Law of Original Horizontality
Most sediments were deposited beneath the seas and oceans of the world. Because of gravity and the more or less horizontal ocean floor, sediments are originally laid down in flat, horizontal layers. This nearly universal process has become known as the Law of Original Horizontality. These layers may get pushed, folded, erupted on, and other things, but they started out flat.
Principle of Stratigraphic Superposition
Have you ever been in an ancient city and seen evidence of many civilizations who built their idea of the latest architecture on top of older structures? When the basements of towering, new office buildings are dug, often times an area’s history is revealed as well. Even cross-sections of older city streets sometimes expose older and earlier layers of brick, cobblestones, and dirt beneath recent concrete paving.
When this happens with layers of sediment and sedimentary rock, it is called the Principle of Stratigraphic Superposition. This principle says that in any strata, which has not been folded or overturned, the oldest sediments will be found at the bottom of the sample with the most recent sediments layered on top.
The Principle of Stratigraphic Superposition says that the deeper you go into the Earth, the older the sedimentary rock.
This law applies to sedimentary, igneous, and metamorphic rock layers, as well, that haven’t been mangled by any other outside forces after their first layering.
The principle was first described by William Smith, a civil engineer who did a lot of surveying work for canal construction in western England. During the construction of the canals, he noticed that there were different layers of sedimentary rock in a predictable order of layering. In 1816, Smith published Strata Identified by Organized Fossils in which he listed 17 strata with specific plants and organisms unique to each for periods between Jurassic and Tertiary.
The next year, he added 10 more strata downward until he reached bedrock granite. Smith got so used to seeing the different rock layers that after a while, he could name the layer, the region it came from, and its position in the rock sequence. He was a regular rock detective! During Smith’s study of sedimentary rock layers, he also found there were certain fossils that seemed to be connected to specific layers. This fact helped him identify the layers and their most common order of deposition. Later, this fossil and sediment relationship became known as the Law of Faunal Succession.
The Law of Faunal Succession explains how fossil faunas and floras follow one another in a definite, identifiable order.
Geologists use the Law of Original Horizontality, the Principle of Stratigraphic Superposition, and Law of Faunal Succession to figure out the age, scattering, and order of different layers of sedimentary strata.
