A mold is the cavity left behind in the rock after an organism's hard part has dissolved.
Explanation;
When plants and animals die they undergo decay. when the soft tissues of an organisms decay in sediment, the hard parts such as bones may be left behind.
Carbonization or distillation is the process that yields detailed carbon impression of dead organisms on a sedimentary rock.
When an organisms completely dissolves in a sedimentary rock, an impression of its exterior may be left in the rock. This impression is called an external mold.
When the mold gets filled with minerals, it becomes a cast. when sediments or minerals fills the internal cavity of an organism such as skull, an internal mold forms.
