Glaciers move slowly, but Bear Glacier seems to have racing stripes. Glaciers pick up dirt and debris from the rocks they pass. They deposit that material in accumulations called moraines. A lateral moraine is the material on the sides of a glacier.
Two glaciers flowing together form a medial moraine in the middle where they join and show up as those dark stripes. When a glacier has a medial moraine, it’s made up of more than one ice flow.