15.2 Evidence through Geology

Absolute Dating

Dating of igneous rock layers through radiometrics.

Relative Dating

Placing rocks in sequence of Formation

Principle of Original Horizontality

Sediment layers are usually laid down in horizontal positions. Any layers in a fold or incline are modified.

Cross-Cutting Relationships

Any land feature that cuts into another is younger than the feature it cut into.

Unconformity

When deposition of rock stops, erosion occurs, then deposition starts back up.

Angular unconformity

Previously deformed layer overlaid by new horizontal layer

Nonconformity

Older intrusive igneous formations overlaid by younger sedimentary rocks

Conformity

Older igneous rock contained within a younger sedimentary layer

Index Fossils

Common and widespread fossils that hasn’t existed for awhile.

Index Fossils and Use

Index fossils used for correlalation between sedimentary rock layers.

Biozone

In stratigraphic units, a body of rock whose lower and upper boundaries are defined by the ranges of one or more species in stratigraphic record