In this manner, glaciers scraped a vastness of geological history off the Earth’s continents and sideways into ocean basins, where plate tectonics could hide the evidence. They push immensities of accumulated ice down on landscapes and then spread sideways, pulverizing and scraping against adjacent landscapes, bulldozing their ways seaward through mountains and across hills and plains. The relevant fact about glaciers is that they don’t sit still. Every continent was a Greenland or an Antarctica.
A more accurate name for this time would be Glacierball Earth, because glaciers, all formed from accumulated snow, apparently covered most or all of Earth’s land during the Great Unconformity-and most or all of the seas as well. Lots of it: enough to turn the planet into one giant snowball, informally called Snowball Earth. Many theories attempt to explain what erased such a large span of Earth’s history, but the prevailing guess is perhaps best expressed in “Neoproterozoic glacial origin of the Great Unconformity”, published on the last day of 2018 by nine geologists writing for the National Academy of Sciences. That’s when the Cambrian period arrived and with it an amplitude of history, written in stone. Because of that unconformity, geology knows close to nothing about what happened in the world through stretches of time up to 1.6 billion years long.Īll of those absent records end abruptly with the Cambrian Explosion, which began about 541 million years ago. In the decades since Powell made his notes, the same gap has been found all over the world and is now called the Great Unconformity. Geology calls that nothing an unconformity. The walls of the Grand Canyon say nothing about what happened during that time. The answer turned out to be more than a billion years.
He didn’t know how much time had passed between basement and flooring, and could hardly guess. Observing this, he correctly assumed that the upper layers did not continue from the bottom one, because time had clearly passed between when the basement rock was beveled flat, against its own grain, and when the floors of rock above it were successively laid down. Floating down the Colorado River, he saw the canyon’s mile-thick layers of reddish sedimentary rock resting on a basement of gray non-sedimentary rock, the layers of which were cocked at an angle from the flatnesses of every layer above. John Wesley Powell recorded the greatest example of gone rock in 1869, on his expedition by boat through the Grand Canyon. In the library of Earth’s history, there are missing books, and within books there are missing chapters, written in rock that is now gone.
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