Wanderlust World Wanderlust World The most reliable basis for correlating a shale layer in New York with one in California—and concluding they were deposited simultaneously—is the presence of the same index fossils in both layers.
Index fossils are remains of organisms that were:
When identical index fossils appear in separate rock layers, it strongly indicates those layers formed during the same narrow window of geological time, even if the outcrops are thousands of miles apart.