Amazing how few people will check out a story for themselves.
From another part of the NPS site:
The Grand Canyon is considered one of the natural wonders of the world largely because of its natural features. The exposed geologic strata - layer upon layer from the basement Vishnu schist to the capping Kaibab limestone - rise over a mile above the river, representing one of the most complete records of geological history that can be seen anywhere in the world. Geologic formations such as gneiss and schist found at the bottom of the Canyon date back 1,800 million years. This geologic incline creates a diversity of biotic communities, and five of the seven life zones are present in the park.
...The arid climate has been a benefit to the Paleontological resources of the park. The dry climate has been instrumental in preserving many prehistoric fossils deep within caves in Grand Canyon's geologic formations.
But hey, let's just go with what some nutball activist group claims, and run with it.