Since glacier movement is gravity-controlled, the ice never retreats. It keeps coming. But most glaciers move from colder to warmer temperatures, so there comes a time when the ice melts as fast as it moves forward. The ice margin, or glacier front, is then stationary. Should the climate become warmer, melting exceeds the supply of new ice and the glacier front retreats.
Water is heavy, but it is far lighter than the rocks of the Earth's crust.
The ocean basins are the results of vertical and lateral movements of crustal plates. The water is there because of the basins, not the other way around. Fractures in the limestone above a cavern make ideal passages for the entry of surface water. On its way down, the water picks up calcium carbonate in solution from the walls of the passage. Presently it emerges on the cavern ceiling, from which it drips to the floor.