The rocks on Earth are divided into igneous rocks, sedimentary rocks, and metamorphic rocks. These rocks are weathered by factors such as wind, rain, sun exposure, and waves, and the eroded debris, sand, and soil are carried by water into low-lying rivers, lakes, or oceans to form sediments. At the same time, chemical substances such as calcium and salt also deposit in rivers, lakes, and oceans. At that time, there were many animals and plants living on land, in lakes, and in the sea, and their dead bodies were also buried in the water along with various sediments. After a long period of dehydration, lithification, and fossilization, these sediments and biological remains formed sedimentary rock layers such as conglomerates, sandstones, sandy shales, shales, limestones, and salt layers, as well as fossils in these sedimentary rock layers. Layers of sedimentary rocks add up to form strata, just like the pages of a book.
The oldest strata on Earth may date back to 3.9 billion years ago. If a layer of sediment is deposited at a rate of 1–2 centimeters per year, the strata formed over hundreds of millions of years would exceed 10,000 meters in thickness. To study such a thick "book of the Earth," geologists need to divide the strata into "chapters" according to their formation ages, which are known as chronostratigraphic units and correspond strictly to geological eras. Strata formed during the same era can be classified into corresponding chronostratigraphic units. The entire history of the Earth is divided into the Precambrian and Phanerozoic eras, with the Phanerozoic divided into the Paleozoic, Mesozoic, and Cenozoic eras. The Paleozoic can be divided into the Early Paleozoic and Late Paleozoic, with the Early Paleozoic including the Cambrian, Ordovician, and Silurian periods, and the Late Paleozoic including the Devonian, Carboniferous, and Permian periods; the Mesozoic includes the Triassic, Jurassic, and Cretaceous periods; and the Cenozoic includes the Paleogene, Neogene, and Quaternary periods. The strata formed during each era are named after the era. The corresponding strata units of the eras, epochs, and periods are called eons, realms, systems, formations, and stages. For example, the Phanerozoic is called the Phanerozoic Eon, and the Paleozoic is called the Paleozoic Realm. The Cambrian is called the Cambrian Formation, and the Early, Middle, and Late Cambrian periods are correspondingly called the Lower Cambrian, Middle Cambrian, and Upper Cambrian Formations. The strata formed during the Devonian period are called the Devonian Formation, and the Early, Middle, and Late Devonian periods are correspondingly called the Lower Devonian, Middle Devonian, and Upper Devonian Formations. The strata formed during the Triassic period are called the Triassic Formation, and the Early, Middle, and Late Triassic periods are correspondingly called the Lower Triassic, Middle Triassic, and Upper Triassic Formations. The strata formed during the Jurassic period are called the Jurassic Formation, and the Early, Middle, and Late Jurassic periods are correspondingly called the Lower Jurassic, Middle Jurassic, and Upper Jurassic Formations. The strata formed during the Cretaceous period are called the Cretaceous Formation, and the Early, Middle, and Late Cretaceous periods are correspondingly called the Lower Cretaceous, Middle Cretaceous, and Upper Cretaceous Formations. The strata formed during the Paleogene period are called the Paleogene Formation, and the Early, Middle, and Late Paleogene periods are correspondingly called the Lower Paleogene, Middle Paleogene, and Upper Paleogene Formations. The strata formed during the Neogene period are called the Neogene Formation, and the Early, Middle, and Late Neogene periods are correspondingly called the Lower Neogene, Middle Neogene, and Upper Neogene Formations. The strata formed during the Quaternary period are called the Quaternary Formation, and the Early, Middle, and Late Quaternary periods are correspondingly called the Lower Quaternary, Middle Quaternary, and Upper Quaternary Formations. Due to the earlier development of geological research in Western countries, many geological era names were given by foreign scholars based on local geographical names, such as Cambrian, Ordovician, Silurian, and Devonian. Some geological era names were also named according to the characteristics of the strata, such as Carboniferous (this period's strata contain a lot of coal, and coal was formerly called Carbon), Permian (this period's strata are obviously divided into upper and lower parts, but now the three-part division is used), etc. In China, geological research started relatively late, but many Chinese strata contain a lot of fossils and are well-preserved, so some geological eras are also named by Chinese scholars. For example, the Late Permian is also called the Leping Formation (Formation), which is further divided into the Changxing Formation (Stage) and Wujiaping Formation (Stage), and the Late Cambrian is called the Furong Formation (Formation), under which there are the Jiangshan Formation (Stage) and Paiibi Formation (Stage).