#B. PRINCIPLE OF ORIGINAL HORIZONTALITY FULL#
#B. PRINCIPLE OF ORIGINAL HORIZONTALITY SERIES#

One of the first to question this time scale was a Scottish geologist named James Hutton (1726-1797). At the time, most Europeans believed that the Earth was around 6,000 years old, a figure that was based on the amount of time estimated for the events described in the Bible. Steno discovered the rules for determining the relative age of rock beds, but he did not have a good understanding of how long it would take for these rock formations to form. Weathering and erosion occurred, forming a layer of soil on top of layer A.Fault E formed, shifting rocks A through C and intrusion D.When layers A-B-C were present, intrusion D formed.

#B. PRINCIPLE OF ORIGINAL HORIZONTALITY FULL#

So the full sequence of events is as follows: Based on this, layer C is oldest, followed by B and A. The principle of superposition states that the oldest sedimentary rock units are at the bottom, and the youngest are at the top.

The intrusion (D) cuts through the three sedimentary rock layers, so it must be younger than those layers. So the fault must be the youngest formation that is seen. The fault labeled ‘E’ cuts through all three sedimentary rock layers (A, B, and C) and also cuts through the intrusion (D). The principle of cross-cutting relationships states that a fault or intrusion is younger than the rocks that it cuts through. Based on the principle of cross-cutting relationships, the river must be younger than all of the rock layers that it cuts through.įigure 11.14: Cross-section of sedimentary layers: (A-C) igneous intrusion, (D) cross-section, (E) fault. Finally, the Colorado River cuts through all the layers of sedimentary rock to form the canyon. We know these rock layers were once connected, which is described in the rule of lateral continuity. Distinctive rock layers, such as the Kaibab Limestone, can be matched across the broad expanse of the canyon. The youngest rock layers are at the top of the canyon, while the oldest are at the bottom, which is described by the law of superposition. 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.

#B. PRINCIPLE OF ORIGINAL HORIZONTALITY SERIES#

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.įigure 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. Figure 11.10: Tyrannosaurus rex fossil resembling a living organism.

B. principle of original horizontality