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.
41.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
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
43.The word “scarce” in line 4 is closest in meaning to
(A) ancient
(B) tiny
(C) available
(D) rare
44. It can be inferred from the passage that the principle of uniformitarianism indicates that
(A) similar conditions produce similar rock formations
(B) rock layers in a given region remain undisturbed over time
(C) different kinds of sedimentary rocks may |
