The Song or Sung Dynasty was actually one dynasty that lost much territory from Mongol attacks. It was originally both the North and South Sung dynasty as shown on the map, but in 1126 C.E., the Mogol invasion began and the Sung people had to retreat to lower parts of China. Some major geographic features in the South that kept them alive were rivers such as the Xi River and the Yangzte River. These rivers were both very large and could be used to produce large amounts of crops. These rivers helped to keep the people alive in tough times. The oceans to the right of the dynasties area was also a plentiful water source, but not for drinking. It was used for other uses such as fishing. The flat land of China Proper, where the Song dynasty was located, was rich and fertile for farming as well.

Credit to: Mr. Miller's History Lecture in Class and http://www.chaos.umd.edu/history/imperial2.html#song