Table for Individual Question Feedback Points Earned: 2.0/2.0 Correct Answer(s): A 3.
This block of rock fell off the Kaibab cliff near the top of the Bright Angel Trail, just below the South Rim of the Grand Canyon, and landed as shown. (No one was hurt by the fall.) Before the material in this block was hardened to rock, the material was soft sediment. Soft sediment often dries out, cracks, and then is buried by more sediment when the water returns. When a limestone is forming, such as this Kaibab Limestone, the drying and wetting may happen with the tides, or in other ways. The sun was high and hot when the picture was taken, but slanting in from the right as shown, and we have provided arrows to direct your eye to the shadow from Dr. Alley’s boot toe as well as to another useful shadow. Are you looking at the side that was down when the sediment was soft, or the side that was up? A) Side that was down B) Side that was up Feedback: Mud cracks extend downward into soft sediment. When more sediment is washed in, this second layer will fill the cracks beneath. Later, after the layers have hardened, the rock may be cracked apart. If you see troughs in a mud-crack pattern, you are looking at the side of the second layer that originally was up. You can tell that this picture shows troughs, and not ridges, by the shadows—troughs have the light and the shadow on the same side, as shown here, whereas ridges have light and shadow on opposite sides. Table for Individual Question Feedback Points Earned: 2.0/2.0 Correct Answer(s): B
4. Dinosaurs once stomped across the Yukon, leaving tracks in mud that were buried in more mud and hardened to stone. Some may have been turned on end or turned over by mountain building; others were split apart and turned over by humans looking for dinosaur tracks and putting them in museums. The light was shining in from the upper right, as indicated, making lighter-looking and darker-looking places
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- Fall '08
- Correct Answer, Individual Question, geologist