26 Original Horizontality and Superposition

Elizabeth Johnson

In order to understand the structural diagrams we will use in this module, it is necessary to remember two principles of stratigraphy.

Stratigraphy is the study of layered sedimentary rocks.

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.

The Principle of Original Horizontality states: Layers of rocks deposited from above, such as sediments and lava flows, are originally laid down horizontally.

How to use 3d Block Diagrams

To view block diagrams, you will need to use a compatible brower.  Chrome is free and a good option.  The Safari browser is NOT compatible with the 3D models.

Figure 1A is a block diagram of horizontal layers or strata.  The different colors represent layers or beds of different types of rock.  In a real cross section or geologic map, each layer is color coded to be a specific type of rock.  We do not specify which types of rocks form the layers in these cartoon diagrams.  The “N” arrow indicates the North direction on this diagram.  The arrow is pointing towards the north.  You can rotate this view in the 3D model to better see the arrow.

Figure 1B is the map view of the block diagram from Figure 1A.  It shows what the surface of the Earth above these rocks looks like, as though we were looking down from space. The “N” arrow indicates the North direction on this diagram.  North is “up” in this view.  East is to the right, south is at the bottom, and west is to the left.

Figure 1. Horizontal Strata/Beds.

Link to the interactive 3D model of Figure 1 (Please do NOT use the Safari browser to view interactive diagrams- it won’t work):


http://opengeology.org/textbook/7-geologic-time/ CC-BY_SA


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Physical Geology Laboratory Copyright © by Elizabeth Johnson is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License, except where otherwise noted.

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