Of course, all strata eventually end, either by hitting a geographic barrier, such as a ridge, or when the depositional process extends too far from its source, either a sediment source or a volcano. Principle of Lateral Continuity: Within the depositional basin, strata are continuous in all directions until they thin out at the edge of that basin. The exception to this principle is at the margins of basins, where the strata can slope slightly downward into the basin. Principle of Original Horizontality: Layers of rocks deposited from above, such as sediments and lava flows, are originally laid down horizontally. Principle of Superposition:In an otherwise undisturbed sequence of sedimentary strata, or rock layers, the layers on the bottom are the oldest and layers above them are younger. Lower strata are older than those lying on top of them. This section discusses principles of relative time used in all of geology, but are especially useful in stratigraphy. Stratigraphy is the study of layered sedimentary rocks. James Hutton (see Chapter 1) realized geologic processes are slow and his ideas on uniformitarianism (i.e., “the present is the key to the past”) provided a basis for interpreting rocks of the Earth using scientific principles. The principles of relative time are simple, even obvious now, but were not generally accepted by scholars until the scientific revolution of the 17th and 18th centuries. 7.1 Relative DatingĪ Geologic Time Scale Relative dating is the process of determining if one rock or geologic event is older or younger than another, without knowing their specific ages-i.e., how many years ago the object was formed. The specific events within Earth history are discussed in Chapter 8.
Using this time scale, geologists can place all events of Earth history in order without ever knowing their numerical ages. The figure of this geologic time scale shows the names of the units and subunits. Eventually he produced the first national geologic map of Britain, becoming known as “the Father of English Geology.” Nineteenth-century scientists developed a relative time scale using Steno’s principles, with names derived from the characteristics of the rocks in those areas. William Smith (1769-1839), working with the strata of English coal mines, noticed that strata and their sequence were consistent throughout the region. Nicolas Steno (1638-1686) introduced basic principles of stratigraphy, the study of layered rocks, in 1669. Working out Earth’s history depended on realizing some key principles of relative time. The geologic time scale and basic outline of Earth’s history were worked out long before we had any scientific means of assigning numerical age units, like years, to events of Earth history.