A Separate Peace is written as a flashback - the first chapter starts with Gene returning to Devon fifteen years after graduation, remembering incidents that had occurred while he was a student. The marble stairs and the tree raise memories of powerful, fearful events.
As Gene discovers upon entering the First Academy Building, the marble stairs are old, with "worn moons in the middle of each step" that "were not very deep." The only unusual feature Gene notes is that "the marble must be unusually hard." In light of the event that took place on those steps, the hardness of the material is significant.
Gene feels a little ridiculous looking for a tree - "at any other time I would have felt like a fool slogging through mud and rain, only to look at a tree." However, the significance of the events connected with one particular tree along the river made the process of finding it worth the inconvenience. In visiting it, Gene finds resolution of another fear from the past - "Nothing endures, not a tree, not love, not even a death by violence."