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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 will

  not 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 fact

  that 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 to

  increased 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

  7. According to the passage, the material used to determine

  past climatic conditions must be widespread for which

  of the following reasons?

  Ⅰ.Paleoclimatologists need to make comparisons

  between periods of geological history.

  Ⅱ. Paleoclimatologists need to compare materials that

  have supported a wide variety of vegetation.

  Ⅲ. Paleoclimatologists need to make comparisons with

  data collected in other regions.

  (A) Ⅰ only

  (B) Ⅱ only

  (C) Ⅰ and Ⅱ only

  (D) Ⅰ and Ⅲ only

  (E) Ⅱ and Ⅲ only

  8. Which of the following can be inferred from the passage

  about the study of past climates in arid and semiarid

  regions?

  (A) It is sometimes more difficult to determine past

  climatic conditions in arid and semiarid regions than

  in temperate regions.

  (B) Although in the past more research has been done on

  temperate regions, paleoclimatologists have

  recently turned their attention to arid and semiarid

  regions.

  (C) Although more information about past climates can

  be gathered in arid and semiarid than in temperate

  regions, dating this information is more difficult.

  (D) It is difficult to study the climatic history of arid and

  semiarid regions because their climates have tended

  to vary more than those of temperate regions.

  (E) The study of past climates in arid and semiarid

  regions has been neglected because temperate

  regions support a greater variety of plant and animal

  life.

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