Answer to Question 1Rock types can be matched up on the basis of similar rock type and superposition, but correlation of
this type can be done only in a limited area where beds can be traced from site to site. To correlate
over larger areas or rocks of different composition, fossils must be used. Fossils are useful because
many organisms lived for a certain length of time. The overlapping geologic ranges of fossil species
allow definition of time intervals because each species is unique and had a definite duration.
Answer to Question 2No single location in a region has a geologic record of all events that occurred in its history;
correlation allows geologists to determine the complete geologic history of the region. The entire 2-
billion-year record of the Colorado Plateau is not preserved in a single location due to erosion. By
correlating the uppermost rocks at one location with the lowermost equivalent rocks of another area,
the entire geologic sequence can be put together
Discovering Fossils 11-30
Discovering Fossils 10-30
Earth’s history spans 4.5 billion years. Geologists and paleontologists have pieced together the his
Metamorphic Rocks
In plant fossils, stomatal density is used to estimate CO2 concentration in the atmosphere
Detrital Sedimentary Rocks