| Themes > Science > Paleontology / Paleozoology > About Paleontology / Paleozoology, Generalities > Geologic Time > Correlation |
One apparently obvious way to correlate (tie together) these local sequences might be to trace the layers exposed in our outcrop laterally and thus directly link them to the equivalent layers in other, nearby local sequences. If the layers are continuous, this might even work for short distances. Unfortunately, by tracing out our layers laterally, we introduce other problems. First of all, rock layers do not stay constant in character over long distances. Rocks are a reflection of their environment of formation. Because there are many different environments accumulating sediments at any given moment in time, it stands to reason that we will also form many different types of rocks at any given time and that, therefore rocks of the same age will change in appearance over distance (facies). Secondly, the same rock layer will only rarely be of the same age in different places. If, for instance, we were to find a deposit of beach sand covering several thousand square miles, it would stand to reason that there was no beach of that size in the past, just as there is none that size today. A much more likely explanation is that the beach migrated over time in response to sea level changes, and that the rock layers (deposits) associated with this beach were deposited in different areas at different times. Reliance on lateral continuity for equivalence of time is more likely to lead to false interpretations than to correct ones. To correlate these sequences we need distinctive unique time markers which will enable us not only to tie things of similar ages together, but also to compare and arrange sequences which are not in the same area in chronological order. Occasionally there are events which are distinct, unique, and affect such large areas as to provide us with an almost instantaneous time marker. An example might be a bed of volcanic ash that settled over a large area as a result of a volcanic explosion. Such marker beds, as they are called, do exist, and are extremely useful. Unfortunately, they are rare, random, and non-universal events and therefore have only limited application in a history which spans nearly five billion years. Only ONE record in all of geologic time has been continuous and ever changing while being common enough to have left traces over virtually the entire earth and that is the record of life, as represented by fossils. |
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