Complex sentence: These are mostly pyroclastic deposits from explosive volcanic eruptions, but the "Yarralumla Formation" is a sedimentary mudstone/siltstone formation that was formed around 425 million years ago during the Silurian Period.
Split into simpler sentences: These are mostly pyroclastic deposits from explosive volcanic eruptions. The "Yarralumla Formation" is a mudstone/siltstone formation that was formed around 425 million years ago during the Silurian Period.