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

much grander scale.In places they can be traced for hundreds of kilometers, winding through the countryside and marking an ancient glacial boundary.But the ice sheets of the Pleistocene Ice Age didn’t deposit their rubbly burden only as terminal moraines, easily recognized by their ridgelike shape.Some of that material was simply scattered across the landscape as a layer of gravel and boulders without any particular form.Sometimes the drift was shaped by the moving ice into features such as strange teardrop-shaped hills called drumlins, which usually occur in swarms, lined up parallel with one another.Exactly how drumlins form is unclear, but they apparently take shape beneath the flowing ice, their orientation reflecting the direction of ice movement.In other places, drift occurs as long, sinuous ridges of sand and gravel called eskers, which have occasionally been put to use as beds for railway lines in lowlying marshy areas.Eskers are thought to be essentially “negative streams”—rocky material built up in a confined stream that flowed beneath a glacier.When the glacier finally melted away, they were left standing above the surrounding countryside.
    Starting soon after Agassiz published his Études sur les glaciers, geologists began to map out these features wherever they existed.A primary goal of this mapping was to determine the extent of the ice age glaciers, another to discover how they had flowed.Even now, details are being added to the general picture, which emerged quite quickly.It has become clear that the ice age glaciers did not form a single, gigantic ice sheet that extended southward from the North Pole, as Agassiz and his supporters had initially assumed.Instead, there were centers of ice accumulation, located where temperatures were low and the snow supply was ample.In North America alone, there were several centers of thick ice accumulation, with ice flowing out in all directions and inplaces coalescing with the glaciers of other centers.But some parts of the far north—for example, parts of Alaska—had no glaciers at all, even during the coldest part of the ice age, because of low snowfall.
    It was the mapping that revealed the multiple glacial episodes of the Pleistocene Ice Age.There is an exceedingly simple but very powerful concept in geology, first formalized in the 1600s and still taught to beginning students in the earth sciences: any geological feature that cuts into or across another is younger than the one it cuts across, and any material deposited on top of something else is younger than the underlying material.To beginning geologists, it often seems silly to formalize such a commonsense principle, yet even quite complex sequences of geological events can often be unraveled by applying this concept.It has been used for everything from exploration for oil to working out the cratering history of the moon.When it was applied to the moraines and other deposits left by glaciers of the Pleistocene Ice Age, it showed that there had been several distinct glacial episodes, separated from one another by significant amounts of time.
    The principle of superposition, as the concept just described is sometimes called, provides information about relative time—one deposit is older than another, or some process occurred before another—but not absolute time in years.That only became possible more than a century after the ice age theory was proposed, after the discovery of radioactivity and the development of techniques that used radioactivity for dating.But even in the nineteenth century, geologists were able to determine that there had been at least three and perhaps as many as five separate expansions of ice far south into Europe and North America during the Pleistocene Ice Age, and that these had been separated by long periods of time with much warmer climates.European and North American scientists gave these episodes different names, and it was not possible to correlate them precisely between