A billion years is a long way to walk

My husband and I both had goals for our visit to the Grand Canyon at the beginning of this week. He wanted to give himself a workout that would leave him feeling sore all week. I wanted to check out the Trail of Time, an exhibit that some of my colleagues from New Mexico and Arizona had been developing. I didn't know whether it was complete, or where it started, but I'd been hearing Karl Karlstrom and Laurie Crossey talk about it for years.

The rangers working at the main visitor center had heard about it at a briefing, and had some pamphlets hidden behind their desk, but weren't quite sure exactly where it started. They knew it was along part of the Rim Trail , and it sounded like it was near the Bright Angel trailhead (where my husband wanted to go, to start burning out his calves), so we parked, sent my husband on his way, and started wandering along the trail.

The trail isn't very obvious. Right now, it consists of a series of metal plates put in the ground every ten meters, and smaller metal circles every meter. The plan is to create a hikeable geologic time scale, at a scale of 1 meter = 1 million years, complete with signs about the geology. They've got the scale in place now, and since I had heard Karl and Laurie talk about the geology, I could fill some of the story in for myself.

The oldest plates that we found were for about 1940 million years, somewhere near Vercamp's Visitor Center. They get gradually younger toward the east, so as we hiked, we walked from the past towards the present day.

The oldest rocks in the Grand Canyon are around 1840 million years old. They, and similar rocks (such as the oldest metamorphic rocks in the mountains north of Durango) record the growth of ancestral North America by collisions with chains of volcanic islands.

Sometime around 1700 million years ago, a mountain-building event called the Yavapai Orogeny occurred. Well, we assume it built mountains. It certainly deformed and metamorphosed the rocks, and melted them to form some granites. And from the 1720-million-year marker, you can see some of those old rocks, way down at the bottom of the canyon, but the view of them is better further along the trail.

After that... well, there's evidence for other stuff happening elsewhere in the Southwest, but in the Grand Canyon, those metamorphic rocks were gradually being unroofed. And I mean gradually. It's easy to throw around Precambrian ages, but it's harder to think about what they mean. The next youngest rocks, the Grand Canyon Supergroup, were deposited starting around 1200 million years ago. That's around 500 million years after the older rocks were metamorphosed. For comparison, if we went back 500 million years before today, there would be trilobites doing whatever squiggly things trilobites did, and Pangea wouldn't even have begun to come together. And if that perspective doesn't help, you can walk that 500 million years with a five-year-old who wants ice cream.

You can't actually see the Grand Canyon Supergroup from the 1200-million-year marker, but it's visible below all the flat-lying rocks from viewpoints further to the east.

And then there's another gap. There are other tilted rocks above the Grand Canyon Supergroup, rocks related to the separation of North America from whatever was west of it around 800 million years ago. But we didn't make it to 800 million years ago. We didn't make it to the Cambrian explosion, a little after 600 million years ago. We didn't make it to the first land plants. We didn't make it to the age of the rocks were were walking on, at around 250 million years. We didn't make it to dinosaurs, or the formation of the Rocky Mountains, or any of the ages proposed for carving the Grand Canyon itself.

We got to 1000 million years - one billion years - and that was enough.

One billion years is a long way to hike when you've been promised ice cream.