Problem 3
This scenic image is from Monument Valley in the northeastern corner of Arizona. The bedrock in this region consists of layers of sedimentary rocks. Although the prominent rock exposures ("monuments") in this photo are widely separated, we can infer that they represent a once-continuous layer. Discuss the principle that allows us to make this inference.
Problem 5
These polished stones are called gastroliths. Explain how such objects can be considered fossils. What category of fossil are they? Name another example of a fossil in this category.
Problem 8
This scene in Montana's Glacier National Park shows layers of Precambrian sedimentary rocks. The darker layer contained within the sedimentary layers is igneous. The narrow, light-colored areas adjacent to the igneous rock were created when molten material that formed the igneous rock baked the adjacent rock. a. Is the igneous layer more likely a lava flow that was laid down at the surface prior to the deposition of the layers above it or a sill that was intruded after all the sedimentary layers were deposited? Explain. b. Is it likely that the igneous layer will exhibit a vesicular texture? Explain. c. To which group (igneous, sedimentary, or metamorphic) does the light- colored rock belong? Relate your explanation to the rock cycle.
Problem 9
Solve the problems related to the magnitude of Earth history below. To make calculations easier, round Earth's age to 5 billion years. a. What percentage of geologic time is represented by recorded history? (Assume 5000 years for the length of recorded history.) b. Humanlike ancestors (hominids) have been around for roughly 5 million years. What percentage of geologic time is represented by these ancestors? c. The first abundant fossil evidence does not appear until the beginning of the Cambrian period, about 540 million years ago. What percentage of geologic time is represented by abundant fossil evidence?
Problem 10
A portion of a popular college text in historical geology includes 10 chapters ( 281 pages) in a unit titled "The Story of Earth." Two chapters (49 pages) are devoted to Precambrian time. By contrast, the last two chapters \((67\) pages) focus on the most recent 23 million years, with 25 of those pages devoted to the Holocene Epoch, which began 10,000 years ago. a. Compare the percentage of pages devoted to the Precambrian to the percentage of geologic time that this span represents. b. How does the number of pages about the Holocene compare to its percentage of geologic time? c. Suggest some reasons the text seems to have such an unequal treatment of Earth history.
