<P> The first hedgehogs were built of reinforced concrete with the shape similar to later metal version . But the concrete hedgehogs proved ineffective during the tests as they could be substantially damaged by machine - gun fire . Moreover, even when turned into debris it provided more cover for the enemy infantry than the metal counterpart . Therefore, only the oldest sections of Czechoslovak defensive line built in 1935--1936 were equipped with concrete hedgehogs and usually only in the second line . </P> <P> The Czech hedgehog was widely used during World War II by the Soviet Union in anti-tank defense . They were produced from any sturdy piece of metal and sometimes even wood, including railroad ties . Czech hedgehogs were especially effective in urban combat, where a single hedgehog could block an entire street . Czech hedgehogs thus became a symbol of "defense at all cost" in the Soviet Union; hence, the memorial to Moscow defenders, built alongside the M - 10 highway in 1966, is composed of three giant Czech hedgehogs . </P> <P> Czech hedgehogs were part of the German defenses of the Atlantic Wall . During the invasion of Normandy the Allies cut up sizable numbers of intact and wrecked hedgehogs, and welded them to the front of their M4 Sherman tanks and M5 Stuart tanks . Known as Rhino tanks, they proved very useful for clearing the hedgerows that made up the bocages across Normandy . </P> <P> The postwar tests conducted by Czechoslovak army proved the low efficiency of the metal hedgehogs against heavy armored vehicles such as the Soviet ISU - 152 and T - 54 or German PzKpfw V Panther . As many as 40% attempts for breakthrough were successful for which the army developed new anti-tank obstacles for the purpose of the border fortifications activated during the Cold War . But the metal hedgehog was still used as the quick road - block against wheeled vehicles . </P>

What are those things on the beach at normandy