I'm no geological expert, but I don't think there's much liquid water involved in the moving forward of the glacier. It slides down the mountains and then once it gets warm enough it starts to melt. The melting causes it to recede. It's not actually moving back, the front is just melting off. When the glacier forms rocks get stuck inside it and when it melts the rocks stay where they are. I believe a hill made of glacier debris is called a terminal moraine. Moraine, Ohio is named after this.