unit D must be the youngest rock unit and the order of formation must be A, B, C, D
The Law of Cross Cutting Relations states that any feature (such as a fault or a like) that cuts across a rock unit must be younger than the rock unit.
In the picture, the dark colored rock (unit D) cuts across all of the other rock units (A, B, and C).
Therefore, unit D must be the youngest rock unit and the order of formation must be A, B, C, D.
Therefore, the order of formation is A, B, C, D.
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