Dating and correlating rock layers
Principle of Original Continuity If layers are deposited horizontally over the sea floor, then they would be expected to be laterally continuous over some distance.
Thus, if the strata are later uplifted and then cut by a canyon, we know that the same strata would be expected to occur on both sides of the canyon.
In fact, sedimentary rocks are, in a sense, trash from the Earth's surface deposited in basins.
Principle of Original Horizontality Sedimentary strata are deposited in layers that are horizontal or nearly horizontal, parallel to or nearly parallel to the Earth's surface.
In either case, the inclusions had to be present before they could be included in the younger rock, therefore, the inclusions represent fragments of an older rock.
This principle is often useful for distinguishing between a lava flow and a sill.
From the beginning of this course, we have stated that the Earth is about 4.6 billion years old.
Geologist in the 1800s worked out 7 basic principles of stratigraphy that allowed them, and now us, to work out the relative ages of rocks.
Principle of Inclusions If we find a rock fragment enclosed within another rock, we say the fragment is an inclusion.
If the enclosing rock is an igneous rock, the inclusions are called xenoliths.
Once these age relations were worked out, another principle fell into place - the principle of fossil succession.
We discuss the 7 principles of stratigraphy first and then see how these apply to fossils.