The Rules of Relative Adverb
I'm currently studying English for grammar test.
And I have a question about relative adverb.

"His studies of ice-polished rocks in his Alpine home land, far outside the range of present-day glaciers, led Louis Agassiz in 1837 to propose the concept of an age [when/in which] great ice sheets existed in what are now temperate areas."

I thought the relative adverb "when/in which" modifies "the concept", so it should be "in which" since I thought "when" modifies only the word "time".
But the book says that either "when" or "in which" is fine since this relative adverb modifies the word "an age".

Is it true that the relative adverb modifies "the age"??
If so, can the relative adverb "when" modify words other than the word "time"??

I appreciate any kinds of your helps!! Thank you so much as always :)

Question

The Rules of Relative Adverb
I'm currently studying English for grammar test.
And I have a question about relative adverb.

"His studies of ice-polished rocks in his Alpine home land, far outside the range of present-day glaciers, led Louis Agassiz in 1837 to propose the concept of an age [when/in which] great ice sheets existed in what are now temperate areas."

I thought the relative adverb "when/in which" modifies "the concept", so it should be "in which" since I thought "when" modifies only the word "time".
But the book says that either "when" or "in which" is fine since this relative adverb modifies the word "an age".

Is it true that the relative adverb modifies "the age"??
If so, can the relative adverb "when" modify words other than the word "time"??

I appreciate any kinds of your helps!! Thank you so much as always :)

Answer

First off, it is "relative pronoun" (關係代名詞), not relative adverb - I doubt there is such a thing as relative adverb.

And yes, the relative pronoun "[when/in which]" relates to "an age".  Great ice sheets existed in that age, which is natural and exact than ice sheets existing in the concept.

And again yes, "when" can be used with anything that has some notion of time in it.  Ex:  "Those were the good old days when I didn't have to work so hard".  "That was the moment when[that] I realized I was alone".  Neither of these has the word "time".