The last ice age has left its tell-tales written quite clearly across the landscape. When Louis Agassiz first promulgated his theory that ice had once covered the Swiss countryside, he looked to the valleys there that retain glaciers to this day. Like other observers, he noted the presence of strange boulders, called "erratics," tossed down in valleys like flotsam after a flood had drained away. He saw the strange polish along the bedrock—a sheen imparted as if by some massive swipe of sandpaper; he saw the debris of rocks and boulders fringing the margin of existing glaciers. He saw what can be seen still, markings in stone that indicated that ice once flowed over vast stretches of land now clear and verdant.

The Australian climate historian L.A. Frakes has prospected through various theories proposed to account for those early ice ages. He isn‘t terribly enthusiastic about any of the possible culprits, but his choice for the least unlikely of them all emerges out of the recent revival of what was once a radically unorthodox idea: that continents drift over the face of the planet. Frakes argues that the glaciers originated at sites near the poles and that the ice ages began because the continents of the early earth had drifted to positions that took more and more of their land nearer to the polar regions.

More land near the poles meant that more precipitation fell as snow and could be compacted on land to form glaciers. With enough glaciers, the increase in the amount of sunlight reflected back into space off the glistening white sheen of the ice effectively reduced the amount by which the sun warmed the earth, creating the feedback loop by which the growth of glaciers encouraged the growth of more glaciers. Rocks have been found in North America, Africa and Australia whose ages appear to hover around the 2.3 billion-year-old mark. That date and their spread are vague enough, however, to make it almost impossible to determine just how much of the earth was icebound during the possible range of time in which each of the glacial deposits was formed.

Uncertainties about both the timing and the extent of these glaciers also muddy the search for the cause of the ancient ice ages. The record is so spotty that geologists are not sure whether areas near the equator or nearer the poles were the coolest places on earth. It‘s also possible that volcanic eruptions had tossed enough dust into the atmosphere to screen out sunlight and cool the earth.

Such traces are the currency of science—data—and like money, a richness of data both buys you some credibility and ties you down, eliminating at least some theoretically plausible explanations. For this early period, theorists have come up with a variety of ideas to explain the ancient ice ages, all elegant and mostly immune to both proof and criticism. For example, a change in the earth‘s orbit could have reduced the amount of sunlight reaching the planet. However, the only physical signature of such an event that would show in the rocks would be the marks of the glaciers themselves.

1. There is an implicit assumption in the statement that geologists don‘t know whether the coolest places on earth were near the poles or near the equator. The assumption is that:

A. both polar and equatorial glacial deposits have been found.

B. certain geological information can be considered lost forever.

C. it is more important to determine the date of the ice ages than the extent of the glaciers.

D. the glaciers were extremely mobile in spite of their mass.

E. areas around the equator are usually hotter than those around the poles


2. Suppose that an advocate of the "change in orbit" theory of the ancient ice ages criticizes a defender of the "volcanic eruption" theory on the grounds that only some of the glacial records contain evidence of prior volcanic activity. The defender might justifiably counter this attack by pointing out that:

A. a change in the earth‘s orbit would have increased rather than reduced the sunlight reaching the planet.

B. volcanoes could not possibly release enough dust to block the atmosphere.

C. a theory that has some supporting evidence is better than a theory that cannot be proved.

D. a theory should be so constructed as to be immune from proof.

E. the ̳advocate‘ had got his facts from a dubious source


3. Suppose paleobotanists discover that during geological periods of reduced sunlight, ancient forests died away, leaving fossilized remains. What is the relevance of this information to the passage?

A. It supports the claim that dust from volcanic eruptions caused the ice ages.

B. It weakens the claim that dust from volcanic eruptions caused the ice ages.

C. It supports the claim that ice ages were accompanied by widespread loss of vegetation.

D. It weakens the claim that the only evidence of a change in orbit would be glacier marks.

E. It has no relevance to the passage