* Limestone is primarily composed of calcium carbonate (CaCO3). This compound is the main component of the shells and skeletons of many marine organisms like corals, clams, and foraminifera.
* Over time, these shells and skeletons accumulate on the ocean floor. As they are buried under layers of sediment, they become compressed and cemented together, forming limestone.
* The presence of fossils within limestone is direct evidence of its marine origin. These fossils are remnants of the organisms that contributed to the formation of the rock.
Let's look at why other options might not be as strong:
* Presence of layers in limestone: While layers are common in sedimentary rocks, they don't specifically indicate an ocean origin.
* Limestone is a sedimentary rock: This is true, but many sedimentary rocks can form in various environments.
* Limestone is often found near the ocean: While this is true, it's not definitive proof. Limestone can also be found in areas that were once covered by oceans, but are now inland.
Therefore, the presence of marine fossils is the most conclusive evidence that limestone formed from ocean sediments.
