Layer 4: Mudstone – sediments laid down in deep water. Which dating method is best for rocks with fossils in?Layer 5: Loess – a rock made from very fine wind-blown dust. Which dating method is best for rocks with feldspar crystals buried in dust?

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Conveniently, the vast majority of rocks exposed on the surface of Earth are less than a few hundred million years old, which corresponds to the time when there was abundant multicellular life here.

Look closely at the Geologic Time Scale chart, and you might notice that the first three columns don't even go back 600 million years.

When you talk about the Precambrian, Paleozoic, Mesozoic, and Cenozoic on Earth, or the Noachian, Hesperian, and Amazonian for Mars, these are all relative ages.

Relative-age time periods are what make up the Geologic Time Scale.

Just like a stack of sedimentary rocks, time is recorded in horizontal layers, with the oldest layer on the bottom, superposed by ever-younger layers, until you get to the most recent stuff on the tippy top.

On Earth, we have a very powerful method of relative age dating: fossil assemblages.

A few days ago, I wrote a post about the basins of the Moon -- a result of a trip down a rabbit hole of book research.

Here's the next step in that journey: the Geologic Time Scales of Earth and the Moon.

The Geologic Time Scale is up there with the Periodic Table of Elements as one of those iconic, almost talismanic scientific charts.

Long before I understood what any of it meant, I'd daydream in science class, staring at this chart, sounding out the names, wondering what those black-and-white bars meant, wondering what the colors meant, wondering why the divisions were so uneven, knowing it represented some kind of deep, meaningful, systematic organization of scientific knowledge, and hoping I'd have it all figured out one day.

You can help us choose the best absolute dating method for each rock. To learn more about the dating methods, select the image from the lab.