Pictured above is a bluff of St. Peter Sandstone exhibiting some spectacular black staining. The bluff is exposed near the confluence of Sylamore Creek and the White River north of Mountain View, Arkansas. Bluffs with this staining are referred to as “painted” because it looks like paint has been poured over the face of the rock.
The stains, which are manganese oxide, were deposited by groundwater as it seeped from the sandstone. The St. Peter Sandstone contains a minute amount of manganese that gets picked up by water as it flows through the rock. When the groundwater flows out of the sandstone, some of it evaporates leaving the manganese behind. Over time, a coating of manganese builds up on the bluff face.
The St. Peter Sandstone is also found along certain reaches of the Buffalo National River. The “Painted Bluff” – as it is known locally to river folk – is another great example of manganese staining.
Pictured above is a travertine falls. It looks like a waterfall except that, rather than being water, it’s composed of solid rock.
Travertine is made of calcite which also forms stalactites and stalagmites. Like those familiar cave features, travertine falls form by precipitation from water; the water is flowing in a creek, over a ledge instead of dripping from a cave ceiling. As the travertine precipitates in layer upon layer, it begins to take on the appearance of flowing rock.
Dripstone features like these only form in areas where the groundwater carries a high load of dissolved carbonate minerals. This one was photographed in Searcy County, Arkansas, not far from the Buffalo National River, near the contact between the St. Peter and Plattin Formations.
For another view of this travertine falls click here
Springs are abundant in the Ozark Plateaus Region in northern Arkansas. The spring above flows to the surface along a bedding plane between the Plattin Limestone (upper half of picture) and the St. Peter Sandstone (covered in lower half of picture). It is common to see springs at the base of limestone units. Limestone is more easily solutioned than sandstone or shale, allowing water to travel downward from the surface by cracks and through openings in the rock. Once the water reaches the sandstone (as pictured above) and can no longer travel vertically, it will flow laterally along the bedding plane between the limestone and the sandstone until it reaches an outlet such as a spring along a hillside or in a valley.
Sandstone pipes are vertical cylindrical features that are commonly preserved in the St. Peter Sandstone in northern Arkansas. They are made up of the same sand as the surrounding rock. These features were observed in Ordovician-aged sandstone in Arkansas by geologists as early as 1916. Research by other scientists showed that these pipes formed in sand that was slightly deformed by a column of water rising through it from a lower horizon and feeding a spring at the surface. This sand then lithified into the rock we see today which includes the sandstone pipe. A modern-day example of sandstone columns forming in springs is present in the Dismal River, in the Nebraska Sand Hills. At this location, boiling (motion from water pressure, not temperature) sand springs have developed, fed by groundwater moving upward along cylindrical conduits. In the picture above, the sandstone surrounding the pipe has eroded away leaving the sandstone pipe standing in relief.
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.