A glacier is a slow-moving mass of ice. It may be as small as an average steamship; it may be less than a mile wide and several miles long; or it may cover hundreds of square miles. It may be less than a hundred feet, or a thousand feet or more, in thickness. It may move only an inch or two a day, or it may move several feet. Commonly it moves downward, but occasionally one moves upward. The movement is due to gravity and to the plasticity or rubbery nature of the ice when under sufficient pressure or weight. In a large glacier the weight of the superimposed icy stratum is immense; it is greater than the bottom layers can support. Under the enormous pressure the bottom layers crawl or flow from beneath like pressed dough. This forced mass moves outward in the direction of the least resistance--commonly down the slope. Glacier ice is formed by snow accumulating at a given point more rapidly than it melts. This is due chiefly to wind, snowslides, and heavy snowfall. The glacier, heavy and powerful, planes, polishes, and reshapes the surface over which it travels, or the walls with which it comes in contact. Most of the lake-basins were gouged out by glaciers. Mountain-ranges have been worn down to hills or plains; cañons and depressions have been filled, and extensive areas overlaid with ground-up rocky material. The gentle snowflake has been the earth's chief maker of scenery and soil. Snowflakes, working _en masse_ and through long periods of time, have formed glaciers and as such have wrought wonders. A moraine is an embankment or delta of boulders and crushed rock deposited by a glacier or ice river. Though commonly at the end, it may be both along the side and at the end of a glacier, or of the channels which the glacier once filled. All the mountainous National Parks have important glacial records or ruins that almost entirely cover them. These are moraines, soil-deposits, glaciated cañons, and lake-basins. Vast is the quantity of material picked up and transported by glaciers. Mountains are moved piecemeal, and are ground to boulders, pebbles, and rock-flour in the moving. Besides the material the glacier gathers up and excavates, it carries the wreckage thrown down upon it by landslides, and also the eroded matter poured upon it by streams from the heights. Most of the material that falls upon the top of the upper end of the glacier gradually works its way to the bottom. At last, with the other gathered material, it is pressed against the bottom and sides and used as a cutting, rasping, or grinding tool till worn to pebbles or powder.