cambridge university press pp ref ref name rafferty p rafferty john p ''geological sciences geology landforms minerals rocks'' new york britannica educational publishing pp ref furthermore promoted theory gradual climate change centuries time ancient petrified bamboos found preserved underground dry northern climate zone ''yanzhou'' modern day yan'an shaanxi province ref name nj ref name rafferty p rafferty john p ''geological sciences geology landforms minerals rocks'' new york britannica educational publishing p ref previous chinese authors also presented ideas changing landforms scholar official du
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