Correlation of rock layers
To develop a geologic time scale that is applicable to the entire Earth, rocks of similar age in different regions must be matched Such a task is referred to as correlation. Within a limited area, correlating the rocks of one with those of another may locality cropping be done simply by walking along the out edges. However, this might not be possible when the rocks are mostly concealed by soil and vegetation. Correlation over short distances is often achieved by noting the position of distinctive rock layer in a sequence of strata. Or, a layer may be identified in another location if it is composed of very distinctive or uncommon minerals. By correlating the rocks from one place to another, a more comprehensive view of the geologic history of a region is possible. Figure 11.11, for example, shows the correlation of strata at three sites the Colorado Plateau in southern Utah and northern Arizona. No single locale exhibits the entire sequence, but correlation reveals a more complete picture of the sedimentary rock record.