At Fenway Park, long before there was Pesky's Pole in the right field corner, and before the Green Monster was the Green Monster in left field, there was Duffy's Cliff - a 10-foot incline just before the wall. It was nicknamed after George "Duffy" Lewis, Red Sox outfielder from 1910-1917 who had worked hard to learn the physics of the incline and eventually tamed the beast like no other outfielder.
Some sources say it was a 15-degree incline while many others say it was 45 degrees. From this early photo, it looks closer to 45 to me. At least toward the foul line it does. Maybe it tapered off toward center. For reference, the incline in the outfield that used to adorn the ballpark formerly known as Minute Maid Park in Houston was 30 degrees.
"Duffy's Cliff" remained a part of the playing field at Fenway from 1912-33, but was often used to seat overflow crowds. When it wasn't occupied, well, Lewis knew the point of being an outfielder was to put batters out, not to wait for fly balls to hit the incline and roll back to him. So he studied and experimented and learned the best angles to approach the hill while chasing fly balls, and even how best to throw while coming down off it.
During 1934 renovations of Fenway Park, the outfield was flattened. It would be another 13 years before the ads were removed from the great wall in left field which was then painted its namesake green.