Gathering Blue depicts a post-apocalyptic society. The exact form the apocalypse took remains cryptic in the novel, but as the story begins, the "civilization of the ancestors" had ended long ago. The society is now primitive, operating without any technology to speak of, and most people in the society are illiterate. The method they have for retaining their history is oral: A Singer presents a Ruin Song annually to the entire community which tells the history of people. The part of the Ruin Song that refers to the apocalypse speaks of "smoky, poisonous fumes, of great fractures in the earth itself, of the way huge buildings toppled and were swept away by the seas." The Singer's robe depicts through embroidery the scenes mentioned in the Song. Kira repairs an area of the robe that shows "tall buildings ... against a background of fiery explosions." These were skyscrapers, destroyed in a war.
The words to accompany that section of the robe say, "Burn, scourged world / Furious furnace, / Inferno impure." That the destruction was widespread, even worldwide, is implied by the verse, "Ravaged all, / Bogo tabal / Timore toron / Totoo now gone." Destruction ranged from Bogata to Baltimore to Toronto, meaning that cities on both the North and South American continents were destroyed. The fact that many humans are physically deformed in the village where Christopher lives suggests either nuclear or chemical weapons were used that caused congenital defects for generations.
Though Lowry never specifically says so, one assumes that a World War, including nuclear weapons, wiped out civilization as we know it. Kira's community rebuilt, but without technology and without many of the markers that define our culture, such as religion, education, philosophy, and the arts.