Well-preserved wrecks abound in the Baltic, where low salinity and cold water slow down decay. These include the Vasa, which san

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问题     Well-preserved wrecks abound in the Baltic, where low salinity and cold water slow down decay. These include the Vasa, which sank on her maiden voyage in 1628 and was raised from Stockholm Harbour in 1961. Another is the Kronan, which blew up and sank during a battle with the Dutch in 1676. Not all wrecks are now under water. Many have been found in reclaimed parts of Holland’s Zuider Zee, after the land has been drained. At Skuldelev in Denmark a group of five Viking ships, sunk to form a blockage at the entrance to a fjord.
    Although they have to work in a hostile environment, underwater archaeologists follow the same principles as their counterparts on land. A site must first be surveyed. This is done with tape-measures and grids using simple geometry, supplemented by photography, video, and geophysics. The aim is to produce an analytical map so that the wreck can be assessed. Often this will enable archaeologists to record the site and interpret its significance without having to disturb it. Only in exceptional cases are historic shipwrecks excavated, and their contents recovered.
    Well-preserved wrecks have usually reached a state of equilibrium with their environments. Having survived for centuries in this state, they will probably remain in much the same condition for centuries to come. But if they are disturbed, the stability which has protected them will be broken. Archaeologists normally excavate shipwrecks only if they are under some kind of threat—a shift in the environmental balance, perhaps, or plans to dredge a harbor entrance where historic sites are known to lie. An example of this kind of "rescue" excavation is provided by the Cromwellian warship Swan wrecked in 1653 off Mull, which is now threatened by seabed erosion.
    Very occasionally archaeologists excavate wrecks which are not under threat to recover them and their contents for public exhibition. Examples of these include the Swedish warship Vasa at Stockholm(1628)and the Mary Rose at Portsmouth(1545). In such cases the damage to their archaeological integrity is offset by careful records made by the archaeologists, the conservation of the ships and their contents, and subsequent publication and display.
    Parts of the hull tell the marine archaeologist how the ship was designed and built. Tool-marks reveal woodworking techniques used by the shipwrights. Fragments of rigging and rope—sometimes even pieces of sails—help to show how the ship was operated by its crew. Other finds throw light on activities such as navigation, medical care, everyday crafts, food storage, cooking, and domestic routines. If the ship was a merchant vessel parts of its cargo may have survived, perhaps revealing where it came from. Warships contain evidence of the weapons they carried. Above all, items of clothing and personal possessions bring us into immediate contact with people who lived and died long ago.
What is the main idea of the 1st paragraph?

选项 A、Plenty of well-preserved shipwrecks are available in the Baltic.
B、How to survey underwater wrecks.
C、The sinking of five Viking ships formed a blockage at the entrance to a fjord.
D、The importance of the wreck recovery.

答案A

解析
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