What are Glaciers?

In high mountains there are places that are packed with ice.  These ice packs are called 'Glaciers' these resemble gaint frozen rivers.  Many thousands of years ago most of the Earth's surface was covered with moving glaciers.
It takes a very long time to make a glacier.  First, snow falls on the mountains.  It collects year after year, until there is a thick layer called a snow field.
During summer months the snow field melts and sinks into the snow below it.  There it freezes and forms a layer of ice.  This happens year after year, until most of the snow field is converted to ice.  The snowfield is now a glacier.
The snow and ice ina glacier can become very thick and heavy.  The glacier then begins to actually move under its own weight and creeps down the mountain valley.  It has now become a valley glacier.
The valley glacier moves slowly but with enormous force.  As it moves, it scrapes the slides of the mountain and tears off pieces of it.  Sometimes it tears off chunks as big as a house.  As the glacier moves down warmer regions, the water melts and becomes source of water for rivers and lakes.