The moss covered, jasper walls of Avalanche Creek are one of my favorite aspects of Glacier National Park. That and how this creek is a torrent of colors, particularly after rain.
The stone walls are jasper, a stone that polishes well and usually breaks with a smooth surface.en.wikipedia.org/wiki/Jasper
When jasper (which forms the majority of the bedrock in Glacier National Park), or really any other surface, is wet or polished, the color is much more brilliant. In this case, the stream has been carving out this canyon for quite some time [understatement perhaps] and all the surfaces are polished smooth. Between ice fractures and all the complex eddies, some rather unique rock formations have formed.