tpo-ground water - 英檢

在TPO1 裡ground water一篇當中的某一段，我想請問代名詞"they"是代表什麼?

In lowland country almost any spot on the ground may overlie what was once the
bed of a river that has since become buried by soil; if "they" are now below
teh water's upper surface (the water table), the gravels and sands of the
former reiverbed, and its sandbars, will be struated with groundwater.


我看了很多次都看不出來they指的是哪個名詞。
在they前面沒有一個名詞是複數。

如果硬要選的話，我覺得是spot。
請問大家認為they是代表哪的名詞，謝謝。

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