Valley glaciers are long, narrow streams of ice that originate in the snowfields of high mountain ranges and flow down preexisting stream valleys. They range from a few hundred meters to more than 100 kilometers in length. They receive an input of water (in the form of snow) in the higher reacher of mountains, and they have a system of tributaries leading to a main trunk system. Their flow direction is controlled by the valley the glacier occupies, and as the ice moves, it erodes and modifies the landscape over which it flows. The ice may be as much as 1,000 meters deep and flow slowly.
Valley glaciers are responsible for some of the most rugged and scenic mountainous terrain on earth. The Alps, the Sierra Nevadas, the Rockies, and the Himalayas were all greatly modified by glaciers during the last ice age, and the shapes of their valleys, peaks, and divides retain the imprint of erosion by ice.