NOTES ON FINDING OUT HOW OLD THINGS ARE:
Stratigraphy: The science of analyzing ages of rocks using a combination of relative age laws, index
fossils and absolute dating methods.
Correlation: Using stratigraphy to relate different sets of rock layers that are exposed in different
places. Example: The layers of rocks in the Grand Canyon, Bryce Canyon and several other places in
that region can be correlated using relative age laws, index fossils and absolute dating methods.
Unconformity: A gap in layers of rock that is caused by erosion or a pause in deposition.
- Some unconformities are obvious
- Because the layers are at different angles from each other (Angular Unconformity)
- Or because sedimentary rocks are laid down on top of VERY DIFFERENT igneous or
metamorphic rocks that clearly formed in a different environment (Nonconformity),
- Others are very very sneaky because they look just like a change from one layer to the next
Relative Age Laws:
1) Law of Superposition: If rock layers are undisturbed, the youngest rocks will be on top of the older
rocks. Connection: A stack of pancakes will have the youngest pancake on the top and the oldest on
2) Law of Original Horizontality: Sedimentary rocks and sediments are originally deposited
horizontally. The same is true with lava flows.
3) Law of Crosscutting: If rock layers have been crosscut by a fault or an igneous intrusion (a
magma batholith, a dike, or a sill), the crosscutting is younger than the rock layers that were crosscut.
With magma, one clue to look for is whether or not the rock that is intruded into has been cooked
4) Law of Inclusion: If pieces of one layer are included in another layer (chunks of rock that fell into
magma or chunks of bedrock that were picked up by erosion and included in sedimentary rock), the
inclusions are older than the rock in which they are included.
5) Law of Folding: Tectonic forces often bend, and fold rock layers that were originally flat. The
folding or deforming of those rock layers is younger than the original rock layers that were folded or
6) Cole's Law: Thinly sliced cabbage. I prefer it with a cream sauce, raisins and grated carrots.
- Weathering and Erosion and USUALLY happen when land is above water.
- Sedimentary Rocks USUALLY form when land is under water.
Index Fossils: Fossils that are from an organism that was geographically widespread, abundant, and
lived for a short period of time can be used as an index to identify rock layers of the same age.
Absolute Dating Methods: (to be added to soon)