Story, first published: Threads of Time (1974)
The story is told using the device of an unfinished manuscript. Two professors, one in geology and the other in Greek history, are vacationing in the Wisconsin hills. The geologist narrator is looking for a lost mine deserted by the miners because they found an object that started ticking each time they went into the cave "like it was trying to talk to them." The history professor finds a cube containing a holograph of the Battle of Marathon, evidence that someone from the future has traveled in time.
The plot rapidly thickens as the professors meet the time travelers from the future who are searching for the cause of the nuclear wars that destroyed most of their past history. The ticking comes from a cylinder that the time travelers explain is a space capsule sent by an alien race whose sun went nova. One of the time travelers, Angela, treats the geologist to a time trip by joining minds and also to a psychic sexual experience. The story ends with the history professor disappearing into the past, and the geologist finding two more cubes. One is a photo of the coronation of Charlemagne, and the other of a crucifixion - but the cross is not tall, the ankles are tied not nailed, and there is no placard, no crown of thorns, and no two companions. Here the manuscript ends.
Ewald, Robert J.: When the Fires Burn High and the Wind is from the North, p.113-114