Complex sentence: On June 4 the mountaineers were able to start their summit bid starting on 6:40 am; the late start resulted from a water bottle having spilled its content on their sleeping bags and the two climbers had to again melt snow for drinking water.
Split into simpler sentences: On June 4 the mountaineers were able to start their summit bid at 6:40 am, later than orignally planned. A spilled water bottle caused the delay since the two climbers had to again melt snow for drinking water.