![]() Image of rocks: three are cut, the rocks on top are not Walther's Law By carefully examining which rock units are cut by faults or intrusions, or which rock units have been weathered, geologists can further determine the relative ages of rocks. Similarly to the principle of superposition, a rock must already be in place to be cut by a fault, igneous intrusion or erosion. A, b, c, d, e, f Cross-Cutting Relationships Knowing this, geologists can figure out the relative ages of rocks on top of each other. In order to deposit a sandstone on top of a limestone, the limestone has to already be there. If the table isn’t already there and you put the book down, it falls to the floor (and note! The floor had to be there for the book to land on it.). A simple way to think about this is that for something to be on top of something else, for example in order to put a book on top of a table, the table has to be there. This principle states that a sequence of rocks in their original orientation will have the oldest rock on the bottom and the youngest rock on the top. The rocks remain horizontal until a force acts on them, pushing (or pulling) them out of their original orientation. More sediment is deposited on top, and over time the whole sequence lithifies (sort of like the jello did in the fridge). As water moves sediment from high regions, like mountains, to low regions, like the ocean, the energy of the system decreases until the sediments are deposited in a basin, like a lake or an ocean. This is similar to how sedimentary rocks form. Now imagine that you have a jello mixture in the bowl - if you chill it and it solidifies, and then pour a different color on top, You have the two flat layers of jello, one on top of the other. If you dump that water into a bowl, the surface remains flat. The surface of the water is perfectly flat - horizontal. This is sometimes easier to envision with liquids: imagine pouring water into a cup. The principle of original horizontality states that sediment is deposited horizontally. Nooreen Meghani 2015 Original Horizontality
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