Rock relative dating to determine fault

This gap in the rock record of the Grand Canyon is known as “The Great Unconformity.” An unconformity is a break in the rock record caused by some interruption to deposition.Unconformities can happen for a variety of reasons.

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By the end of the 19th century, geologists had used these principles to put together an outline of the geological history of the world, and had defined and named the eons, eras, periods, and epochs of the geologic time scale.

They did not know how many thousands, millions, or billions of years ago the Cambrian period began, but they knew that it came after the Proterozoic Eon and before the Ordovician Period, and that the fossils unique to Cambrian rocks were younger than Proterozoic fossils and older than Ordovician ones.

In the Grand Canyon, for example, there is a layer of 1.7-billion-year-old igneous rock right under a layer of 550-million-year-old sedimentary rock.

What happened in the intervening 1.2 billion years? We just don’t, at the Grand Canyon, have a record of it.

Perhaps the sea levels retreated and what was once a perfect environment for deposition (shallow sea) was then several miles inland.

Or maybe uplift exposed once safely buried rock layers.

The geologic record is like a big puzzle that (frankly) is really fun to sort out, especially if you are a little OCD and like things to be in order (not that I’d know anything about that…).

Perhaps the biggest impediment to a neat and orderly rock record are our old friends weathering and erosion.

Recall that most fossils are housed within sedimentary rock, which results from the compaction and cementation of deposited sediments.

A principle of geology, called the states that in a series of undisturbed layers, any layer was deposited after the layers below it (that is, it’s younger than those layers) and before the layers above it (that is, it’s older than those layers).

In relatively close locations, the same actual rock layer may be present.

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