How Do You Stack Up?

Procedure

1) Look at the four “rock samples” you have been given, paying close attention to the contact between layers – straight lines or wavy.  Straight lines represent places where sediment was continuously deposited in a bedding plane.  Wavy lines represent unconformities, where rock layers may be missing.  The top of each rock sample is incomplete, so it should be a jagged line.  Assume that the bottom of the lowest layer is a bedding plane. 

Look at the symbols representing fossils to the layers in your drawings.  Pay attention to the shapes and the layers that they are in.

2) Write the rock sample number on the back of each section.

3) Carefully cut the rock samples out of the paper, and lay the individual samples next to each other on your desk or table.

4) Find layers that have the same rocks and contain the same fossils.  Move each sample up or down to line similar layers up next to each other. 

5) If unconformities appear in the samples, there may be rock layers missing.  You may need to examine other sections to find out what fits between the layers above and below the unconformities.  Leave room for these layers by cutting the samples along the unconformities (wavy lines).

6) Eventually, you should be able to make a geologic column that represents all four of the samples.  It will show rock types and fossils for all the known layers in the area.

7) Use glue to arrange the samples together on your paper in a pattern that represents the complete geologic column.

Click here for another set of the geologic column samples and key to use... 

Analysis

Answer the following questions on your own paper, under your geologic column.

1) How many layers are in this part of the geologic column you modeled?

2) Which is the oldest layer in your column?  Which rock layer is the youngest?  Describe these layers in terms of rock type and the fossils they contain.

3) Which (if any) fossils can be used as index fossils for a single layer?  Why are these fossils considered index fossils?  What method would be required to determine the absolute age of these fossils?

4) List the fossils by symbol in your column from oldest to youngest.

5) Look at the unconformity in Sample 2.  Which rock layers are partially or completely missing?  Explain how you know.