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Mapping Deep Time: About Geologic Time


1854 penny

American large cent, 1854

Photo (c) Andrew Alden, licensed to About.com (fair use policy)

Geologists are notorious for the way they talk about time—millions of years for this, billions of years for that. The numbers are huge, incomprehensible, ridiculous! How did they get so unmoored from the present?

From Centuries to Megamillennia

It's a stretch to grasp a mere century, let alone more time. For instance, my grandmother was born about 100 years ago. Measuring my own life against hers, I can sometimes get a sense of how long a century is.

Near my desk I have an old American penny dated 1854; perhaps Grandma's own grandmother once had it in her pocket when she was a young girl. Maybe she told Grandma that in 1854, she heard an old woman talk of seeing George Washington with her own eyes in 1776.

You can't get much further back in time through direct oral transmission. Yet beyond lie thousands of years of recorded human history—and if not for written records, only ruins and a few rare antiques and artifacts would testify that real people lived real lives back then. The only historians would be archaeologists.

Geologic time is vastly greater, so much greater that human history is invisible in comparison. There's a passage in H. G. Wells's The Time Machine that describes the time traveler accelerating into the far future. At first he sees days and nights flash past one by one, then flicker and blend into gray, until the buildings and the very landscape around him fade into a silent, timeless nothingness.

Deep Time Discovered

James Hutton, the father of modern geology, must have felt something like that sensation in the 1780s as he pondered his years of observations in the Scottish countryside. His geologic evidence showed that whole ranges of mountains had eroded grain by grain, with unimaginable slowness, into the sea. There their sediment was gently covered with sediments from younger mountain ranges, turned to rock, raised again into new ranges and eroded again. This had happened over and over. And even the youngest mountains were old.

In the record of the rocks, he wrote, "there is no vestige of a beginning—no prospect of an end." Hutton had discovered deep time.

Geologists after Hutton began to place a few signposts in that endless record by picking out areas where the rock strata had certain distinctive sets of fossils in them. By mapping which rocks lay above and which below, they gradually established a time line, a chronology for parts of the record. No one could tell the true ages of rocks, only their relative ages.

Geologic Ages

Geologic age was a new concept suited for deep time. Imagine that American coins had no dates, only their particular designs, to tell their age—"Now this ruin dates from the Buffalo Nickel Era in the Age of Broken Glass," we might say. Likewise, all that geologists could do was to give names to the rock layers and the ages they represented, and figure out their proper order.

One of the first formally named geologic time periods was the Cambrian Period, named by Adam Sedgwick using the ancient Roman name for Wales, where these rocks are well exposed. The Jurassic was another, named for the Jura mountains by Alexander von Humboldt. Today we have names for every part of geologic history from beginning to end—a fearful jumble, a jigsaw puzzle of names, called stratigraphy.

Perhaps you've taken a geology course and learned, then forgotten, the eons, eras, periods, epochs, and ages of Earth history. Here's a secret—nobody knows them all. There are too many, and there's no simple system, no numbered centuries. The geologic record is often likened to the pages of a large book that has been torn apart, scattered, scorched, and crumpled. And there are no page numbers. It's names all the way down.

Consider, let's say, a body of limestone on the French-Swiss border, in the Jura Mountains. The fossils in it might show that it formed in the Jurassic Period of the Mesozoic Era, more specifically in the Bathonian Age of the Middle Jurassic Epoch. That label sums up a large amount of information—a particular arrangement of the continents, a specific assemblage of living things, a distinctive climate pattern. But on the whole we have only a general description of the 3 million years of the Bathonian Age. When it comes to really envisioning the land in detail, the French countryside at that time, we still aren't much better off than Wells's man in the time machine. (But you can see paleogeographic maps with some detail.)

For hundreds of years, Earth scientists have lived in this welter of names without knowing any of the numbers. Nowadays the numbers are coming, thanks to advances in age dating that allow us to measure deep time. Earth, it turns out after generations of work, really is billions of years old. But we'll always need the names—probably even more of them.

PS: How do geologists keep those names in the right order? With mnemonics like "Put Eggs On My Plate Please Honey."

Recent books on geologic time include:

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