Painted Bluff at Buffalo Point along the Buffalo National River
Painted Bluff gets its name from water seeping over the top portion of the bluff. This darkens the rock giving it a painted look. The rock formation that is painted is the St. Peter Sandstone. The rock formation below the painted portion of the bluff is the Everton Formation. Thin bedded limestone and dolostone layers make up the lower portion of the bluff. The rock formations are both Ordovician (485-444 million years ago) in age, however there is an unconformity between the two formations. An unconformity is a rock surface that represents a gap in the geologic record either due to a period of erosion or non-deposition. Notice the wavy line halfway down the bluff. This wavy line separates the sandstone from the limestone and is the unconformity surface. The top of the limestone was at one time the rock exposed at the earth’s surface in this area. The limestone was eroded, and then the sandstone was deposited upon it.
Well, much drier this week, but of course the heat is back!
Ferns on Nubbin Ridge
Paw paws on Middle Fork
Mushrooms on Lost Creek
We’re still working on the Shirley quad this week. Started out Monday by dropping down a steep side drainage off Nubbin Ridge nearly to the Middle Fork, then right back up another one. Got good points at the base of the Witts Springs and Cane Hill. The Imo is at the bottom of the valley but the very large boulders eroding at the top cover most of it. Relief was around 400 feet.
Typical Witts Springs on Lost Creek
near Cane Hill top (1)
Cane Hill (2)
Cane Hill (3)
Cane Hill (4)
Cane Hill (5)
Cane Hill (6)
Cane Hill (7)
near Cane Hill bottom (8)
near top of Imo
Tuesday and Wednesday we were back in the headwaters of Lost Creek. There are many branches to this creek, and there seems to be a fairly large structure crossing them approximately east/west. Could be a fault, or maybe just a fold of some kind—will take much more data collection to be sure. We did observe a repeating sequence from higher elevations to lower—now we just have to figure out which formation they are in! In the Ozarks, the Pennsylvanian-aged rocks are a series of sandstone and shale units deposited in a shallow marine environment. That means they are all very similar to each other and the differences are subtle and vary from place to place. To differentiate one formation from another, we depend a lot on context, which means we must take meticulous notes on each outcrop in the hopes we can see a pattern in the stratigraphy that matches something known. We’re starting in the northwest corner of Shirley because we’ve already mapped the surrounding quads in that area, so have a pretty good idea of the geology there. Things are getting more complicated as we reach the structure because the strata are displaced somewhat from one side to the other. We will continue to add points next week in order to discover what the rocks are doing there.
Since I’ve mentioned some of the formations in the area and how difficult they are to differentiate, let me discuss one that has stayed fairly consistent across the Boston Mountains Plateau. The Cane Hill is a Member of the Hale Formation, and its type section is in northwest Arkansas. It is typically very thin- to thin-bedded, ripple-bedded, very fine-grained sandstone with interbedded shale and thicker shale units. There are abundant trace fossils in the Cane Hill including asterosoma which is a somewhat star-shaped feeding burrow.
Bluffs of this sandstone often affected by a particular weathering phenomenon where the cement holding the sand together actually solutions out which lets the individual grains fall away eventually forming a concave structure. I have included a good set of photos of the Cane Hill and a few of its characteristic structures. It is getting to be over 300 feet thick as we continue to map toward the southeast.