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
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