Frozen Earth: The Once and Future Story of Ice Ages (10 page)

Figure 4.
A large erratic boulder in a field near Örebro, Sweden.
Boulders like this one, very different in makeup from other rocks in the vicinity and much too heavy to have been carried by water, convinced Louis Agassiz that large tracts of Europe had once been covered by thick, flowing ice that carried the boulders far from their place of origin.
Note the woman to the right of the erratic for scale.
A boulder of this size probably weighs close to ten thousand metric tons.
Photograph copyright Dr.
John Shelton.

Before the ice age theory was generally accepted, most geologists and naturalists argued that the erratics had been transported by water to their current resting places.
They realized that even fairly small boulders would sink instantly in normal streams, but they also understood enough about rare natural phenomena such as tsunamis (“tidal waves”) and great storms to know that water could transport heavy objects under extreme conditions.
In the late eighteenth and early nineteenth centuries, memory of the great Lisbon earthquake of 1755 was strong.
It had actually occurred off the coast of Portugal, not in Lisbon itself, but it had generated large tsunamis that scattered heavy objects far inland as though they were matchsticks.
It was also well known that a raging mountain stream could carry very large rocks, especially when
swollen with the downpour of a violent storm.
But even such extreme events could not easily explain the massive granite erratics in the Jura Mountains, especially the ones that were perched high on valley walls, far above the streams below.
Nor could they account for the presence in northern England of erratic boulders that appeared (based on their mineral makeup) to have originated across the North Sea in Norway, or those in the German lowlands that were hundreds of kilometers from their source.
Compounding the problem of interpreting these deposits, however, was the fact that at a few localities in Britain, where much of the most detailed research into the ice age controversy was being conducted, the fine-grained drift that accompanied the erratic boulders contained seashells.
Critics of the ice transport hypothesis seized on this; they claimed it was conclusive evidence that the ocean was involved.
They argued that the erratics must have been transported by great, violent floods coursing over the land, and they said that there were simply no modern-day counterparts.
They knew that the sea had covered parts of the continents in the past, because fossilized fish were found throughout Europe.
The marine shells in “glacial” drift, they asserted, were proof that the sea had invaded the land yet again and left the drift behind when it receded.
Actually, until about the 1820s, there was widespread belief that
all
of the loose sand and boulders strewn across the land surface had been deposited there by one or more floods, probably by the one described in the Bible.
It was not until much later that the true origin of the seashells in drift was realized.
James Croll, a Scottish scientist whom we shall encounter later in this book, deduced that they too had been transported by glaciers, scraped up by the ice along with sediments from the shallow seas around Britain and carried inland.
However, before their origin was understood, the shells were a serious difficulty for those who argued that drift and erratics were ice age deposits.

Still, notwithstanding the seashell argument, even some of the opponents of the glacial theory had to admit that it would be difficult, if not impossible, to transport large erratic boulders in water over long
distances, no matter how violent the storm or flood.
It could be shown by simple physics that it couldn’t be done.
So they came up with the ingenious solution mentioned in chapter 2: the erratics might indeed have been carried by ice, but ice that was floating on formerly more extensive seas, transporting boulders from a northerly source.
If parts of the continents had been submerged in the past, they reasoned, the icebergs could have floated over the sunken land, dropping their rocky burden as they melted.
That would explain the presence of ocean shells in the drift.
It was the idea of drifting icebergs that first led to use of the term “drift” for the characteristically chaotic sediments left behind by glaciers—sediments that have neither the well-defined layers nor the uniformity of grain sizes that characterize those deposited in water.
The term is still in use today.
Geologists also refer to such material as being unsorted, because it encompasses materials ranging in size from grains of sand and occasional shells to the erratic boulders themselves.

Drift and erratic boulders were not the only glacial features studied by early ice age researchers, but because essentially identical materials could be observed directly associated with glaciers in the Alps, these deposits were among the most persuasive evidence of past glaciation.
Every existing mountain glacier carries a large amount of rock debris that will eventually become glacial drift.
Some of it falls onto the glacier surface from the surrounding valley walls, and some is actually plucked from the bedrock below by the ice itself.
Beginning with Agassiz’s systematic studies at his glacier observatory, a series of investigations also showed that glaciers flow, and do so at significant rates.
The rock debris is carried along with the flowing ice, and, at the snout of the glacier, dumped in a chaotic pile of large and small boulders, gravel, sand, and silt—a feature known as a moraine.
Actually, glaciologists distinguish many types of moraines, but in its most general sense, the term—like the term “glacial drift”—just refers to the debris carried by a glacier.
Terminal moraines mark the farthest extent of a glacier, lateral moraines form along the sides of mountain glaciers, and medial moraines in their middles, the result of tributary glaciers entering the main ice
flow.
Figures 5 and 6 illustrate a few varieties of moraines.
Some types—for example, medial moraines—may exist on the ice of an active glacier, but can also be distinguished long after the glacier has melted away, because they form a longitudinal ridge in the middle of a glacial valley.

Figure 5.
That glaciers flow is particularly apparent from the air.
This glacier in Greenland flows toward the observer, carrying on its surface ribbons of rocky material that have fallen onto its surface from the valley walls, forming moraines.
Because many tributaries join the main glacier, it becomes more and more banded with moraines downstream as each tributary adds its contribution of debris.
Photograph courtesy Professor Michael Hambrey, Liverpool John Moores University.

Ice in a glacier, like water in a stream, flows under the influence of gravity.
A mountain glacier accumulates snow at its upper end, where the average temperature is low, and loses ice by melting at its snout.
In places like Greenland and Alaska, some glaciers flow directly into the sea, where gigantic chunks break off and form icebergs.
If snow accumulation and melting are more or less in balance over a significant
length of time, the size of a glacier and the location of its lower end will remain approximately constant, in spite of the fact that the ice is flowing and transporting rock debris all the while.
When this occurs, the glacier is in a steady state, and very large terminal moraines can be built up.
If the climate warms and the glacier melts away, the moraine remains as a distinctive landform—a great ridge composed of pebbles and boulders, marking the previous terminus of the glacier.
Sometimes there is a whole series of these features, tracing out positions where the glacier front remained stationary for varying lengths of time before melting back further.

Figure 6.
A cross section through a lateral moraine in Switzerland illustrates the great range of sizes of material it contains.
Someone has sorted out piles of sand and boulders of various sizes near the bottom of the picture.
Notice that a forest and a layer of soil has developed on the moraine.
Glacial deposits from each cycle of the Pleistocene Ice Age have such soils developed on them, indicating that the ice advances were separated by long and relatively warm interglacial periods.
This particular moraine can be traced for many kilometers.
Photograph copyright Dr.
John Shelton.

Once the nature of moraines formed by contemporary glaciers was understood, it became clear that much of the enigmatic “drift” so common in northern Europe and North America must have an analogous origin in the now-vanished glaciers of the ice age.
The moraines left behind by continental-scale ice sheets are really not much different from those of alpine glaciers, except that they exist on a 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 in
places 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 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 separated
parts 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.