G10 is fiberglass and epoxy laminate that is highly compressed and I believe is heat cured. It looks like thick plastic sheet. The mast step, as designed, was supposed to be bolted at each corner. Modifying it by cutting the corners required the redrilling of mounting holes on either side of the mast. I assume there was flex fore and aft -faulty rig tuning or poor design?- and that's why it cracked at the mounting holes, but really not sure. Maybe just spending it life on San Francisco Bay took its toll. I've seen one other with the same issue, though not as severe as mine. It had in mast furling as well.