Many USGS scientists worked on the team monitoring the volcano, but it was graduate student Harry Glicken who had been manning the Coldwater II observation post for the two and a half weeks immediately preceding the eruption. The evening before the eruption he was scheduled to be relieved by USGS geologist Don Swanson, but something came up, and Swanson asked Johnston to take his place. Johnston agreed. That Saturday, the day before the eruption took place, Johnston ascended the mountain and went on a patrol of the volcano with geologist Carolyn Driedger. Tremors shook the mountain. Driedger was supposed to camp on one of the ridges overlooking the volcano that night, but Johnston told her to head home and said that he would stay on the volcano alone. While at Coldwater II, Johnston was to observe the volcano for any further signs of an eruption. Just prior to his departure, at 7 p.m. on the evening of May 17, 13½ hours before the eruption, Glicken took the famous photograph of Johnston sitting by the observation-post trailer with a notebook on his lap, smiling.