Original Horizontality

Sedimentary rocks and volcanic rocks are deposited in horizontal layers. This phenomenon can be observed in the picture of the Grand Canyon.

Lateral Continuity
Sediment forms into continuous lateral layers in every direction. This layering effect continues until the sediment thins to nothing or terminates at the edge of a depositional basin. Can you trace the layers across shown in the photograph of the Grand Canyon?

Superposition
The principle of superposition states that sediment is deposited in upward stacking layers, like a brick wall with the older layer being on the bottom and new additions made to the top. Look at the diagram of this stacking. Note how the rock layers progress upward from limestone to shale to sandstone. Unless these rocks have been overturned (there are ways to tell if this is so), it is obvious that the limestone is the oldest of the layers.

Cross-Cutting Relationships
Geologists can determine whether one rock is older than another by observing cross-cutting relationships. If a rock layer or fault cuts across another rock layer or fault, then the feature that is cut across must be older. For example, the photo at right shows two igneous dikes cutting across older igneous rock.

Inclusion
Think about rocks you might have come across that seem to have pieces of rocks of all different shapes and sizes in them. This is no coincidence. Geologists explain that if a rock contains fragments of another rock, then the included fragments are older than the rock that encloses them. Inclusion happens two main ways:

1. Pieces of preexisting rocks get included as sedimentary particles in sedimentary rocks
2. Pieces of rock get bombarded with hot magma. These rocks do not melt completely, but instead when the magma cools the preserved bits of rock are visible.

Faunal Succession
Life forms on earth have evolved in a recognizable order. And, once extinct, they never evolve again. Fossils are geologists' illustrations of the progression of the sequence of life. They compare fossils found in ancient rocks to modern species.

The Importance of Fossils:

• The regularity of changes in fossil species is one of the very strong lines of evidence supporting the theory of evolution.
• It follows that any two sedimentary layers with exactly the same fossil assemblage must be of the same age.
• Even if sedimentary rocks are different types from different parts of the world, they are time-equivalent if they have the same fossil assemblage. This offers a global "picture" of sedimentary rocks for any given time period.