The principle of superposition therefore has a clear implication for the age of a vertical succession of strata.
There are situations where it potentially fails -- for example, in cave deposits.
For example, wave ripples have their pointed crests on the "up" side, and more rounded troughs on the "down" side.
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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Much of the Earth's geology consists of successional layers of different rock types, piled one on top of another.
A few principles were recognized and specified later.
An early summary of them is found in Charles Lyell's .
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).
The layers of rock are known as "strata", and the study of their succession is known as "stratigraphy".