[scriptkiddy]

You have a layer of ice covered with a layer of snow. Each year, that snow melts a little and then re-freezes on top of the ice. Then, more snow falls on top of that snow. This happens year over year for a long time. Each layer of snow puts more and more pressure on the previous layers of snow and ice, compacting them. Eventually, these layers of snow become so compacted that they form new layers of ice and fuse into the existing layers of ice. Does that make sense?