"There was a little creek here for a while," said Frank. "The water has all run off now, but it has spoiled the path."
"Will the gully get deeper every time it rains?" asked Susie.
"Of course," said Donald. "That's what makes it."
"Why does the water run along the path?" asked Uncle Robert.
"Because it is lower than the ground on each side," said Frank.
"How deep do you think the water will dig into the path if we do not fill it up?" asked Uncle Robert.
"Oh, way, way down. I suppose," said Donald.
"But if grass grew on the path," said Frank, "the water wouldn't wear the ground away. We will have to fill it up with stones."
"See these pebbles, uncle," said Susie. "How did they get here? They look just like those we saw on the island."
"Do you remember what I told you about the bowlders on the island?"