In region experiencing snowfall, the snow keeps on accumulating in layers one above the other.
Its overlying pressure is applied to the underlying snow.
It is so great that snow in lower layers becomes granular, hard and compact.
The pressure also quickens the melting of some of the snow, which on refreezing starts turning into a granular ice.
Again it is the pressure of the overlying layers which makes this solid mass of ice mobile.
This great mass of ice moving more under its own weight is called a glacier.
Its velocity is very low and it moves from a few centimetres to a few metres in a day.
Types of Glaciers
On the basis of their location or area of origin, glaciers are divided into two types:
 Continental Glaciers
A thick ice sheet covering vast area of land is called a continental glacier.
The thickness of ice in such regions goes up to thousands of metres.
Glaciers of this type build up at the centre and move outward in all directions.
Continental glaciers of today are found mainly in Antarctica and Greenland.
The precipitation in these regions occurs in the form of snow.
It gets accumulated year by year because of relatively slower rate of its melting.
 Valley Glaciers
When a mass of ice from the high mountainous regions starts moving down into the pre-existing valleys, it is called a valley glacier or a mountain glacier.
The shape of the valley glaciers depends on the valley it occupies.
Where the valley is broad, the glacier spreads outwards and where the valley is narrow, the glacier contracts.
The longest glacier in India is the Siachen Glacier in Karakoram Range which is 72 kilometres long.
Gangotri Glacier in Uttarakhand is 25.5 kilometres long.
There are many smaller glaciers in other parts of the Himalaya.
Their length varies from 5 to 10 kilometres.
The two important rivers of India, the Ganga and Yamuna, origin nate from Gangotri and Yamunotri glaciers respectively.