For reasons not fully understood, the field itself reverses periodically every million years or so.
由于一些尚不完全清楚的原因，该领域本身大约每隔百万年就会周期性地翻转一次。
What is still not fully understood, however, is why some trees resort to producing red pigments while others don't bother, and simply reveal their orange or yellow hues.
然而，至今仍不完全清楚，为什么有些树木倾向于制造红色素，而有些树木不去刻意制造色素，但还是会呈现出橙色或黄色。
The environmental changes associated with this transition are not fully understood, but people living closest to the San Juan Mountains, to the northeast of Mesa Verde, were affected first.
与这一变迁相关的环境变化并未得到人们的充分认识，但是离位于梅萨维德东北部的圣胡安山脉最近的居民却首先受到了影响。
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