Why do geologists correlate rock layers?

Within a small area, you can correlate the rocks of one locality with those of another by simply walking along the outcropping edges. However, this might not be possible when the rocks are covered by soil and vegetation. This problem can be corrected if you note the position of a distinctive rock layer in a sequence of strata. You may be able to identify a rock layer in another location if it's composed of very distinctive or uncommon minerals.