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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.

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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.

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.

For example, the principle of superposition is based, fundamentally, on gravity.

In order for a layer of material to be deposited, something has to be beneath it to support it.

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.

Much of the Earth's geology consists of successional layers of different rock types, piled one on top of another.

The most common rocks observed in this form are sedimentary rocks (derived from what were formerly sediments), and extrusive igneous rocks (e.g., lavas, volcanic ash, and other formerly molten rocks extruded onto the Earth's surface).

Many other indicators are commonly present, including ones that can even tell you the angle of the depositional surface at the time ("geopetal structures"), "assuming" that gravity was "down" at the time, which isn't much of an assumption :-).

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