[求助]2001年5月阅读的第7题，第44题

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.