barrier that surrounded West (West Berlin) Berlin and prevented access to it from East Berlin and adjacent areas of East Germany (German Democratic Republic) during the period from 1961 to 1989. In the years between 1949 and 1961, about 2.5 million East Germans had fled from East to West Germany, including steadily rising numbers of skilled workers, professionals, and intellectuals. Their loss threatened to destroy the economic viability of the East German state. In response, East Germany built a barrier to close off East Germans' access to West Berlin (and hence West Germany). This barrier, the Berlin Wall, was first erected on the night of August 12–13, 1961, as the result of a decree passed on August 12 by the East German Volkskammer (“Peoples' Chamber”). The original wall, built of barbed wire and cinder blocks, was subsequently replaced by a series of concrete walls (up to 15 feet 【5 metres】 high) that were topped with barbed wire and guarded with watchtowers, gun emplacements, and mines. By the 1980s this system of walls, electrified fences, and fortifications extended 28 miles (45 km) through Berlin, dividing the two parts of the city, and extended a further 75 miles (120 km) around West Berlin, separating it from the rest of East Germany.