A glacier is an accumulation of snow that forms when more snow falls each winter than melts the following summer.
As the layers grow year after year, their weight causes the lower layers to compress and turn into a dense ice called firn.
When this ice begins to move slowly over the land or down a mountainside, it becomes a glacier.
Glaciers have carved out many of the world’s lakes and mountains over the eons.
The glaciers in Glacier National Park are only a few thousand years old, which is relatively young in geological terms.
They are slowly shrinking in size because more of their snow melts each summer than accumulates each winter.