'Chatter marks' are crescent shaped fractures and gouges occasionally found on de-glaciated bedrock. Under a thick glacier, hundreds of feet of ice press boulders down onto bedrock, creating tremendous pressures where they make contact. Scientists attribute chatter mark formation to the irregular movement of boulders against bedrock under a glacier. I usually think of glacier motion as regular and gradual, almost 'fluid', but even in my few years exploring I have witnessed events such as glacier outburst floods and rock avalanches onto glaciers that might cause sudden glacier surges. I wonder if these sorts of events could cause the irregular glacial movements that produce chatter marks. Regardless, simply witnessing these events reminds me that glacier change is not always predictable and gradual. Photographer: Bill Eichenlaub