TPO7The Geologic History of the Medite - 英檢

在原文第4段:
In the central deeper part of the basin, the last of the brine evaporated to
precipitate more soluble sodium chloride (salt). Later, under the weight of
overlying sediments, this salt flowed plastically upward to form salt domes.
Before===>this<===happened, however, the Mediterranean was a vast desert 3,000
meters deep. Then, about 5.5 million years ago came the deluge.


請問一下,文章中的"This"指的是什麼?
這個地理構造的形成順序看了好幾次都看不出所以然...
麻煩各位幫忙了,謝謝

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