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They are applied by geologists in the same sense that a "null hypothesis" is in statistics -- not necessarily correct, just testable.
In the last 200 or more years of their application, they are valid, but geologists do not assume they are.
They are the "initial working hypotheses" to be tested further by data.
This document is partly based on a prior posting composed in reply to Ted Holden.
My thanks to both him and other critics for motivating me.
There are situations where it potentially fails -- for example, in cave deposits.
In this situation, the cave contents are younger than both the bedrock below the cave and the suspended roof above.
The layers of rock are known as "strata", and the study of their succession is known as "stratigraphy".
Fundamental to stratigraphy are a set of simple principles, based on elementary geometry, empirical observation of the way these rocks are deposited today, and gravity.
This orientation is not an assumption, because in virtually all situations, it is also possible to determine the original "way up" in the stratigraphic succession from "way up indicators".
For example, wave ripples have their pointed crests on the "up" side, and more rounded troughs on the "down" side.
The simplest situation for a geologist is a "layer cake" succession of sedimentary or extrusive igneous rock units arranged in nearly horizontal layers.Tags: Adult Dating, affair dating, sex dating