Abrasion is the mechanical scraping of a rock surface by friction within rocks and moving bits during their transportation by wind, glacier, waves, gravity, flowing water or erosion. After friction, the changing particles remove loose and weak mess from the side of the rock.
You take any of the "signs of change in populations of organisms and their environments". For imagine, Australopithecus (early man) turning to Homo sapiens. In that way they could tell that you took this journey.
