Respuesta :
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.
The rocks have stumped scientists because some roll, some take unexplainable routes, and some have traveled past ones that have stayed still, but scientists have theories that the rocks are pushed by wind or that they slide on ice or mud.