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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