Frozen Earth: The Once and Future Story of Ice Ages by Doug Macdougall Page A

continents; however, it was generally agreed that on each continent, the glacial deposits recorded the same series of cold and warm episodes.The changes in ice age climate had been global, or at least they had affected widely separatedparts of the Northern Hemisphere similarly.We now know that the glacial periods identified by mapping their deposits were only the last few of a long string of cold and warm cycles stretching back several million years.This knowledge comes not from studies on land, but rather from evidence of a quite different type contained in deep-sea sediment cores.On land, the evidence for the earlier glacial cycles has been almost completely obliterated by the more recent ones, but in the oceans each layer of sediment buries and preserves the ones that preceded it.
    How did the early investigators, without the help of radioactive dating methods, conclude that long time intervals separated the glacial periods?It was a task that required a certain amount of ingenuity.In many localities, it was fairly straightforward to use the principle of superposition to determine that there had been several different glacial advances.In places, younger drift could be observed deposited on top of earlier glacier debris, and in other localities, older moraines had been broken through and partly scoured away by more recent glaciers, which had deposited their own debris far beyond.However, determining just how much time had elapsed between these various events was a difficult problem.An important clue was that between successive glacial advances, soil had developed on the moraines and drift deposits.Soil forms anywhere rocks are exposed to rain—water is an effective solvent, and it also promotes chemical reactions with the minerals contained in rocks.The result is that solid rock dissolves and crumbles and is transformed into the soft clay of soil.Plants, insects, and microbes appear, churning the soil, facilitating even more chemical reactions and adding organic matter.In tropical climates with heavy rainfall, soils have been observed to form on fresh volcanic lava flows within a few hundred years or even less.But in the colder regions from which the ice age glaciers retreated, soils formed much more slowly.Soil layers that developed on moraines and drift between glacial advances indicate that the cycles were separated by relatively long periods of moderate climate.Fossils of plants and animals in the soil paint a similar picture.The interglacial periods were long enough for there to be a completechange of fauna and flora, and the new species were characteristic of temperate rather than arctic regions.When radioactive dating methods became available, it was discovered that through the last six or seven glacial cycles, the times of maximum ice advance were separated by roughly one hundred thousand years, and the warmer periods, with temperatures similar to today’s, lasted ten to twenty thousand years.
    When temperatures rise above freezing, large amounts of meltwater flow across the surfaces of glaciers, along their edges, at their bases, and even through the ice itself.During one of his field sessions on an Alpine glacier, the ever-curious Louis Agassiz had himself lowered down an almost vertical tunnel that had been cut by summer meltwater.It was one of his more foolhardy experiments; the tunnel got narrower and narrower, and eventually bifurcated, and Agassiz lost voice contact with his colleagues on the surface.They kept lowering him, right into an icy torrent deep in the glacier.Fortunately for Agassiz, the glacier-bound stream wasn’t very deep, and eventually his friends became concerned and hauled him up.But it could have been much worse—the amounts of water coursing through melting glaciers can sometimes be huge.
    The meltwater flowing away from a glacier carries with it grains and fragments of rock that were originally embedded in the ice.Ice is indiscriminate about what it carries, but the running water is