unconformity A surface that represents a break in the geologic record, with the rock unit immediately above it being considerably younger than the rock beneath. East is to the right, south is at the bottom, and west is to the left. A principle or law stating that within a sequence of undisturbed sedimentary rocks, the oldest layers are on the bottom, the youngest on the top. 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. In an undisturbed sequence of sedimentary rocks, older rocks lie beneath younger rocks, according to the principle of superposition. 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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