Generally speaking, the middle of a valley glacier moves more rapidly than its sides (Fig. 236), but in some cases, especially in large glaciers, there are found to be two or more main lines of movement, with belts of lesser movement between. The top of a glacier moves, on the whole, more rapidly than the bottom, though the observations made do not show that the rate of movement diminishes regularly downward, and it probably does not so diminish in many cases. In Switzerland, where the glaciers have been studied more carefully than elsewhere, the determined rates of movement range from one or two inches to four feet or more per day. Some of the larger glaciers in other regions move more rapidly, but it does not follow that large glaciers always move faster than small ones. The Muir glacier of Alaska has been found to move seven feet or more per day,[126] and some of the glaciers of Greenland have been found to move, in the summer time, 50, 60, or even more feet per day. A single estimate as high as 100 feet per day has been made; but these high rates have been observed only where the ice of a large inland area crowds down into a comparatively narrow fjord, and debouches into the sea, and then only in the summer. In the case of the glacier with the highest recorded rate of summer movement, 100 feet per day, the advance was only 34 feet at about the same place in April. [Illustration: +Fig.+ 236.—Diagram to show the rate of movement of the Rhone Glacier at various points in its course at centre and sides. It also shows the fluctuations in the positions of the end of the glacier between 1874 and 1882, and the profile of the ice. (Heim.)] The _average_ movement of the border of the inland ice of Greenland is very small. Rink says that “between 62° and 68° 30′, the edge of the inland ice is almost stationary for a remarkably long distance.”[127] The observations of the authors between 77° and 78° were of like import. Probably the average movement of the border of the Greenland ice-cap is less than one foot a week. =Conditions affecting rate of movement.=—The rate of glacier movement appears to depend on (1) the depth of the moving ice; (2) the slope of the surface over which it moves; (3) the slope of the upper surface of the ice; (4) the topography of the bed over which it passes; (5) the temperature; and (6) the amount of water which falls upon it or is carried to it by the drainage of its surroundings, in addition to that produced by the melting of the glacier itself. Great thickness, a steep slope, much water, smoothness of bed, and a high (for ice) temperature favor rapid movement. Since some of these conditions, notably temperature and amount of water, vary with the season, the rate of movement for any given glacier is not constant throughout the year. Other conditions, especially the first of those mentioned above, vary through longer periods of time, and occasion periodic variations in the rate of movement.