The snow never melts on some mountains. And at the north and south poles it stays cold all year. Over the years snow piles up in these places. The weight of new snow packs down the old snow. That turns snow to ice. The huge sheets of ice are called glaciers.
As a glacier grows,its weight causes it to slide. Mountain glaciers may slide into warm valleys and melt. Sometimes glaciers near ther sea slide into the water and melt.