collected a sample, designated as Sample 76535, at geology station 6 near the base of the North Massif; the sample, a troctolite, was later identified as the oldest known "unshocked" lunar rock, meaning it has not been damaged by high-impact geological events. Scientists have therefore used Sample 76535 in thermochronological studies to determine if the Moon formed a metallic core or, as study results suggest, a core dynamo. Before concluding the moonwalk, the crew collected a breccia rock, dedicating it to the nations of Earth, 70 of which were represented by students touring the U.S. and present in Mission Control Center in Houston, Texas, at the time. Portions of this sample, known as the Friendship Rock, were subsequently distributed to the nations represented by the students. A plaque located on the LM, commemorating the achievements made during the Apollo program, was then unveiled. Before reentering the LM for the final time, Cernan remarked, Cernan then followed Schmitt into the LM; the final lunar excursion had a duration of seven hours and fifteen minutes. Following closing of the LM hatch and repressurization of the LM cabin, Cernan and Schmitt removed their spacesuits and reconfigured the cabin for a final rest period on the lunar surface. As they did following each of the previous two EVAs, Cernan and Schmitt discussed their geological observations from the day's excursion with mission control while preparing to rest. Solo activities While Cernan and Schmitt were on the lunar surface, Evans remained alone in the CSM in lunar orbit and was assigned a number of observational and scientific tasks to perform while awaiting the return of his crewmates. In addition to the operation of the various orbital science equipment contained in the CSM's SIM bay, Evans conducted both visual and photographic observation of surface features from his aerial vantage point. The orbit of the CSM having been modified to an elliptical orbit in preparation for the LM's departure and eventual descent, one of Evans' solo tasks in the CSM was to circularize its orbit such that the CSM would remain at approximately the same distance above the surface throughout its orbit. Evans observed geological features visible to him and used handheld cameras to record certain visual targets. Evans also observed and sketched the solar corona at "sunrise," or the period of time during which the CSM would pass from the darkened portion of the Moon to the