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If a horizontal layer of rock or sediment is underlain by a sequence of tilted rocks, you know two things:
first, the underlying rocks, if they are sedimentary or if they are lava flows, used to be horizontal - so after they were laid down, they were somehow tipped.
second, you know that you are looking at an angular unconformity - that is, there is a gap in the rock record between the tilting of the lower rocks and the deposition of the upper (horizontal) rocks.
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Jolyn Wells-Moran |