Page 246 - Laboratory Manual in Physical Geology

C. Analyze this outcrop, photographed by geologist, Thomas McGuire. It is another natural geologic cross section with red
sandstone layers on the bottom and a yellow conglomerate (gravel) rock layer on top. Notice that the red rock layers are not
horizontal. They are bent up on the left and right, and down in the middle, as wave-like folds (like a crumpled rug).

1. Using a pen, trace two of the contacts between layers of the red sandstone as well as you can. Assuming that the red
sandstone layers were originally horizontal, what may have caused them to be folded in this way?

2. On both sides of the picture, use an arrow to label the exact location of the contact between the red sandstone and the
horizontal yellow conglomerate above it. This surface is an unconformity —a surface (contact) representing erosion of
layers or a break in deposition of layers, like a place where pages are missing from a book. Something happened at the time
represented by the surface, but no rock layer remains as a record of the event. What sequence of events may have happened
to form the unconformity?

D. REFLECT & DISCUSS In all of your work above, you had to figure out the relative ages (from oldest to youngest) of rock
layers, fractures, folds, and clasts included in soil. Based on your work, write down three rules that a geologist could follow to
tell the relative ages of rock layers, fractures, clasts, and folds in geologic cross sections.

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