Comment: 10:14 - 11:49 (01:35)
Source: Annenberg/CPB Resources - Earth Revealed - 17. Sedimentary Rocks: The Key to Past Environments
Keywords: "James Sadd", "sedimentary rock", "clastic sediment", geologist, "Principle of Uniformity", "geologic time", weathering, water, wind, tide, "sea level change", lithification, deposition, "Earth's surface", limestone, "calcium carbonate", mud, quartz, sand, fossil, "tropical sea", Bahamas, "Florida Keys"
Our transcription: Geologists interpret the characteristics of sedimentary rock using the "Principle of Uniformity."
This principle is a model of the way sedimentary rocks form.
According to this model, we accept that sedimentary rocks have formed throughout geologic time in exactly the same way that sediments are forming today.
Throughout Earth history the same geologic processes, such as weathering, running water, wind, tides, changing sea level, have created and deposited sediments that eventually were hardened to rock.
The Principle of Uniformity says that the present is the key to the past, so as we study sediments being deposited right now and associate their characteristic with the conditions at the Earth's surface, we're creating a model for interpreting environments in the geologic past.
This principle can be very easy to use.
This limestone, for example, is made of gray, calcium carbonate mud, a little quartz sand, and the fossils of marine organisms.
Sediments being deposited in shallow tropical seas, such as the Bahamas and the Florida Keys, would look very much like this limestone, if they were hardened into rock.
So in this case, uniformity is telling us that this area was covered by a shallow tropical sea about 250 million years ago.
When geologists used the Principle of Uniformity to analyze an entire sequence of sedimentary rock, the changing environments recorded in that sequence of rocks shows us how the Earth's surface itself changed and evolved through time.
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