How are casts formed by decaying organisms? Organisms turn into rock over time. Organisms leave carbon residue on a rock. Organisms leave impressions in sediment that hardens into rock. Impressions left by organisms are filled in with sediment that hardens into rock.

Casts are a type of fossil formed through a specific process involving the decay of organisms and the subsequent filling of their impressions. Here's how casts are formed: 1. **Impression Formation**: When an organism is buried in sediment (like mud, sand, or volcanic ash), it leaves an impression or mold in the soft sediment as it decays. This mold captures the shape and surface details of the organism. 2. **Filling the Mold**: After the organism has decayed and the mold remains, additional sediment or minerals may accumulate within the mold. Over time, these materials harden and solidify, filling the space where the organism once was. 3. **Rock Formation**: The filled mold becomes a cast, a replica of the original organism, preserved in rock. The surrounding sediment layers also harden, preserving the cast as part of the geological record. **Key Point:** - **Casts** specifically refer to the stone replicas formed when the impressions left by organisms are filled in with minerals or sediment that later hardens into rock. Among the options you listed, the most accurate description of how casts are formed is: - **Impressions left by organisms are filled in with sediment that hardens into rock.** This process ensures that the detailed shape of the original organism is preserved in the cast fossil. **Summary:** Casts form when an organism leaves an impression in sediment; as the organism decays, sediment fills the impression, and this fill hardens into rock, creating a replica of the original organism.