阅读理解 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 historycomes 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 fossilsof the Proterozoic, mostly single-celled bacteria, provide little evidence in this regard.However, the rocks themselves do include the earliest evidence for glaciation, probablya 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 nowat work on and within the Earth operated in the same manner in the distant past. Thedeposits associated with present-day glaciers have been well studied, and some of theircharacteristics are quite distinctive. In 2.3-billion-year-old rocks in Canada near LakeHuron (dating from the early part of the Proterozoic age), there are thin laminae offine-grained sediments that resemble varves, the annual layers of sediment deposited inglacial lakes. Typically, present-day varves show two-layered annual cycle, one layercorresponding to the rapid ice melting and sediment transport of the summer season, andthe other, finer-grained, layer corresponding to slower winter deposition. Although it isnot easy to discern such details in the Proterozoic examples, they are almost certainlyglacial varves. These fine-grained, layered sediments even contain occasional largepebbles or “dropstones,” a characteristic feature of glacial environments where coarsematerial is sometimes carried on floating ice and dropped far from its source, intootherwise very fine grained sediment. Glacial sediments of about the same age as thosein Canada have been found in other parts of North America and in Africa, India, andEurope. 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 haveBeen 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 ofGlacial episodes between about 850 and 600 million year ago, near the end of theProterozoic con.