A muddy drive down Mountainside: Seeing the Hartsell up close
The USGS briefly describes the Hartsell Fm. in this general area as “Thin-bedded, fine-grained sandstone and greenish-gray shale interbedded with coarse limestone. Thickness 0 to 60 feet.” After returning to HPMORV park, I took a drive down the trail called “Mountainside”, which drops from ~850 feet to ~600 feet elevation. I collected a sample and took some photos, which I will show in this post, along with some (quite different) photos of this formation on the other side of Thompson’s Draw. This image shows my traverse downhill to join “Big Hill” trail.
There is a steep, muddy, drop down the top of the hill on Mountainside but then the trail (road, actually when weather is dry..never!) levels off. I climbed back up “Powerline Cutoff” and took this photo, which shows thin bedding in sandy rocks.
This is a pretty easy climb in the 4×4; the steps are like an asphalt highway for going uphill. The hand sample I collected was not particularly interesting.
I think the original top of this layer is to the left in the photo. It has a reddish (buff…LOL) hue and shows some fine cross bedding. The reddish color suggests that the sediments were deposited above sea level ~320 mya, and the fine layering only occurs in fairly quiet water with rhythmic variations in water depth (e.g. tides) and low energy from waves. I drove a little further (location 2 in the above photo) and took this photo, which suggests an intertidal location because of the lack of wave ripples, and the irregular surface.
I will jump ahead to the drive up the southern flank of Thompson’s Draw, where I found these great exposures of massive bedded sandstone with layers of thin-bedded sandstone with cross bedding. This is where the buggies go…there are trails over this ledge!
This doesn’t quite fit the USGS description, but rocks vary rapidly over short distances. The reason in this case is probably the environment in which these rocks were deposited, and the time interval over which the sediments were deposited. This was a shallow-water environment a lot like the Gulf coast, with lagoons, barrier islands, sand bars, and mud flats. My drive down Mountainside also covered several million years of history, but we cannot date these sediments…we only know for sure that the rocks get older downward…LOL!