Clast caught in the act of glacial plucking

This image, from the Beartooth Plateau of southcentral Montana, shows a granitic boulder detached, but not removed, from its bed. Strongly developed joint sets trending approximately vertically and horizontally are obvious in the image. A less well developed set of joints trending diagonally across the photo has loosened the clast, so only gravity holds it in place. Ice moving towards the camera may have allowed meltwater to freeze to both the clast and the glacier bed on the down-ice (low-pressure) side of the obstruction. Continued flow had apparently begun to pull the rock free, but stopped short of complete removal. This process provides clasts to the glacier bed to accomplish abrasion.