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Tilting and erosion of the older rocks took place during this time, and if there was any deposition going on in this area, the evidence of it is now gone.Figure 8.2.3 The great angular unconformity in the Grand Canyon, Arizona.The lower sandstone layer is disrupted by two faults, so we can infer that the faults are younger than that layer.But the faults do not appear to continue into the coal seam, and they certainly do not continue into the upper sandstone.New layers are formed later more recently than the older ones, so they sit on top.Stratigraphy, or layers of soil, at an archaeological site in the Islands of Orkney, Scotland.The Law of Superposition isn’t only used by archaeologists, though – it is also very important to other scientists like paleontologists and geologists.
Say one day you need to find a letter from three weeks ago in those layers of envelopes, how will you know where to look in the pile when there are two weeks’ worth of newer mail on top?
(The near-vertical stripes are blasting drill holes.
The image is about 7 m across.) [SE photo] An unconformity represents an interruption in the process of deposition of sedimentary rocks.
The tilted rocks at the bottom are part of the Proterozoic Grand Canyon Group (aged 825 to 1,250 Ma).
The flat-lying rocks at the top are Paleozoic (540 to 250 Ma).The lava flow took place some time after the diorite cooled, was uplifted, and then eroded.