Folds in the lower reaches of valley glaciers can be caused by powerful surges of tributary ice streams. Susitna Glacier in the Alaska Range, seen in this Terra image from 2009, displays this phenomenon. Vegetation appears in shades of red and snow is white. The glacier’s surface is marbled with dirt-free blue ice and debris-coated brown ice. Infusions of relatively clean ice push in from tributaries in the north. The glacier surface appears especially complex near the center of the image, where a tributary has pushed the ice in the main glacier slightly southward. Steep cliffs and slopes exist in the glacier surface, with most of the jumble the result of surges in tributary glaciers.