Sara was studying the methods scientists use to determine the ages of the rock layers. She was particularly interested in the use of index fossils. Four illustrations showing one to four rock layers Location 1 has 4 layers from top to bottom: D, C, B, A Location 2 has 3 layers from top to bottom: C, B, A Location 3 has only layer A Location 4 has these layers from top to bottom: D, C, A

After looking at the picture, she noticed that the rock layers at Location 4 differed from the other three locations. What likely occurred at Location 4?