The River Loop Trail at Black Hawk State Historic Site in Rock Island, Illinois, has exposures revealing nearly 130 million years of geologic history
At the base of the hill is Devonian-aged limestone containing brachiopods and bryozoans. Above the limestone is Pennsylvanian-aged shale, with an unconformity between the two. The black shale is rich with fossils of ferns and marsh grasses. Directly on top of the shale is Pennsylvanian-aged sandstone. Using the geologic materials described, determine the environment of deposition for each unit. How has the overall environment changed from the Devonian to the end of the Pennsylvanian?
What will be an ideal response?
Answer: The limestone represents a shallow marine environment, most likely a reef. Sometime after the deposition of the limestone, erosion created an unconformity. The shale represents a coastal swamp, complete with swamp grass. The sandstone represents a beach. The overall environmental change is that sea level is lowering.
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