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 possi- ble. Figure 11.11, for example, shows the correlation of strata at three sites the Colorado Plateau in southern Utah and north ern Arizona. No single locale exhibits the entire sequence, but correlation reveals a more complete picture of the sedimentary rock record. Many geologic studies involve relatively small areas. Such studies are important in their own right, but their full value