Here's why:

* Cretaceous rocks are indeed where you find many dinosaur fossils. The Cretaceous Period (roughly 145 to 66 million years ago) was the last period of the Mesozoic Era, and dinosaurs were thriving during this time.

* Tertiary rocks, now called Paleogene and Neogene rocks, do contain some dinosaur fossils, but very few. This is because the Cretaceous-Paleogene extinction event wiped out most dinosaurs about 66 million years ago. While some small, bird-like dinosaurs survived, they were not the giant, familiar types we usually think of.

In summary:

* Dinosaurs are primarily found in Cretaceous rocks, but a few fossils exist in Paleogene rocks, though they are significantly rarer.

* The statement is not entirely accurate because it implies a complete absence of dinosaur fossils in Tertiary/Paleogene/Neogene rocks.