Comment: 02:04 - 03:02 (00:58)
Source: Annenberg/CPB Resources - Earth Revealed - 11. Evolution Through Time
Keywords: mold, bone, shell, "sedimentary rock", groundwater, cast, fossil, track, burrow, nest
Our transcription: These clues exist in the fossilized remains of past life.
Fossils can be formed in several ways.
After some organisms die, they are quickly buried by sediment.
As the sediment turns to stone, the remains decay or dissolve, and a hollow cavity called a "mold" is left behind.
Molds are among the most common fossils, but other types also occur in abundance.
Buried bone and shell material may survive intact within sedimentary rock for millions of years; however, much of this material crystallizes or is replaced by minerals deposited from groundwater.
Casts preserving the original forms of the organism result. Like molds, casts are numerous.
Evidence showing the activity of past organisms is also found in the form of fossil tracks, burrows, and nests.
Geology School Keywords