![]() The principle of Uniformitarianism states that the geologic processes observed in operation that modify the Earth's crust at present have worked in much the same way over geologic time. Geologists still use the following principles today as a means to provide information about geologic history and the timing of geologic events. Methods for relative dating were developed when geology first emerged as a natural science in the 18th century. Sixteen years after his discovery, he published a geological map of England showing the rocks of different geologic time eras. Due to that discovery, Smith was able to recognize the order that the rocks were formed. He also found that certain animals were in only certain layers and that they were in the same layers all across England. As he continued his job as a surveyor, he found the same patterns across England. ![]() While digging the Somerset Coal Canal in southwest England, he found that fossils were always in the same order in the rock layers. The regular order of the occurrence of fossils in rock layers was discovered around 1800 by William Smith. The Law of Superposition, which states that older layers will be deeper in a site than more recent layers, was the summary outcome of 'relative dating' as observed in geology from the 17th century to the early 20th century. Relative dating by biostratigraphy is the preferred method in paleontology and is, in some respects, more accurate. Though relative dating can only determine the sequential order in which a series of events occurred, not when they occurred, it remains a useful technique. Prior to the discovery of radiometric dating in the early 20th century, which provided a means of absolute dating, archaeologists and geologists used relative dating to determine ages of materials. ![]() In geology, rock or superficial deposits, fossils and lithologies can be used to correlate one stratigraphic column with another. Relative dating is the science of determining the relative order of past events (i.e., the age of an object in comparison to another), without necessarily determining their absolute age (i.e., estimated age). Photo from Glen Canyon National Recreation Area, Utah. 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 and the Law of Superposition, two important ideas used in relative dating. For the relative dating of words and sound in languages, see Historical linguistics.
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