Why does the colour of the ice vary from Summer to Winter? There seem to be very distinct layers through the ice. Is it due to freeze thaw?

The surface of glaciers is covered with debris, silt, sand and other particles. As the ice melts, this debris comes up to the surface, giving the surface of the glacier a grayer color.

Does that apply as you descend a crevass/ice cave? The variations are merely a build up of debris? I don't understand why the colour variations are in such pronounced bands?
Jersey Tomater (or anyone) can you explain?

http://www.blueiceberg.com/nature/nature38.htm

Click onto the Blue Ice Berg story, after the story there are facts about ice bergs and glaciers.