Question

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.

Explanation:

A cast fossil is formed when an organism leaves an impression (mold) in sediment. Then, sediment or minerals fill in that impression. Over time, this filled - in sediment hardens into rock.

• Option 1: When an organism turns into rock over time, it is called petrification (like petrified wood), not a cast.
• Option 2: When an organism leaves a carbon residue on a rock, it is a carbon - film fossil, not a cast.
• Option 3: When an organism leaves an impression in sediment that hardens into rock, it is a mold fossil, not a cast.

Answer:

Impressions left by organisms are filled in with sediment that hardens into rock.