An Early Scientist Discovers Some Basic Rules
Nicholaus Steno (a.k.a. Niels Stensen) was a Danish physician living in Florence, Italy in the 1600s. He observed sedimentary rocks and formulated three basic principles of historical geology, which are commonly referred to as "Steno's Laws."
Principle of Superposition
Oldest rocks on the bottom Younger rocks on top
Principle of Original Horizontality
Sediments are deposited in flat, horizontal layers.
(Steeply dipping or folded rock layers indicate tectonic disturbance after deposition.)
Principle of Original Lateral Continuity
Sediments are deposited over a large area in a continuous sheet. Rock layers extend continuously in all directions, until they thin out at the edge of the depositional basin, or grade into a different type of sediment.
These three principles are the basis of stratigraphy, the study of layered rocks. Stratigraphy allows geologists to place rock units into a sequence, helpng them interpret Earth history.
The Principle of Cross-Cutting Relationships
Discussed by English geologist Sir Charles Lyell, 1830s.
Applies to geologic features like faults, or intrusions of igneous rock or veins of minerals, or ancient erosion surfaces called unconformities that cut across pre-existing rocks.
Examples:
The fault is younger than the rocks it cuts.  The intrusion is younger than the rocks it cuts.  Note the irregular erosional surface. This is an unconformity. The unconformity is younger than the rocks that have been eroded. 
The Principle of Inclusions
Note that there are fragments of eroded rock overlying the unconformity in the diagram above. These are clasts or inclusions.
The inclusions are older than the rocks which contain them.
The clasts (in the bed above the unconformity) are derived from the underlying (older) bed. The gravel clasts are older than the layer which contains them. The layer containing the gravel must be younger than the layer from which the clasts originate.
There are two fundamentally different types of inclusions.
Inclusions in sedimentary rocks - the gravel clasts in the layer above an unconformity.  Gravel clasts in a sedimentary rock above an unconformity. The gravel clasts are older than the bed of sedimentary rock which contains them. Inclusions in igneous rocks. A xenolith is a fragment of the surrounding rock which has broken off during an intrusion and fallen into the magma. The xenolith is older than the igneous rock which contains it.  A xenolith in a granite intrusion at Stone Mountain, Georgia. The xenolith is older than the granite.
The diagram below compares inclusions in a sedimentary rock (A) with inclusions in an igneous rock (B).  In the diagrams above, which are the gravel clasts and which are the xenoliths? In each diagram, determine the origin of the inclusions and whether they are older or younger than the rocks that contain them.
Interpreting the Sequence of Events
The sequence of geologic events in an area can be solved by applying the fundamental principles of geology. In this diagram, which shows a hypothetical exposure of rock, such as might be seen in a roadcut or in the walls of the Grand Canyon, it is possible to determine the order in which the various geologic events occurred.  The sequence of events is as follows:
Deposition of sedimentary rocks D.
Fault B.
Intrusion of igneous rock C.
Erosion, forming the unconformity.
Deposition of sedimentary rocks E.
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