Can someone explain a few things about "What"

1) Is what a modifier? In some cases it looks like it can replace "which" or "that"
2) Does to modify a noun or a clause?

From OG:

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