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Laboratory Manual for Introductory Geology 4e

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EXERCISE 12.4

Name:

Course:

Using Sedimentary Structures to Unravel Geologic History

Section:

Date:

(a) Sketch diagrams showing how the three sedimentary structures indicated below would appear in beds that had

been turned upside down.

Mud cracks. Graded beds. Symmetric ripple marks.

(b) Are the sedimentary features in Figure 12.5 right-side up or upside down as shown? Explain your reasoning.

The figure below shows the value of sedimentary structures in relative dating. A cliff face exposing horizontal rock

layers (left side) might be incorrectly interpreted as undeformed horizontal strata without the top and bottom features

that prove that the layers have been folded, causing some to be overturned. The right side of the figure shows what the

geologist might have seen had the adjacent rocks not been eroded away or covered by glacial deposits.

(c) Number the layers, with 1 representing the oldest rock. Which is the youngest?

Reversals of top-and-bottom features reveal folding.

Upright symmetrical

ripple marks

Upright graded

bedding

306 CHAPTER 12 INTERPRETING GEOLOGIC HISTORY

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