The Origin and Evolution of the Earth System:

A Rising Tide Project for Grades 9 and 10

The laws of stratigraphy are some of the best tools we have for understanding rock and sediment layers. These laws help with relative dating, which cannot determine the age of these layers, simply the relative order in which the layers formed. In order to understand these laws, we must assume that the geologic processes of today were the same in the past.  This is called uniformitarianism.  

The first law of stratigraphy is the law of superposition, which states that the oldest layer is at the bottom of the sequence, and the youngest at the top.  The second law is the law of original horizontality.  This law states that sediment or rock layers were deposited in a horizontal orientation*.  If units are not horizontal, an event occurred subsequent to the deposition which caused the layers to fold or tilt.  The third law is the principle of lateral continuity.  This states that deposits originally extended in all directions.  Another way to determine relative age of rock is by examining cross-cutting relationships. This describes the relationship between existing rock and rock which intrudes by magma flow into existing rock.  This creates an intrusion, and the intrusion is always younger than the rock it invades. Sometimes layers of rock contain unconformities which represent a time interval where erosion and no deposition has occurred.

*Note: topography controls the angle at which sediments are deposited locally.  For example, sands deposited on a sand dune will result in angled layers. Original horizontality generally remains true when observing layers over broader regions.

Law of Superposition		Law of Original Horizontality (originally like figure to the right)
Law of Lateral Continuity		Cross-Cutting Relationship