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In choosing a method for determining climatic condi-tions that existed in the past， paleoclimatologists invoke four principal criteria. First， the material-rocks， lakes，vegetation， etc.-on which the method relies must be（ 5 ）widespread enough to provide plenty of information，since analysis of material that is rarely encountered willnot permit correlation with other regions or with other periods of geological history. Second， in the process of formation， the material must have received an environ-（10） mental signal that reflects a change in climate and that can be deciphered by modern physical or chemical means. Third， at least some of the material must have retained the signal unaffected by subsequent changes in the environment. Fourth， it must be possible to deter-（15） mine the time at which the inferred climatic conditions held. This last criterion is more easily met in dating marine sediments， because dating of only a small number of layers in a marine sequence allows the age of other layers to be estimated fairly reliably by extrapola-（20） tion and interpolation. By contrast， because sedimenta-tion is much less continuous in continental regions， esti-mating the age of a continental bed from the known ages of beds above and below is more risky. One very old method used in the investigation of past （25） climatic conditions involves the measurement of water levels in ancient lakes. In temperate regions， there are enough lakes for correlations between them to give us a reliable picture. In arid and semiarid regions， on the other hand， the small number of lakes and the great （30） distances between them reduce the possibilities for corre- lation. Moreover， since lake levels are controlled by rates of evaporation as well as by precipitation， the interpreta- tion of such levels is ambiguous. For instance， the factthat lake levels in the semiarid southwestern United （35） States appear to have been higher during the last ice age than they are now was at one time attributed toincreased precipitation. On the basis of snow-line eleva- tions， however， it has been concluded that the climate then was not necessarily wetter than it is now， but rather （40） that both summers and winters were cooler， resulting in reduced evaporation.Another problematic method is to reconstruct former climates on the basis of pollen profiles. The type of vege- tation in a specific region is determined by identifying （45） and counting the various pollen grains found there. Although the relationship between vegetation and climate is not as direct as the relationship between climate and lake levels， the method often works well in the temperate zones. In arid and semiarid regions in （50） which there is not much vegetation， however， small changes in one or a few plant types can change the picture dramatically， making accurate correlations between neighboring areas difficult to obtain.

1. Which of the following statements about the

difference between marine and continental

sedimentation is supported by information in the

passage？

（A） Data provided by dating marine sedimentation is

more consistent with researchers\' findings in

other disciplines than is data provided by dating

continental sedimentation.

（B） It is easier to estimate the age of a layer in a

sequence of continental sedimentation than it

is to estimate the age of a layer in a sequence

of marine sedimentation.

（C） Marine sedimentation is much less widespread

than continental sedimentation.

（D） Researchers are more often forced to rely on

extrapolation when dating a layer of marine

sedimentation than when dating a layer of

continental sedimentation.

（E） Marine sedimentation is much more continuous

than is continental sedimentation.

2. Which of the following statements best describes the

organization of the passage as a whole？

（A） The author describes a method for determining past

climatic conditions and then offers specific

examples of situations in which it has been used.

（B） The author discusses the method of dating marine

and continental sequences and then explains how

dating is more difficult with lake levels than with

pollen profiles.

（C） The author describes the common requirements of

methods for determining past climatic conditions

and then discusses examples of such methods.

（D） The author describes various ways of choosing a

material for determining past climatic conditions

and then discusses how two such methods have

yielded contradictory data.

（E） The author describes how methods for determining

past climatic conditions were first developed and

then describes two of the earliest known methods.

3. It can be inferred from the passage that

paleoclimatologists have concluded which of the

following on the basis of their study of snow-line

elevations in the southwestern United States？

（A） There is usually more precipitation during an ice age

because of increased amounts of evaporation.

（B） There was less precipitation during the last ice age

than there is today.

（C） Lake levels in the semiarid southwestern United

States were lower during the last ice age than they

are today.

（D） During the last ice age， cooler weather led to lower

lake levels than paleoclimatologists had previously

assumed.

（E） The high lake levels during the last ice age may have

been a result of less evaporation rather than more

precipitation.

4. Which of the following would be the most likely topic

for a paragraph that logically continues the passage？

（A） The kinds of plants normally found in arid regions

（B） The effect of variation in lake levels on pollen

distribution

（C） The material best suited to preserving signals of

climatic changes

（D） Other criteria invoked by paleoclimatologists when

choosing a method to determine past climatic

conditions

（E） A third method for investigating past climatic

conditions

5. The author discusses lake levels in the southwestern

United States in order to

（A） illustrate the mechanics of the relationship between

lake level， evaporation， and precipitation

（B） provide an example of the uncertainty involved in

interpreting lake levels

（C） prove that there are not enough ancient lakes with

which to make accurate correlations

（D） explain the effects of increased rates of evaporation

on levels of precipitation

（E） suggest that snow-line elevations are invariably

more accurate than lake levels in determining rates

of precipitation at various points in the past

6. It can be inferred from the passage that an

environmental signal found in geological material

would not be useful to paleoclimatologists if it

（A） had to be interpreted by modern chemical means

（B） reflected a change in climate rather than a long-

term climatic condition

（C） was incorporated into a material as the material was

forming

（D） also reflected subsequent environmental changes

（E） was contained in a continental rather than a marine

sequence

KEYS

Passage 1： ECEEB D

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