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Geologic Time Chapter Prepared by Iggy Isiorho for Dr Isiorho Time and Geology The Key to the Past Uniformitarianism Principle that geologic processes operating at present are the same processes that operated in the past The principle is stated more succinctly as The present is the key to the past Actualism The principle that the same processes and natural laws that operated in the past are those we can actually observe or infer from observations as operating at present Under present usage uniformitarianism has the same meaning as actualism for most geologists Numerical age Age given in years or some other unit of time Relative time The sequence in which events took place not measured in time units Relative Time The geology of an area may seem at first glance to be hopelessly complex A non geologist might think it impossible to decipher the sequence of events that created such a geologic pattern However a geologist has learned to approach seemingly formidable problems by breaking them down to a number of simple problems As an example the geology of Grand Canyon can be analyzed in four parts 1 horizontal layers of rock 2 inclined layers 3 rock underlying the inclined layers plutonic and metamorphic rock and 4 the canyon itself care into these rocks Principles Used to Determine Relative Age Original Horizontality Superposition Lateral continuity Cross cutting relationships Original Horizontality Original Horizontality The deposition of most water laid sediment in horizontal or near horizontal layers that are essentially parallel to the earth s surface Back Superposition Superposition A principle or law stating that within a sequence of undisturbed sedimentary rocks the oldest layers are on the bottom the youngest on the top Back Lateral Continuity Lateral Continuity Principle that states that an original sedimentary layer extends laterally until it tapers or thins at its edge Back Cross cutting Relationships Cross cutting Relationships A principle or law stating that



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