Contents Casts & Molds





Molecular fossils



Drying & Dessication

Wax & Asphalt

Coprolites &Gastroliths

Trace fossils


Window to the past

Casts & Molds

What are cast and mold ?

Cast and mold are a type of fossilization where the physical characteristics of organisms are impressed onto rocks, especially coarse porous rocks such as sandstones. Typically, the hard parts of an organism (e.g. shells of mollusks, skeletal structures of coelenterates, bones and teeth of vertebrate, chitinous exoskeleton of arthropods, trunks of trees, and many sphenophyte) leave the best impression, because they are usually composed of calcium carbonate, calcium phosphate, silica, or chitin, which provide a rigid structure and do not decay as easily. The rigidity of the hard body parts allows the sediment to form around the organism, as opposed to soft body parts which decay too fast for impression to form as well as being too soft for the mold to set.

The fossilization process

The fossilization process begins when the whole organism or hard body part is trapped in sediments. Because most of these body parts are typically composed of substances which are soluble in carbonated water, this entrapment usually occurs in coarse and porous rock such as sandstones. The porous nature of the rock enables the carbonated ground water to permeate and dissolve the original tissue leaving a detailed mold of the organism.

Two types of mold result from this process external and internal. An external mold is created with the dissolution of the organic which then leaves an empty cavity imprinted with the external details of the organism. With hollowed structures, an internal mold may form. The "shell" of the organism is filled with various inorganic materials such as sediment or crystals; when the shell dissolves, it leaves an impression of the interior surface of the shells (e.g., muscle scars) on the material.

A cast of the organism can then be made using the two types of mold. Natural casts can be formed when minerals are deposited within the mold. Casts can also be synthetically created when the molds are filled or covered with synthetic material such as latex or plaster of paris to generate a replica of the organism. Thus in this manner, cast and mold fossilization enable us to "recreate" the structure of the organism. However in the creation of the cast, some details of the skeletal structures are lost.


Amber || Casts & Molds || Compactions || Compressions || Coprolites & Gastroliths

Drying & Dessication || Freezing || Impressions || Molecular Fossils || Permineralization

Reference || Trace Fossils || Wax & Asphalt

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