Coal seams of the Carboniferous usually occur in cyclic sedimentary sequences. In 1912, J.A. Udden of the Illinois Geolgical Survey was the first to this cyclic character. He identified four stages of development:
  1. accumulation of vegetation
  2. deposition of calcareous material
  3. sand deposition
  4. erosion to seal level and soil making
This pattern was later called a cyclothem. These cyclothems can be accounted for by cyclic sea level fluctuations. Around the world the cyclothems record sea level changes of 150 to 200 m. Sea level changes this large, which happened almost every 300 000 years can only be explained by the advance and retreat of large continental glaciers positioned over the South Pole.