The law of superposition is an axiom that forms one of the bases of the sciences of geology, archaeology, and other fields pertaining to geological stratigraphy. Principle of cross-cutting relationships.Thus the principle of original horizontality is widely, but not universally, applicable in the study of sedimentology, stratigraphy, and structural geology. Also, sedimentary beds may pinch out along strike, implying that slight angles existed during their deposition. Similarly, sediments may drape over a pre-existing inclined surface: these sediments are usually deposited conformably to the pre-existing surface. This is known as the angle of repose, and a prime example is the surface of sand dunes. For instance, coarser grained sediments such as sand may be deposited at angles of up to 15 degrees, held up by the internal friction between grains which prevents them slumping to a lower angle without additional reworking or effort. However, it is now known that not all sedimentary layers are deposited purely horizontally. Īs one of Steno's Laws, the principle of original horizontality served well in the nascent days of geological science. Picture from Glen Canyon National Recreation Area, Utah.įrom these observations is derived the conclusion that the Earth has not been static and that great forces have been at work over long periods of time, further leading to the conclusions of the science of plate tectonics that movement and collisions of large plates of the Earth's crust is the cause of folded strata. From top to bottom: Rounded tan domes of the Navajo Sandstone, layered red Kayenta Formation, cliff-forming, vertically jointed, red Wingate Sandstone, slope-forming, purplish Chinle Formation, layered, lighter-red Moenkopi Formation, and white, layered Cutler Formation sandstone. These strata make up much of the famous prominent rock formations in widely spaced protected areas such as Capitol Reef National Park and Canyonlands National Park. The Permian through Jurassic stratigraphy of the Colorado Plateau area of southeastern Utah is a great example of Original Horizontality. It was first proposed by the Danish geological pioneer Nicholas Steno (1638–1686). The principle is important to the analysis of folded and tilted strata. Horizontal Strata/Beds.The principle of original horizontality states that layers of sediment are originally deposited horizontally under the action of gravity. East is to the right, south is at the bottom, and west is to the left. The “N” arrow indicates the North direction on this diagram. It shows what the surface of the Earth above these rocks looks like, as though we were looking down from space. You can rotate this view in the 3D model to better see the arrow.įigure 1B is the map view of the block diagram from Figure 1A. We do not specify which types of rocks form the layers in these cartoon diagrams. In a real cross section or geologic map, each layer is color coded to be a specific type of rock. The different colors represent layers or beds of different types of rock. The Safari browser is NOT compatible with the 3D models.įigure 1A is a block diagram of horizontal layers or strata. To view block diagrams, you will need to use a compatible brower. The Principle of Original Horizontality states: Layers of rocks deposited from above, such as sediments and lava flows, are originally laid down horizontally. The Principle of Superposition states : In an otherwise undisturbed sequence of sedimentary strata, or rock layers, the layers on the bottom are the oldest and layers above them are younger. Stratigraphy is the study of layered sedimentary rocks. In order to understand the structural diagrams we will use in this module, it is necessary to remember two principles of stratigraphy. 26 Original Horizontality and Superposition
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