Are We a Match? Correlation is the process used by geologists to "match" rock layers and formations of one outcrop to rock layers and formations of another outcrop that are of similar age. Correlation is done by matching rock layers by rock type, grain size, the presence of fossils and other distinctive features. By interpreting the sequence of geologic events in a single outcrop, a geologist can determine the geologic history of that single locale. By interpreting the sequence of geologic events in a nearby outcrop and then correlating the rock layers between the two outcrops the geologist can infer the geologic history of a larger area. By continuously correlating the rock layers and formations from one outcrop to the next, it is possible to develop a complete geologic history of all of the exposed bedrock. Methods Used To Correlate Walking the Outcrop- The most direct method for observing a correlation between rocks or events at separate location is by actually following a particular layer or formation from one location to the other. In this case you actually observe the physical continuity of the structure in question. Walking the outcrop can be done across valleys; or other very geographically close regions. Similarity of Rock- Another method of correlation is the direct observation of similarities between layers of rock in different locations. Suppose you are examining an outcrop and notice three layers of rock with particular colors, composition, etc., and in a certain order. In a nearby location you observe three layers of similar color and composition and in the same order. From this evidence you could infer that the rock is the same at both locations. In other words, there is a correlation between the two rocks. Index Fossils-Fossils are the remains or impressions of ancient plants and animals. They are usually found in sedimentary rock. Geologists have developed a system for correlating sedimentary rock based on the presence of particular fossils, known as index fossils. An index fossil is the remains or imprint of a particular type of plant or animal that existed for a relatively short period of time but which found in many different parts of the world. If you think about it, you'll realize why these last two points are important. If the organism existed unchanged for many hundreds of millions of years, its presence would be of no use in dating rock because it would be found in rock layers of many different ages. If the organism were found only in a few, specific locations, it would be of no use in correlating rock from widely separated regions. Volcanic Ash Deposits-When a volcano erupts, large quantities of volcanic ash are shot into the air. This ash, which consists of small pieces of igneous rock, may be carried for great distances before if falls to earth. So ash from a single eruption may be deposited over large areas of the earth's surface over a short period of time. If these layers of volcanic ash can be identified in rock formation at different location, they can be used for correlation. Deposits of volcanic ash are good "time markers", because they were deposited over such a short time period. The letters in the outcrop diagrams represent index fossils. Correlate the rock layers using the index fossils and the ash deposit. List the index fossils and the ash deposit from oldest to youngest. hat did "The Pale Blue general etc? Use infor onse) u expect to learn onomy? #1 V #2 Oldest U T U o ash X о ash X Youngest 1) Why is the T index fossil missing in outcrop #1?