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.

When organisms die, their remains can become buried in sediment, such as mud or sand. Over time, layers of sediment build up on top of them, compressing the lower layers. As the sediment hardens into rock, it preserves the shape and details of the organisms, forming a cast. This process can take millions of years, resulting in stunning fossils that tell us about life long gone. While impressions showcase the external features of an organism, casts fill in the voids left by decay. As the original organic material breaks down, sediment can fill these hollow areas, creating a replica of the organism in rock. This clever trick of nature preserves a snapshot of ancient life, allowing paleontologists to study past biodiversity and ecosystems. Isn't it amazing how time can turn ancient creatures into rock stars?