The historian Frederick J. Turner wrote in the 1890’s that the agrarian discontent that had been developing steadily in the united states since about 1870 had been precipitated by the closing of the internal frontier -- that is, the depletion of available new land needed for further expansion of the American farming system.
解析: only 放在句首时, 主句的情态动词 may 倒装, 这是我们高中就已经学过的东西。 然而句首的 only 与倒装的 may 距离如此之远,这在以前却没有见过,所以读上去让人不习惯。另外一个难点在于, 前面的由 when 所引导的时间状语从句中, 宾语 boundaries 后面跟了一个定语从句, that associate the water within it with the hydrologic cycle,而且从句中的 associate with 又被 water within it 所分开,读者还要分神去考虑 it 所指的是什么,所以读到后面倒装的 may 的时候,难免不知所云。