Law of fossil succession strata dating

05-Dec-2015 00:54 by 4 Comments

Law of fossil succession strata dating

Prior to the late 17th century, geologic time was thought to be the same as historical time.Archbishop James Ussher of Armagh, Ireland, 1654, added up generations from the Old Testament and determined that Earth formed on October 23, 4004 BCE.

This principle is often stated as "the present is the key to the past".

Thus, in a sequence of layers that have not been overturned by a later deformational event, the oldest layer will be on the bottom and the youngest layer on top.

This is the same principle used to determine relative age in the trash pits discussed previously.

The Grand Canyon is particularly good for this because different sedimentary rocks have different colors.

For example, the mudstone, sandstone and shale are cut by the basalt dike, so we know that the mudstone, sandstone, and shale had to be present before the intrusion of the basalt dike.

Sediment deposited on steep slopes will be washed away before it is buried and lithified to become sedimentary rock, but sediment deposited in nearly horizontal layers can be buried and lithified.

Thus rocks that we now see inclined or folded have been disturbed since their original deposition.

Principles of Stratigraphy Stratigraphy is the study of strata (sedimentary layers) in the Earth's crust.

Geologist in the 1800s worked out 7 basic principles of stratigraphy that allowed them, and now us, to work out the relative ages of rocks.

Principle of Original Continuity If layers are deposited horizontally over the sea floor, then they would be expected to be laterally continuous over some distance.

Thus, if the strata are later uplifted and then cut by a canyon, we know that the same strata would be expected to occur on both sides of the canyon.

Principle of Uniformitarianism The principle of Uniformitarianism was postulated by James Hutton (1726-1797) who examined rocks in Scotland and noted that features like mudcracks, ripple marks, graded bedding, etc.