More interesting still -- the mottling is from the remains of the casing and the shot cab (the little building the bomb was housed in at the top of the tower) s

More interesting still -- the mottling is from the remains of the casing and the shot cab (the little building the bomb was housed in at the top of the tower) splashing against the shock front at the fireball's boundary. (http://nuclearweaponarchive.org/Usa/Tests/Tumblers.html http://nuclearweaponarchive.org/Usa/Tests/Tumblers.html)