Geochronological dating website
However, note that because of the "principle of cross-cutting relationships", careful examination of the contact between the cave infill and the surrounding rock will reveal the true relative age relationships, as will the "principle of inclusion" if fragments of the surrounding rock are found within the infill.Cave deposits also often have distinctive structures of their own (e.g., spelothems like stalactites and stalagmites), so it is not likely that someone could mistake them for a successional sequence of rock units. Each of them is a testable hypothesis about the relationships between rock units and their characteristics.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.These are often characterised as the norm, rather than the exception.
In more complicated situations, like in a mountain belt, there are often faults, folds, and other structural complications that have deformed and "chopped up" the original stratigraphy.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).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.
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The simplest situation for a geologist is a "layer cake" succession of sedimentary or extrusive igneous rock units arranged in nearly horizontal layers.