Biaxial compression experiments were carried out to investigate the mechanism of feather crack nucleation in columnar freshwater ice. The tests were performed with a strain rate of 10-3 s-1 at -10 ¿C. It is shown that the nucleation of feather cracks corresponds to a transition when the last of the remaining ligaments bonding the two grains fractures and the grain boundary transforms into a traction-transmitting frictional crack. This is in contrast to the nucleation of wing cracks, proposed earlier, where the nucleation was due to the onset of viscous sliding of grains. In addition to the mechanism, the nucleation stress for feather cracks was measured for different confinement ratios. A fracture-mechanics-based model for the nucleation process is presented by V. Gupta and R. C. Picu . ¿ 1998 American Geophysical Union