earth science

Original Remains

Original, unchanged remains of a plant or animal are preserved

Replaced remains

Original hard parts are dissolved by chemical action and replaced by mineral material

Molds

Fossil shell or bone will dissolve completely leaving a hollow depression in a rock

Casts

New mineral material may fill mold, forming a cast — 3D copy of original fossil

Trace fossils

Evidence of life OTHER than remains of a plant or animal

Ex. Trails, footprints, tracks, burrows, borings

Unconformities

Gap in time

Law of Original Horizontality

Sediments that form sedimentary rocks are deposited in horizontal layers

If they’re not horizontal, must have occurred after rocks formed

Law of superposition

Oldest on bottom, youngest on top

Law of Cross-Cutting Relationships

Faults that cut across preexisting rock layers must be younger than rocks they are found in

Easily recognizable

Large geographic distribution

Existed for short period of time

Three characteristics of index fossils