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.
A Breccia is a rock made up of angular rock fragments mixed with finer sediment. The one above was deposited about 450 million years ago (Ordovician) in a shallow sea in what is now the Ozark highlands, north of Mountain View, Arkansas.
Breccias can form by a variety of processes such as landslides, volcanic eruptions, storm events, cave and sinkhole collapses and others. This one likely formed during a storm. Turbulent ocean waves and currents washed fragments of marine organisms (dark gray) into calmer water where marine mud (light gray) had recently accumulated. The turbulence ripped some of the mud up and mixed it with the organic material.
Breccias are just one of many clues, recorded in rocks, that help geologists understand how the earth has changed through time: a story that itself constantly evolves as new rock is unearthed and studied.
Pictured above is approximately 450 million year old (Ordovician) St. Peter Sandstone photographed near the White River, north of Mountain View, Arkansas. At the center of the picture is a feature not uncommon in this formation: a “quicksand structure”. Though not well understood, this kind of structure is believed to be a preserved conduit through which ground water once flowed to the surface in a boiling spring.
A boiling spring is where ground water rises up through unconsolidated sediment such as sand, keeping the sediment in continuous motion so that it appears to be boiling. Because ground water contains dissolved minerals, the tunnel through which the water is rising can eventually become hardened by precipitated minerals. Today, as the sandstone that formed from that sediment is being eroded away, the conduit stands in relief because it’s a little harder than the surrounding sandstone due to that mineralization.
Clues like these “quicksand structures” offer us a glimpse into what the environment was like when the rock was being deposited.
Above are speleothems- they’re also called dripstones, stalagmites and stalactites, or other names depending on their shape. They can be seen when touring Blanchard Springs Caverns located about 10 miles north of Mountain View, Arkansas. The cave is full of breathtaking features like these which form by precipitation from ground water that drips from the cave ceiling. The groundwater, which has traveled through rock and soil before reaching the cave, is saturated with minerals it dissolved along it’s path. The principal mineral that forms the dripstones is calcite. As the groundwater enters the open air of the cave a pressure drop causes the calcite to come out of solution and precipitate a tiny coating on the outside of the growing speleothem. Over time some of them can reach from the roof to the floor of the cave and elaborate, and marvelous shapes develop.