Questions 41-50
There are only a few clues in the rock record about climate in the Proterozoic con.
Much of our information about climate in the more recent periods of geologic history
comes from the fossil record, because we have a reasonably good understanding of
Line the types of environment in which many fossil organisms flourished. The scarce fossils
5) of the Proterozoic, mostly single-celled bacteria, provide little evidence in this regard.
However, the rocks themselves do include the earliest evidence for glaciation, probably
a global ice age.
The inference that some types of sedimentary rocks are the result of glacial activity
is based on the principle of uniformitarianism, which posits that natural processes now
10) at work on and within the Earth operated in the same manner in the distant past. The
deposits associated with present-day glaciers have been well studied, and some of their
characteristics are quite distinctive. In 2.3-billion-year-old rocks in Canada near Lake
Huron (dating from the early part of the Proterozoic age), there are thin laminae of
fine-grained sediments that resemble varves, the annual layers of sediment deposited in
15) glacial lakes. Typically, present-day varves show two-layered annual cycle, one layer
corresponding to the rapid ice melting and sediment transport of the summer season, and
the other, finer-grained, layer corresponding to slower winter deposition. Although it is
not easy to discern such details in the Proterozoic examples, they are almost certainly
glacial varves. These fine-grained, layered sediments even contain occasional large
20) pebbles or “dropstones,” a characteristic feature of glacial environments where coarse
material is sometimes carried on floating ice and dropped far from its source, into
otherwise very fine grained sediment. Glacial sediments of about the same age as those
in Canada have been found in other parts of North America and in Africa, India, and
Europe. This indicates that the glaciation was global, and that for a period of time in
25) the early Proterozoic the Earth was gripped in an ice age.
Following the early Proterozoic glaciation, however, the climate appears to have
Been fairly benign for a very long time. There is no evidence for glaciation for the
Next 1.5 billion years or so. Then, suddenly, the rock record indicates a series of
Glacial episodes between about 850 and 600 million year ago, near the end of the
Proterozoic con.
【題組】42. According to the passage, the fossil record of the Proterozoic con is
(A) highly regarded because it preserves the remains of many kinds of organisms
(B) less informative than the fossil record of more recent periods
(C) very difficult to interpret due to damage from bacteria
(D) more useful to researchers than other aspects of the rock record
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