Question

what does it mean if a layer of rock has many rounded pebbles (like a cobble)?

it used to be part of a desert

it formed from lava

it was once underwater

it was once at the top of a mountain

Explanation:

Fossils are the preserved remains or traces of ancient organisms. Marine fossils (like shells) are typically formed in underwater environments. When a rock layer has many such fossils, it indicates that the area was once underwater where these organisms lived and were later fossilized.

Answer:

It was once underwater.