Note 3 — THE GEOLOGY OF D.A.R. STATE PARK
Reading edition
The actual remains are usually not preserved in their original state but are represented by molds and casts. Picture an ancient sea. The sea bottom mud slowly hardens around a shell. Water then seeps through the hardened mud and dissolves the shell leaving an open space where the shell once was. This open space is a mold. If the mold is filled a copy of the original shell is formed. This is called a cast.