Why are the glaciers advancing and retreating?
There is no simple answer. If a glacier has a steady source of snow turning to ice in the mountains, a good lense of water on which to slide along the bedrock, enough gravity and momentum in downhill movement, a good moraine of rock and rubble at the front to insulate it from water erosion and cold enough temperatures year-around, a glacier will advance. If it loses enough of these, it will retreat.
