Twenty-two years ago, I was leading students on a tour of the Colorado Plateau for the first time. I was following a number of new roads, and I was learning as much on the trip as my students were. We were discussing rock art, the petroglyphs and pictographs left behind by the ancestral Pueblo people. We were driving down a canyon, and I mentioned how the rock art was emphasized by desert varnish, the dark manganese oxide deposits that often coat sandstone surfaces in the desert. I pointed to the first cliff I saw and said "that's the kind of varnish that makes for nice petroglyphs". I hit the brakes, because as I said it, I could see the rock carvings. I would have missed them.
The first petroglyphs that I saw from the highway on the trip long ago were stylized bighorn sheep. We passed by the same place during our vagabonding journey across the 39th parallel in July. We stopped and got out to have a look.
One of the things that I like about pictographs and petroglyphs is that they are the traces of a living moment. Archaeology and paleontology often deal with death: bones and shells are the remains of people and animals after life has ended. Rock art preserves an action of a living thinking being.
I feel the same way about paleoichnology (the study of trackways and burrows and other traces of living organisms). Living animals leave marks of their activities in the rocks, and finding these traces gives us a feel for action and movement, not death and decay. The second story of serendipity involved dinosaur trackways.
During our long-ago trip to the Colorado Plateau, I could count the total of my known dinosaur trackway sites on one finger...it was a spot near Tuba City, AZ that is marked on all AAA maps. I had come across a paper that suggested that perhaps hundreds of thousands of trackways were preserved along the top of sedimentary layer exposed in and around Arches National Park. We looked at a geologic map of the park, found an old mining road, and set out on a three mile hike in the broiling sun. Our toiling bore fruit...we found dozens of tracks in an isolated outcrop. They were eroded from a contact between a tidal mudflat deposit and a sand dune formation. The tracks were freshly exposed, and clearly would be eroded away within a few decades or centuries. It was a great moment for our students and me.
I was telling Mrs. Geotripper this story as we drove along Highway 191 north of Moab. I was saying something to the effect that any old ranch road branching off the highway probably would lead to a trackway site, and she said "like that one?" I braked and we pulled off onto the gravel access road, and immediately saw the sign in the picture below. The Bureau of Land Management was on the job!
|Photo by Mrs. Geotripper|
|photo by Mrs. Geotripper|
|photo by Mrs. Geotripper|
There was one more oddity about our day. As we walked up the hill towards the tracksite, I realized we weren't alone. There were hundreds of crows swirling in the sky over our heads (in the high resolution photo I counted at least 200 of them). I've seen lots of raven pairs in the region, but I have never seen such a huge gathering of their smaller cousins. Do any of my birder readers have a comment on the flocking behavior of crows in the desert? We have huge murders, or flocks, of crows in the Central Valley where food is plentiful, but in the desert? I was reminded of the Crebain flocks in the Lord of the Rings!