The Mysterious Rock Layer
A student is examining a cliff face with three sedimentary rock layers. The bottom layer is a coarse sandstone with rounded pebbles. The middle layer is a fine-grained shale with fossil fish skeletons. The top layer is a limestone with fossil shells. The student notices that the boundary between the sandstone and shale is uneven, with channels cut into the sandstone. The shale layer contains a thin layer of volcanic ash. The student has a hand lens, a dilute acid bottle, and a geological hammer. They want to interpret the history of the area.
1. Interpret the sequence of events that formed these rock layers, including the environment of deposition for each layer and the significance of the uneven boundary and the volcanic ash.
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The bottom sandstone with rounded pebbles suggests deposition in a high-energy environment, such as a river or beach, where coarse sediments are moved and rounded. The uneven boundary with channels indicates erosion, likely by water, before the shale was deposited. The shale with fish fossils indicates a low-energy environment, such as a deep lake or quiet marine setting, where fine mud settles and fish remains can be preserved. The volcanic ash layer in the shale marks a specific time event, useful for dating. The top limestone with shells indicates a shallow marine environment with abundant shelled organisms. The sequence: first, deposition of sandstone; then erosion (channels); then deposition of shale; then volcanic ash fall; then limestone deposition. The student can test the limestone with dilute acid to confirm it is calcium carbonate (fizzing).
2. The student claims that the fish in the shale lived at the same time as the shells in the limestone. Is this claim supported by the evidence? Explain using the principle of superposition and the presence of the ash layer.
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The claim is not supported. According to the principle of superposition, in an undisturbed sequence, older layers are at the bottom and younger at the top. The shale is below the limestone, so the shale and its fish are older than the limestone and its shells. They did not live at the same time; the fish existed before the shells. The volcanic ash layer within the shale provides a marker that could be used to correlate with other locations, but it does not change the relative ages. The student could use the ash to obtain an absolute age via radiometric dating, which would confirm the shale is older than the limestone. The uneven boundary also indicates a gap in time (unconformity) between the sandstone and shale, so there is missing time there as well.