You put the comma in the wrong place, from Wikipedia: "The release at 08:15 (Hiroshima time) was uneventful, and the gravity bomb known as "Little Boy", a gun-type fission weapon with 60 kg (130 pounds) of uranium-235, took 57 seconds to fall from the aircraft to the predetermined detonation height about 600 meters (1,900 ft) above the city." http://en.wikipedia.org/wiki/Atomic_bombings_of_Hiroshima_and_Nagasakihttp://en.wikipedia.org/wiki/Atomic_bombings_of_Hiroshima_and_Nagasaki
Also from Wikipedia: "The bomb employed a fuse system that was designed to detonate at the most destructive altitude. Calculations showed that for the largest destructive effect, the bomb should explode at an altitude of 580 meters (1,900 feet)."
And: "Because Little Boy was detonated 1,900 feet (580 m) above the ground, as an air burst, there was no bomb crater and no local radioactive fallout. Local Fallout is dust and ash from a bomb crater, contaminated with radioactive fission products. It falls back to the ground downwind of the crater and can easily produce, with radiation alone, a lethal area much larger than that from blast and fire. With an air burst, the fission products remain in aerosol form until they rise into the stratosphere, where they dissipate and become part of the global, rather than the local environment."