Geologic History . . .

Actual Age - the actual age of rocks in years. Relative Age - the age of rock layers in relation to other rock layers. You don't know the rocks exact age, just which one is older or younger. Actual Age - the actual age of rocks in years. In places where layers of rocks are contorted, the relative ages of the layers may be difficult to determine. View near Copiapo, Chile.

The Law of Superposition When you look at layers of sedimentary rock, the older layers are on the bottom, below the younger layers. If you think back to how a sedimentary rock forms, it makes sense. 

The Law of Cross Cutting Relationships The relative age of igneous rocks, due to an intrusion, are younger than the rock layers they cut across. That means that a sedimentary rock is always OLDER than the intrusive rock that rudely shoved its way into it.

Horizontality . . . The Law of Original Horizontality All sedimentary rocks should be in flat layers. If not, something seriously went wrong. The most likely culprit is a mountain building event that created folds, tilts and even faults.

Remember. . . all faults are younger than the rock they cut across, because the rocks must be there first for the fault to occur. Duh.

Conformity Bedding planes are parallel, no physical evidence for erosion or deformation. Beds deposited from the bottom up according to the Law of Superposition of Strata. The beds were probably laid down during spurts of deposition followed by intervals when nothing happened -- no sign of disturbance, uplift, or erosion.

When it’s not horizontal . . . Unconformities- Unconformities are erosional events that break the sequence of deposition. On a cross section, it looks like a thick wavy black line interrupting the parallel layers of sedimentary beds.

Conformity and Non-conformity

Correlating Rock Layers From Different Locations

Outcrops: An outcrop is where the rock layers are exposed, such as on cliff or road cut. Most rock layers are covered in soil so identifying the relative age of rocks can be a detective game. By matching the characteristics of several outcrops, we can determine the relative position of identical rock layers and then figure out which is older.

Index Fossils Index fossils are remains of organisms that are: Common Lived in a wide geographic area existed for only a short period of time. In other words, if you are trying to identify the relative age of a rock, finding a fossil that only lived in a small region or that lived for a really long period of time is useless.

Key Beds… Another interesting key bed item would be Iridium, a radioactive material often found in meteors, but rare on earth. This layer is 66 million years old. Scientists theorize that a huge meteor collided with the earth, and then sent a huge cloud of dust rich in Iridium circling the earth, which then settled and became part of the rock layers. It is the "smoking gun" for the massive dinosaur extinction. Key beds are beds that have a very unique property that is not easily confused with other rock layers. Examples include: volcanic ash deposits nuclear fall-out!

Geologic Time Just as a clock is broken into units of time, so is the geologic record. Instead of being broken into minutes, hours, and seconds, it is broken into different units.