Question

1. which layer above contains the oldests fossils? how do you know? 3. what is an index fossil? how do scientists use them?

Explanation:

1. The principle of superposition in geology states that in undisturbed sedimentary rock layers, the oldest layers are at the bottom. So layer 4 likely contains the oldest fossils.
2. An index fossil is a fossil of an organism that lived for a relatively short period of time but was widespread. Scientists use them to help determine the age of rock layers. If an index - fossil is found in a rock layer, the layer can be dated to the time when that organism lived.

Answer:

1. Layer 4 likely contains the oldest fossils because of the principle of superposition (older layers are at the bottom in undisturbed sedimentary sequences).
2. An index fossil is a fossil of an organism that had a short - lived, widespread existence. Scientists use them to date rock layers as the presence of an index fossil in a layer indicates the layer is from the time when that organism lived.