GEOLOGY

Detailed explanation-1: -Using the principle of faunal succession, if an unidentified fossil is found in the same rock layer as an index fossil, the two species must have existed during the same period of time (Figure 4). If the same index fossil is found in different areas, the strata in each area were likely deposited at the same time.

Detailed explanation-2: -Correlation can be determined by using magnetic polarity reversals (Chapter 2), rock types, unique rock sequences, or index fossils. There are four main types of correlation: stratigraphic, lithostratigraphic, chronostratigraphic, and biostratigraphic.

Detailed explanation-3: -The principle of fossil correlation states that the strata containing a group of fossils that are all the same age must be of similar age to the fossils. Strata are layers of rock, and each single layer is known as a stratum.

Detailed explanation-4: -Field geologists observe texture, hardness and composition of rocks to identify the layers they came from. Typically the harder and more densely packed the particles are, the older the rock and the deeper the layer it came from. Hardness can be tested with a simple fingernail or pocket tool.

Detailed explanation-5: -To correlate rock units, something distinctive must be present in each. This can include an index fossil, a unique rock type, a key bed, or a unique sequence of rocks. A key bed can be global. An example is the iridium layer that was deposited at the time of the Cretaceous-Tertiary extinctions.