A chatter mark is a signature in stone recording the movement of glaciers. They are formed by vibratory chipping of a bedrock surface by boulders carried in the base of a glacier. Chatter marks tend to be crescent-shaped and are commonly arranged in nested series, with the orientation of the fractures at right angles to the direction of glacial movement.
With more than 300 glaciers and countless snowfields, North Cascades National Park is the most heavily glaciated area in the United States outside of Alaska. The North Cascades Glacier Climate Project web page is a great resource for exploring these mighty movers of mountains and John Scurlockâ€™s aerial photography provides an amazing perspective of glaciers from above.
–Geologic information from Encyclopedia Britannica and Wikipedia.org.