This nicely illustrates the principle of original horizontality. Figure 11.13 shows the many horizontal layers of sedimentary rock that make up the canyon. The Grand Canyon provides an excellent illustration of Steno’s laws. For example, if an igneous intrusion goes through a series of metamorphic rocks, the intrusion must be younger than the metamorphic rocks that it cuts through (Figure 11.12). The granite must be younger than the gneiss, because it cuts across the existing gneiss.Ī rock formation or surface that cuts across other rock layers is younger than the rock layers it disturbs. As in the Tyrannosaurus rex Figure 11.10, fossils resemble living organisms.Ĭross-Cutting Relationships Figure 11.12: Cross cutting relationships: older banded gneiss with a white granite intrusion. He then sought to explain how fossil seashells could be found in rocks far from any ocean. Instead of invoking supernatural forces to explain fossils, Steno concluded that fossils were once parts of living creatures. For Steno, the close resemblance between fossils and modern organisms was impossible to ignore. In other words, fossil shells, bones, and teeth were never a part of a living creature! Seeking an alternate explanation, other writers proposed that the fossils had formed within the rocks as a result of mysterious forces. But this explanation could not account for the fact that fossils were not only found on mountains, but also within mountains, in rocks that had been quarried from deep below Earth’s surface. Some religious writers believed that the shells were washed up during the Biblical flood. Two schools of thought explained these fossils. The reason was that the fossils of clams, snails, and other marine animals were found in tall mountains, miles from any ocean. While it may seem obvious today, most people at the time did not believe that fossils were once part of living creatures. \)įigure 11.9: Fossil shark tooth (left) and modern shark teeth (right).įigure 11.10: Tyrannosaurus rex fossil resembling a living organism.
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