The sedimentary rock is called the Tapeats Sandstone. It is typical of the kind of sand that might accumulate along a shoreline, although in this case, the beach existed 525 million years ago. It would have been a strange environment to those of us in the world today...there was no life on land, just barren rock and sediment. There was life in the sea, but few species that would be familiar to us. The fossils most commonly found are trilobites, arthropods that are vaguely similar to pillbugs and horseshoe crabs. There were sponges, and some brachiopods (lampshells), and many strange and wonderful soft-bodied creatures who left little in the way of a fossil record (google 'Burgess Shale' if you want to know what they looked like).
Then the land slowly sank, and sea transgressed across the landscape. The beach sands of the Tapeats buried all but the highest of the hills, which projected as modest islands. They weren't completely inundated until several tens of millions of years later. That's what I found incredible about the Grand Canyon on my first trip all those years ago. When you know what to look for, you can see these islands exposed in the canyon walls.
Many years ago, the first geologists to explore the secrets of the Grand Canyon called this "The Great Unconformity". I agree...
|I place my hand on a billion year gap in time at Frenchman Mountain near Las Vegas. This is the same unconformity as the one seen in the deepest part of Grand Canyon|
The rock samples pictured above are to be found along the Trail of Time between Verkamps Visitor Center and Yavapai Point on the South Rim, a recently completed exhibit. The sponsors and innovators did a great job of bringing alive the concept of geologic time.