Question

Fossils of sea shells are found in a natural bedrock exposure. How did this rock probably form?

Answer 1

Probably in an ancient marine environment, unless the shells are a freshwater species.

Explanation:

If the shells are really a marine variety, this tells us that the animals lived in some ancient sea. It also tells us that the surrounding rock was likely formed in a sedimentary basin. Sedimentary basins often form at the edge of continents and gradually turn to stone, by the process called "lithification". The fact that we see a natural exposure also tells us that there must have been some uplift of the sequence at one time or another AND that erosion muds have cut through the rock section exposing the fossils.

If we can identify the specific species of fossil shell we might know more about which specific marine environment it lived in. However, most bivalves today live in shallow shelf, tidal flats, or shallow marine environments of some kind. We would also look for other pieces of evidence to support our hypothesis as to its original environmental setting, such as sedimentary structures, trace fossil tracks and burrows, or other fossils that might be present.