Studying layered sedimentary rocks is precisely the point of geochronology. Geochronology, of course, is the scientific practice of studying rocks and fossils for the purpose of determining their age and, by extension, the age of the Earth. The more precise the determination of the lowest layers of sedimentary rock, which is believed to represent as much as 70 percent of the Earth’s surface, the more precise the estimate of the Earth’s age. Geochronology is also used in attempting to determine the age of other rock-based formations in the Solar System, particularly the moon, from which rock samples were extracted and brought home by astronauts during the period of Apollo moon landings. Astronomers and other scientists hope to capture an asteroid for the purpose of studying its composition and determining its age, as such knowledge will help in further studies of the broader universe.
Specific to layered sedimentary rocks, geologists long ago determined that studying such layers -- what is called the “superposition” of the rocks -- is the key to tracking the Earth’s evolution, including the ebb and flow of oceans, lakes and rivers, and the shifting of sands and other forms of sediment. While it only makes sense that the lower levels would date to the earliest periods of time, it was only through the science of geochronology and the study of layered sedimentary rocks that scientifically-derived conclusions were found.
In addition to the information available in the documents the links to which are provided below, a good source of information is the website of the Arizona Geochronology Center: www.geo.arizona.edu/agc/home.html.