A fault is a fracture in a piece of rock. A fissure is a crack or a split in something. Geologists are people that study rocks. "It's not my fault" means "I am not responsible", and this references the crack in the cliff face. This joke is playing on the word "fault" and its different meanings.
Two geologists were staring at a huge fissure in a cliff face and one was overheard to say ' It ' s not my fault ' .