Fossil relative dating activity

The Law of Superposition, which states that older layers will be deeper in a site than more recent layers, was the summary outcome of 'relative dating' as observed in geology from the 17th century to the early 20th century.

The regular order of occurrence of fossils in rock layers was discovered around 1800 by William Smith.

For example, in sedimentary rocks, it is common for gravel from an older formation to be ripped up and included in a newer layer.

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These foreign bodies are picked up as magma or lava flows, and are incorporated, later to cool in the matrix.

As a result, xenoliths are older than the rock which contains them.

From top to bottom: Rounded tan domes of the Navajo Sandstone, layered red Kayenta Formation, cliff-forming, vertically jointed, red Wingate Sandstone, slope-forming, purplish Chinle Formation, layered, lighter-red Moenkopi Formation, and white, layered Cutler Formation sandstone.

Picture from Glen Canyon National Recreation Area, Utah.

This principle allows sedimentary layers to be viewed as a form of vertical time line, a partial or complete record of the time elapsed from deposition of the lowest layer to deposition of the highest bed.

The principle of faunal succession is based on the appearance of fossils in sedimentary rocks.

Sixteen years after his discovery, he published a geological map of England showing the rocks of different geologic time eras.

Methods for relative dating were developed when geology first emerged as a formal science.

While digging the Somerset Coal Canal in southwest England, he found that fossils were always in the same order in the rock layers.

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