At the beginning of Devonian the Earth's climate was warm and dry. The erosion of rising landmasses in hot-dry climatic zones produced extensive deposits of terrestrial sediments, made red by oxidized iron (hematite). Plants became firmly established on land and most of the land in the tropics was covered in thick rainforests along the Equator producing large quantities of oxygen through photosynthesis. Oxygen made up 20% of the atmosphere – about today's level – around 350 million years ago, and it rose even higher to between 28 and 35 % over the next 50 million years. Masses of dead plants led to the formation of vast coal deposits that reduced CO₂ in the atmosphere. This and the movement of Gondwana over the South Pole causing a major glaciation in the southern hemisphere that caused the Earth to become cooler leading to an ice age during the Carboniferous.