A thick ice sheet covering a 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. The precipitation in these regions occurs in the form of snow . It gets accumulated year by year because of the relatively slower rate of its melting . 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. As a glacier moves over the land , it drags rock fragments , gravel and sand along with it. These rock fragments become efficient erosive tools . With their help glacier scrapes and scours the surface rocks with which it comes in contact. This action of glacier leaves behind scratches and grooves on rocks .