Question

a river has cut a channel deep into the earth and exposed several layers of undisturbed sedimentary rock as shown in the cross section.
cross section of channel

which layer is the oldest?

layer 1 because it is closest to the surface.
layer 2 because it has the greatest thickness.
layer 3 because it has the most fossils.
layer 4 because it is on the bottom.

Explanation:

In undisturbed sedimentary rock layers, the principle of superposition states that the oldest layers are at the bottom and younger layers are deposited on top. Layer 4 is at the bottom among the layers, so it is the oldest. The other options are incorrect: Layer 1 being close to the surface means it's young; Layer 2's thickness or Layer 3's fossils don't relate to age in the way the options suggest—age is determined by position (bottom is oldest in undisturbed sedimentary rocks).

Answer:

D. Layer 4 because it is on the bottom