Hearing either I would not really see a strong difference or understand the claim made differently. I suppose one might think of the first as putting a little more emphasis on the end point (coral reefs existing) while the second perhaps is more about the process during the period of time. But I'm not at all sure that is how other English speakers would agree with me on that. (US English speaker)

I think the difference lies in how "across" and "through" describe time: - "Across endless eons of time" portrays time as a vast, continuous expanse, like something spread out over ages. - "Through endless eons of time" suggests movement, emphasizing the passage or progression of time. Both are correct, but "across" highlights the immensity of time, while "through" focuses on its flow.

So both sentences indicate that the wave action ended up creating coral reefs, but the main difference is how the movement is carried out Sentence 1: "Across" implies the wave action *moves over* the "endless eons of time". You go across something/a surface from one side to another Sentence 2: "Through" implies the wave action sort of moves *within* "endless eons of time". You move through the inside of something Hope that helps!

The difference lies in the prepositions "across" and "through," which subtly change the nuance of the sentence: 1. "Wave action across endless eons of time created coral reefs." * The preposition "across" emphasizes duration and suggests the action occurred continuously over the vast span of time. It gives the sense that the wave action spanned or extended throughout all those eons. 2. "Wave action through endless eons of time created coral reefs." * The preposition "through" emphasizes passage and suggests the action persisted while time passed. It gives the sense of wave action occurring as time flowed continuously. Both are correct and convey a similar idea, but "across" leans toward an overarching view of time, while "through" emphasizes the passage or progression of time.