![]() Different types of sedimentary rock form in different depositional environments, so geologists can often figure out what existed at a particular place millions of years ago. A depositional environment is simply a place sediment can be deposited. ![]() If two things are right next to each other, they are laterally adjacent. Let’s break it down, starting with laterally adjacent and depositional environments. If there is something missing, there is missing time, or an unconformity. Walther’s law states that depositional environments that are laterally adjacent on the surface of the earth will also appear in succession in a stratigraphic sequence. Instead of dealing only with relative time, Walther’s law deals with relative space through time. Walther’s law is a little different from the previously discussed geologic principles, but it is justĪs important. 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. Other similar relationships include faults being younger than the rock layers they cut and erosional surfaces being younger than the rocks they erode.Nooreen Meghani 2015 Original Horizontality Rather it was intruded as hot magma into the surrounding sedimentary rocks long after they were deposited (Eicher 1976). Hutton showed from the super-heated contacts below and above, and from places where the basalt actually invaded underlying and overlying beds, that the thick basalt body was not merely a flow that had formed in sequence. For example, Salisbury Crag, a prominent Edinburgh landmark known to Hutton, owes its relief to a thick sheet of resistant basalt. ![]() In short an intrusive rock body is younger than the rocks it intrudes. James Hutton’s observations related to uniformitarianism also serve as the basis for another important geologic principle called cross-cutting relationships, which is a technique used in relative age dating. Black Canyon of the Gunnison National Park, Colorado. The light colored dikes are younger than the dark rock. Pegmatite dikes exposed in Painted Wall cliff face. ![]()
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