Prometheus Books, page 232. Other dating methods are also presented in this very useful resource, available at a discount from NCSE.

2. Geologist Jere Lipps (UC Berkeley Museurm of Paleontology) offers the following perspective on the generally accepted view of these varves, If anyone in your class claims that the Green River varves are not annual, share this statement with them, and encourage them to bring in their source.

"The Green River Formation laminae are considered varves, although not
formed under glacial conditions. They consist of a thin, dark winter layer
and a thicker lighter summer layer. These formed by seasonal
events--incresed spring and summer productivity of algae, and accumulation
of finer material in the winter. Some geologists have had a few other
ideas about the Green River Fm, but the seasonal changes accounting for the
laminations seems secure. Thus, they are annual varves."

3. Further verification that Green River varves are annual layers is provided in a detailed letter from USGS geologist John Dyni. This is a pdf file, includes the paragraph above (by Dr. Lipps), and could be printed out for students to read.

4. For any students who maintain that the varves are not annual, and/or the sediments were deposited during the great worldwide Noah's flood, ask them check this site: http://home.entouch.net/dmd/greenriver.htm. This is a very interesting and useful discussion of the geological details which effectively and clearly refute claims that the sediments were deposited during Noah's worldwide flood. It is fairly detailed, in about 15 pages.

NOTICE: If a student ever challenges you with criticisms of the reliability or validity of geological age-dating methods, CLICK HERE

5. Bulk shale can be ordered as pieces of "scrap shale" from Ulrich's Fossil Gallery, Fossil Station, Kemmerer, WY 83101, or call them at (307) 877-6466 to order. Ask for a piece of "scrap shale" that has good , well-defined laminations and is very firm. Flaky shale will crumble when it is cut. A piece about 4" x 6" x 3/8" thick (10x15 cmx1.5cm) will run about 3.00 plus s&h, or $6.85 total (as of Dec. 2003). Special Note: They also sell grade 1 and 2 Eocene fish fossils that are intact, and unexcavated and provide a little "excavation/ preparation " kit for students to prepare a fossil --- just like a museum paleontologist would do (at $7.50, I prefer the cheaper grade 2 fossils). The fish is typically one of three different species.

6. You can usually have your bulk piece of shale cut into tiny billets at a local rock shop. If this is unavailable, you can send your piece to Burnham Petrographics, 5029 West Lodestar Ave., Rathdrum, ID 83858. You can call them at (626) 359-3797, or email to burpet@linkline.com. Request to have the slab cut into a set of approximately 8mm x 20mm x 15mm thick billets. Cost is about .90 each plus s&h. The slab described above would make about 60 billets, making it about $60 total cost for more billets than you will need (a class set would be about 16 or 32 billets), so you can share or sell to other teachers.

7. PICTURES OF VARVES AND RULERS: If you lack the time or resources for getting actual varve billets from the Green River formation, you could try using pictures of these. A collection of 16 varve-billets (edge views) can be downloaded here, along with 24 centimeter-rulers in the same scale as the one with the varve pictures. Students can just use the cm ruler instead of the graph paper. Just print them out and cut them apart. There is also a large 3D view of the slab of varve material (like the one on the left) from which the billets were cut. This could be used for an overhead or PowerPoint presentation for instructions and discussion. Note especially the inclusion cross-sections showing. Have students speculate on what the inclusion was (especially when they see the shape of the surface bulge in the top layer). They could also try to figure out whether the slab is right side up, or upside down (in terms of its original position in the formation), and explain their reasoning.

REMEMBER - The real billets are best, (real stuff!), but the photos can be just as impressive if done well. Consider having the pictures (varve billets and rulers) laminated for durability and convenient re-use.

8. NEW! For an excellent 9' video clip showing paleontologist Michael Bell hunting and exposing stickleback fish fossils in the varves from a fossil-rich former lake site in Nevada, get the HHMI - Evolution DVD set. On disk 1, select the DVD Features: "Fossil Documentary." There is another 3' clip (from the third lecture) showing stickleback changes over about 25,000 years, graphically, in great detail, year by year, including the intermediate fossils formed along the way. This is in the Video Clips: "Fossil Record of Stickleback Evolution" on the disk. Very compelling clips.

 ATTRIBUTION

Some of the ideas in this lesson may have been adapted from earlier, unacknowledged sources without our knowledge. If the reader believes this to be the case, please let us know, and appropriate corrections will be made. Thanks.

1. Original Source: John Banister-Marx

2. Modified by: Larry Flammer

3. Reviewed / Edited by: Martin Nickels, Craig Nelson, Jean Beard: 12/15/97

4. Edited / Revised for website by L. Flammer 1/22/98, 7/7/08