PASSAGE 92

　　There are only a few clues in the rock record about climate in the Proterozoic eon. 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 the types of environment in which many fossil organisms flourished. The scarce fossils 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 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 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 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 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 eon.

　　1. Which of the following does the passage mainly discuss?

　　(A) How patterns in rock layers have been used to construct theories about the climate of the

　　Proterozoic age

　　(B) What some rare fossils indicate about glacial conditions during the late Proterozoic age

　　(C) The varying characteristics of Proterozoic glacial varves in different parts of the world

　　(D) The number of glacial episodes that the Earth has experienced since the Proterozoic age

　　下转第二页