shield, piece of defensive armor, worn on the arm or shoulder to ward off weapons during combat, used prior to the dominance of gunpowder. Originally for individual defense during hand-to-hand combat, it is the most primitive and universal item of defensive armor. Shields were made of hide or wood, often reinforced with metal, and could be round, oblong, or rectangular. As armies developed, soldiers carried matching shields to link together for fighting in formations, such as those used by Assyria (2500 B.C.). A soldier's body armor complemented his shield. Heavy infantry carried larger shields than did skirmishers, cavalry carried smaller shields, and bowmen often carried none. Modern riot police carry plastic shields for protection.

shield, in geology: see continent.

Canadian Shield or Laurentian Plateau, U-shaped region of ancient rock, the nucleus of North America, stretching N from the Great Lakes to the Arctic Ocean. Covering more than half of Canada, it also includes most of Greenland and extends into the United States as the Adirondack Mts. and the Superior Highlands. The first part of North America to be permanently elevated above sea level, it has remained almost wholly untouched by successive encroachments of the sea upon the continent. It is the earth's greatest area of exposed Archaean-age rock; the metamorphic rocks of which it is largely composed were probably formed in the Precambrian. Repeatedly uplifted and eroded, it is today an area of low relief (c.1,000-2,000 ft/305-610 m above sea level) with a few monadnocks and low mountain ranges (including the Torngat and Laurentian Mts.) probably eroded from the plateau during the Cenozoic era. During the Pleistocene epoch, continental ice sheets depressed the land surface (see Hudson Bay), scooped out thousands of lake basins, and carried away much of the region's soil. Drainage is generally very poor on the shield. The southern part of the shield has thick forests while the north is covered with tundra. The region is largely undeveloped but has great water-power potential and is a source of minerals, timber, and fur-bearing animals.

Baltic Shield, the continental core of Europe, composed of Precambrian crystalline rock, the oldest of Europe. The tectonically stable region was not affected by the Caledonian, Hercynian, and Alpine mountain-building periods of Europe, although mountains did rise along the edges. The exposed portion of the Baltic Shield is found in Finland, Sweden, and Norway. During the Pleistocene epoch, great continental ice sheets scoured and depressed the shield's surface, leaving a thin covering of glacial material and innumerable lakes and streams. The ancient rocks have yielded a rich variety of minerals, especially iron and copper. In W former USSR the Russian Platform is that portion of the Baltic Shield buried beneath a great thickness of sedimentary rock.

Angara Shield, Asia: see Siberian Platform.

Any of the large stable areas of low relief (little variation in elevations) in the Earth's crust that are composed of Precambrian crystalline rocks. These rocks are always more than 570 million years old, and some are as old as 2–3 billion years. Continental shields occur on each of the continents.

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One of the world's largest geologic continental shields, centred on Hudson Bay and extending for 3 million sq mi (8 million sq km) over Canada from the Great Lakes to the Canadian Arctic and into Greenland, with small extensions into northern Minnesota, Wisconsin, Michigan, and New York. It is the largest mass of exposed Precambrian rock on earth. The region as a whole is composed of ancient crystalline rocks whose complex structure attests to a long history of uplift and depression, mountain building, and erosion.

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