Question

a geologist finds four layers of sedimentary rock. she determines that no geologic events have shifted the layers. she labels the layers a, b, c, and d from the top to the bottom.
which statement about these layers is accurate?
a is older than b
b is older than c
c is younger than a
b is younger than d

Explanation:

According to the principle of superposition in geology, in undisturbed sedimentary rock layers, the lower layers are older and the upper layers are younger. Here, the order from top to bottom is A, B, C, D. So, each layer above is younger than the layer below it.

• For option “A is older than B”: Since A is above B, A is younger than B, so this option is wrong.
• For option “B is older than C”: Since B is above C, B is younger than C, so this option is wrong.
• For option “C is younger than A”: Since C is below A, C is older than A, so this option is wrong.
• For option “B is younger than D”: Since B is above D, according to the superposition principle, B is younger than D. This option is correct.

Answer:

D. B is younger than D