Monument Valley: Relative Ages of Rocks
Rocks are generally
deposited one layer upon another, with the oldest rock being on
the bottom and the youngest rock layer being on top. A good analogy
is to stack books, pieces of paper, or cardboard on a desk and
show that the book on the bottom of the stack was there first
before the next book was placed on top. The last book placed
on the stack is on the very top.
The exception to the oldest-on-the bottom generalization
is when a magma invades a sequence of rocks from below. The resulting
igneous rock is younger than the rocks it invaded.
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