The dry mountain ranges of the western United States contain rocks dating back 440 to 510 million years, to theOrdovician period, and teeming with evidence of tropical marine life.
美国西部干燥的山脉包含了距今4.4亿至5.1亿年前的奥陶纪岩石，其中充斥着热带海洋生物生存的迹象。
2
TheOrdovician ice age happened 444 million years ago, and records have suggested that CO2 levels were relatively high then.
奥陶纪冰河时期发生在4.44亿年前，有记录表明其CO2含量较高。
3
However, some researchers believe that a GRB was responsible for theOrdovician mass extinction approximately 450 million years ago.