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(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.With this in mind geologist have long known that the deeper a sedimentary rock layer is the older it is, but how old?Although there might be some mineral differences due to the difference in source rock, most sedimentary rock deposited year after year look very similar to one another.The most obvious feature of sedimentary rock is its layering.This feature is produced by changes in deposition over time.

An example of an unconformity is shown in Figure 8.8.

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).

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 8.8 The great angular unconformity in the Grand Canyon, Arizona.

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