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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The oldest form of shield was a protection used to block attacks by hand weapons, such as swords, axes and maces or missiles like spears and arrows. Shields have varied greatly in construction over time and place. Sometimes shields were made of metal, but wood or animal hide construction was much more common; wicker and even turtle shells have been used. Many surviving examples of metal shields are generally felt to be ceremonial rather than practical, for example the Yetholm-type shields of the Bronze Age or the Iron Age Battersea shield.
Size and weight varied greatly, lightly armored warriors relying on speed and surprise would generally carry light shields that were either small or thin. Heavy troops might be equipped with large heavy shields that could protect most of the body. Many had a strap called a guige that allowed it to be slung over the user's back when not in use or on horseback. During the 14th-13th century BCE, the Sards or Shardana, working as mercenaries for the Egyptian pharaoh Ramses II, utilized either large or small round shields against the Hittites. The Ancient Greek hoplites used a round, bowl-shaped wooden shield called an aspis. Examples of German wooden shields c350 BC - 500 AD survive from Weapons sacrifices in Danish bogs. Free standing shields called pavises were used by medieval crossbowmen who needed protection while reloading.
The heavily armored Roman legionaries carried large shields (scuta) that could provide far more protection, but made swift movement a little more difficult. The scutum originally had an oval shape, but gradually the curved tops and sides were cut to produce the familiar rectangular shape most commonly seen in the early Imperial legions. Famously, the Romans used their shields to create a tortoise-like formation called a testudo in which entire groups of soldiers would be enclosed in an armoured box to provide protection against missiles.
Many ancient shield designs featured incuts of one sort of another. This was done to accommodate the shaft of a spear, thus facilitating tactics requiring the soldiers to stand close together forming a wall of shields.
In the early European Middle Ages kite shields were commonly used; these were rounded at the top and tapered at the bottom. They were easily used on horseback and allowed easier leg movement when dismounted. As personal body armour improved, knight's shields became smaller, leading to the familiar heater shield style. Both kite and heater style shields were made of several layers of laminated wood, with a gentle curve in cross section. The heater style inspired the shape of the symbolic heraldic shield that is still used today. Eventually, specialised shapes were developed such as the bouche — which had a lance rest cut into the upper corner of the lance side, to help guide it in combat or tournament.
In time, some armoured foot knights gave up shields entirely in favour of mobility and two-handed weapons. Other knights and common soldiers adopted the buckler (origin of the term "swashbuckler" ). The buckler is a small round shield, typically between 8 and 16 inches in diameter. The buckler was one of very few types of shield that were usually made of metal. Small and light, the buckler was easily carried by being hung from a belt; it gave little protection from missiles and was reserved for hand-to-hand combat. The buckler continued in use well into the 16th century.
The development of plate armour made shields less and less common as plate armour eliminated the need for a shield. Lightly armoured troops continued to use shields after men-at-arms and knights ceased to use them. Shields continued in use even after gunpowder powered weapons made them essentially obsolete on the battlefield. In the 18th century, the Scottish clans used a small, round shield called a targe that was partially effective against the firearms of the time although it was arguably more often used against British infantry bayonets and cavalry swords in close-in fighting.
In the 19th century, non-industrial cultures with little access to guns were still using shields. Zulu warriors carried large lightweight shields made from a single ox hide supported by a wooden spine, these were called Ishlangu. This was used in combination with a short spear (assegai) and/or club.
Although the size of shield would vary between personal preference and role, most were very thin compared to common belief. When used it fighting, shields were best to take advantage of causing glancing blows. By deflecting a sword blow to the side, rather than blocking it head on, you would leave your opponent open to a counterattack with a fast weapon. This method allowed the shield to be lighter and easily wielded, while reducing the amount of energy and risk of injury for the wielder.
Shields for protection from armed attack are still used by many police forces around the world. These modern shields are usually intended for two broadly distinct purposes.
The first type are used for riot control and can be made from metal or synthetics, such as Lexan or Mylar. These typically offer protection from relatively large and low velocity projectiles, such as rocks and bottles as well as blows from fists or clubs. Synthetic riot shields are normally transparent, allowing full use of the shield without obstructing vision. Similarly, metal riot shields often have a small window at eye level for this purpose. These riot shields are most commonly used to block and push back crowds when the users stand in a wall, and to protect against shrapnel, projectiles, molotov cocktails and during hand-to-hand combat.
The second type of modern police shield is typically manufactured from advanced synthetics such as kevlar and are designed to be bulletproof, or at least bullet resistant. These are typically employed by specialist police, such as SWAT teams, in high risk entry and siege scenarios, but are also used on a daily basis in many areas of the US.
Many non-martial devices also employ shielding of a kind--not usually a single device worn on an arm but various protective plates or other insulation positioned where needed. Space craft have heat shields to ensure a safe re-entry. Electronics uses shielding to reduce electrical noise and crosstalk between signals. People and systems that must work in the presence of ionizing radiation are protected with shielding.
Science fiction writers have imagined many futuristic protections they often call "shields," usually using force fields. These include personal shields, as in the Dune series, or larger ones for spacecraft as in Star Trek. See Energy shield for more.