GMAT Verbal Section | Re: WH- clause

Here the 'in which' refers to the word 'age'. And the subject of the 'in which' is 'great ice sheets'.

In general, when you encounter a 'preposition + which' construction, the subject can be found in the clause that follows 'which'.
But, in a 'which' construction, the subject occurs before the 'which'.

For example - 1. the box of toffees, which is on the table, is expensive. Note that the subject of the which clause occurs before the which clause. Also, 'which' modifies that subject (the box).
(the
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