Bones from a single animal lie buried within a single horizontal layer of sediment. A few million years later, erosion starts to expose the layer. Gullies are carved into the soft sediment, and part of the layer is exposed in the slope of the hill. Each time there is a hard rain, a centimeter or more of sediment can be loosened from the edge of the slope, and washed downhill. This rain can free a fragment of one of the bones, which also moves down the hill, and can be found loose on the ground.
The fossil hunter finds such a loose fragment, and then goes directly uphill looking for the place where it weathered out of the ground. When he finds that place, he digs and gets more of that bone and other bones of the same animal in the same layer.
If no one finds the fragment that is washed down the hill, it will be washed away in the next rain. Then another fragment will come out of the layer. Only a few fragments can occur loose on the surface at any one time. Earlier fragments have washed away, and the rest of that bone and other bones are still buried in the layer of sediment. It is impossible that most bones of a million-year-old skeleton will be exposed and lying on the surface at the same time.
On the morning of November 24, 1974, Tom Gray led his eager professor Donald Johanson directly to the place where all the Lucy bones were lying loose on the surface. Nothing more was found in the ground. Those bones could not have been lying there for more than a year or so. Johanson explained this in his book Lucy's Legacy in 2010. He wrote that the fossils were lying in such a way that "a single desert thunderstorm could have washed them off the plateau, over a cliff and into oblivion, forever."
I think that Gray knew those fossils were lying there at Hadar Location 162, where the same team had searched in 1973. Maybe Gray did not know exactly where the fossils would be, or if they would still be there when they arrived that morning. In his first book (Lucy − The Beginnings of Humankind, 1981) Johanson reported that when they returned to camp after finding the Lucy fossils, this is how Gray told the others: “We’ve got it,” he yelled. “Oh, Jesus, we’ve got it. We’ve got The Whole Thing!”
I have done a lot of fossil collecting in such erosional areas, and know how it works. In the soft eroded hills of France, there are many fossils of all kinds. After a hard rain, hobby collectors go to their favorite places on a Sunday afternoon, and pick up the newly exposed fossils − things like small ammonites, brachiopods, snails, corals, and even dinosaur eggshell fragments and dinosaur bone fragments. If one does not pick them up after a hard rainstorm, they will be washed away in the next hard rainstorm.