Above: The Gilgal Gardens in Salt Lake City. Photo from 15 Bytes
Joshua Builds a Monument to the Israelites Crossing of the Jordan River - At the time that Joshua was written, there must have been some sort of monument, or a peculiar grouping of stones, in the Jordan River and also another set on the banks of it (or not too far from shore).
Those who saw the stones probably wondered where they came from. I say this because the author writes that the stones "are still there unto this day." (Joshua 4:4) This suggests that the book was written well after the fact, and the chapter serves as an explanation for these peculiar stones. By the way, the stones are no longer there.
But according to Joshua, the stones acted as a monument to the Israelites 'miraculous' crossing of the Jordan River. The author even writes that this is the story that should be told to children who come later, asking pesky questions.
After this unusual ceremony and the Jordan River filled back up with water, 40,000 Israelite warriors assembled on the plains of Jericho. Another monument was erected with 12 stones Joshua had taken with him.
Perhaps one interesting correlation with this story, is the story of Moses crossing the Red Sea. Twice Yahweh assisted the Israelites with crossing a difficult body of water. But the situations were completely opposite. In Moses' time, the Israelites crossed a large sea and were fleeing an oppressing army; this time the Israelites were the ones on the warpath.