Physical Geology Chapter 6 The Rock Record

Uniformitarianism Is a theory that rejects the idea that catastrophic forces were responsible for the current conditions on the Earth. The theory suggested instead, that continuing uniformity of existing processes were responsible for the present and past conditions of this planet.theory catastrophic

In the mid-seventeenth century, biblical scholar and Archbishop James Ussher determined that the earth had been created in the year 4004 BCE. Just over a century later James Hutton, known as the father of geology, suggested that the earth was much older and that processes occurring in the present were the same processes that had operated in the past, and would be the processes that operate in the future. This concept became known as uniformitarianism and can be summarized by the phrase "the present is the key to the past."James Ussher geology

Sedimentary rocks are formed particle by particle and bed by bed, and the layers are piled one on top of the other. Thus, in any sequence of layered rocks, a given bed must be older than any bed on top of it. This Law of Superposition is fundamental to the interpretation of Earth history, because at any one location it indicates the relative ages of rock layers and the fossils in them.

Very simply put …………..

Law of Original Horizontality states that most sediments, when originally formed, were laid down horizontally. However, many layered rocks are no longer horizontal. Because of the Law of Original Horizontality, we know that sedimentary rocks that are not horizontal either were formed in special ways or, more often, were moved from their horizontal position by later events, such as tilting during episodes of mountain building.

If not horizontal, look for clues to original position of layers and then apply the law of superposition

Clues Graded beds – beds of decreasing sized sediment particles Cross beds – curved shaped beds at an angle to the bedding plane Ripple marks – peaks of ripples oriented upward

Unconformities disconformity Nonconformity Angular unconformity Breaks in the geologic record

Crosscutting relationships A fault or igneous intrusion is always younger than the rock layers it cuts through If through an unconformity, it is younger than all above and below the unconformity

Radioactive elements have half-lives # of half lives may be calculated Years since formation of rock or strata may then be calculated

Absolute Dating Methods

The Fossil Record Fossil – remains of organisms Index fossil – occur only in and are identified with specific strata Paleontology – study of fossils Sedimentary rock – “home” of fossils

How about…….?

Or……?

Dinosaur tracks burrows Worm tubes

Index Fossils lived for a short geologic time but over a large area so are indicative of a particular era in geologic time

Index Fossil example – Indicative of the Silurian and Ordovician periods