Answer:
1. it helps express the song's central idea
Explanation:
The main idea of this paragraph is that the rocks cannot be explained by science, that there is a mystery to them. This sentence contributes to that idea because it gives a specific example of one of the odd things about the rocks. Because they are on a flat surface they should not roll or move on their own, but the fact that they have a trail behind them shows that they have somehow moved. This supporting statement is then supported by more details about the trails.
Every gerund has the same form - verb + -ing. So, having that in mind, the word <em>sleeping </em>is used as a gerund in this sentence, but it is also a subject of the sentence.
